Technical FAQ

The backflow prevention function of load switch IC is a function that prevents backflow of current to input side when the voltage on output side becomes higher than input voltage. It is sometimes mounted on voltage regulator or charger IC, etc. in addition to load switch IC.

Case example that requires the backflow prevention function

1.Preventing the backflow to input wide when voltage higher than input voltage is applied to the output side for Vout "OR" connection.
2.Preventing the backflow from capacitor or power storage device that is connected to output side to input side.
3.Preventing energization of output voltage to input side by cutting off input side and output side when external voltage is applied to output side and the power storage device is connected to output side.

Cautions on backflow prevention function: Backflow may occur even if backflow prevention function is mounted

Although the backflow prevention function seems to prevent backflow from the output side to the input side under any conditions, the current may backflow from output side to input side depending on control system or usage.

For this reason, it is necessary to select a product equipped with appropriate backflow prevention function based on the control system and characteristics of backflow prevention function.
The following explains types and usage of backflow prevention functions.

Types of backflow prevention function: backflow prevention function and complete backflow prevention function

The backflow prevention function is roughly classified into two types.

(1) Backflow prevention function
The backflow prevention function is adopted for general load switch IC, etc.
Only when the switch part of load switch IC is turned OFF, the backflow prevention function surely operates.

With this type of backflow prevention function, whether there is backflow is judged by input voltage becoming lower than output voltage by more than a certain level.
If the input capacity is small or there is no current sinking from input side, the input voltage rises up to output voltage due to backflow current to input side. For this reason, the dropout voltage with which the backflow prevention function operates cannot be ensured and the phenomenon that the backflow prevention function does not work occurs.

As an advantage of this judgment method, when the load switch is turned ON, FET at the switch part can be always fully turned ON and the on-resistance can be reduced.
If the load switch is turned OFF, the input voltage becomes lower than output voltage by more than a certain level before the backflow current flows, so the backflow can be prevented.

(2) Complete backflow prevention function
With this function, the backflow prevention function operates regardless of when the switch part of load switch IC is turned ON and OFF.

With this type of backflow prevention function, the backflow prevention function operates when the output voltage becomes slightly lower than the input voltage.

In this judgment method, the backflow can be prevented even if the switch part of load switch IC is turned ON.
However, when the load switch is turned ON, it is necessary to keep dropout voltage above a certain level in order to prevent the backflow prevention function from malfunctioning. For this reason, voltage drop and conduction loss tend to increase compared to general backflow prevention function.

Here introduces how to calculate pull-up resistance / pull-down resistance when they are not specified for an IC that has built-in pull-up resistance / pull-down resistance.

Step1：Confirmation of block diagram

Before calculating pull-up resistance/pull-down resistance, it is important to confirm the block diagram.
This is because that pull-up/pull-down may not be executed by resistance, but the pull-up/pull-down may be executed by a constant current element.

If pull-up / pull-down is executed by constant current element, the current does not vary depending on the applied voltage and the current becomes constant unlike resistance. Let's refer to the terminal current shown on the data sheet.

Step2：Calculation of resistance value

If the block diagram shows that pull-up / pull-down is executed by resistance, check the terminal current on the data sheet. Here explains it referencing the voltage regulator XC6228 that has CE terminal equipped with built-in pull-down resistance.

As XC6228 is equipped with pull-down resistance, check the terminal current at CE "H" where the current flows into pull-down resistance.

XC6228 CE "H" terminal current

Here, check the measurement conditions (CONDITIONS) specifying CE "H" Current.

" VCE=VIN=5.5V " described for CONDITIONS means that "H" Current when CE = 5.5 V is applied is specified. The voltage to which CE is applied to the pull-down resistance. Thus, if 5.5 V is applied to pull-down resistance, CE "H" Current flows.

If you understand this far, you can calculate the pull-down resistance value using Ohm's law.
The following formula is calculated by Ohm's law "R = V / I".

CE applied voltage = CE “H” Current x “pull-down resistance”

Using the above formula, the pull-down resistor can be calculated as follows.

Pull-down resistance MIN = 5.5V / CE “H” Current MAX = 611kΩ

Pull-down resistance TYP = 5.5V / CE “H” Current TYP = 1000kΩ

Pull-down resistance MAX = 5.5V / CE “H” Current MIN = 1833kΩ

Here calculated the resistance value referencing the pull-down resistance, however, you can also calculate the pull-up resistance in the same way.

What is ship function?

The ship function is a function to prevent the remaining battery power from decreasing during long-term storage after the shipment of product. The ship function is sometimes called shipping mode, etc.

The electronic devices that have no ship function are shipped while consuming current from the battery even after the shipment. In this status, the battery power decreases during long-term storage after the shipment.
When the product is delivered to an end user, the product may not operate because the battery power is low or it may not work for target battery life.

The ship function that suppresses current consumption from battery after the shipment is required to prevent these problems.

The diode whose forward voltage is 0 equipped with characteristics of passing current in only one direction is called ideal diode.

An IC (integrated circuit) that reproduces the characteristics of this ideal diode is called ideal diode IC. As the ideal diode has no forward voltage drop or current leakage, it has no forward voltage drop, loss, or current leakage that have occurred in discrete diode.

With an actual ideal diode IC, it is possible to completely eliminate forward voltage or current leakage because there are on-resistance and current leakage of FET in IC, however, forward voltage drop and power loss can be considerably reduced compared to diode.

Advantages / disadvantages of ideal diode IC

The ideal diode IC has various advantages compared to discrete diode.

Advantages

• Low loss due to such a small VF & low current leakage
  (XC8110/XC8111: VF = 20 mV, discrete SBD: VF = 0.3 to 0.4 V)
• As VF is small, output voltage fluctuation is small and it contributes to stable operation of system
• Equipped with various built-in protection functions
• Small PKG

Disadvantages

• Maximum output current and output voltage are restricted
• Number of products is small / less compatible compared to discrete diode

With a general voltage regulator and a step-down DC/DC, the output voltage when IC is in standby status (when the operation is stopped) decreases to 0V.

With a step-up DC/DC converter, the output voltage when IC is in standby status (when the operation is stopped) varies depending on the IC. Step-up DC/DC converters, types, and functions can be roughly classified into four types.

1. Non-synchronous type

This is a general step-up DC/DC converter that uses diode.
As a diode is used, input side and output side cannot be disconnected in standby status and electrical connection is executed via diode.

A step-up DC/DC converter on which a driver is mounted operates in the same way as non-synchronous type step-up DC/DC converter when parasitic diode of a driver on High Side is not controlled.

2. Load disconnection function (product equipped with built-in Low Side/High Side driver FET)

The driver FET on High Side is controlled so that the output voltage becomes 0V in standby status.
(Driver FET: OFF, parasitic diode: anode Vout, cathode Lx)

The load disconnection function disconnects input side and output side in standby status.
With this function, the output voltage decreases to 0V to prevent malfunction of system and reduce the power consumption.

3. Bypass function (product equipped with built-in Low Side/High Side driver FET)

The driver FET on High Side is turned ON so that output side can be electrically connected with input in standby status.

With the bypass function, the input voltage can be passed through to the output side in standby status.
Utilizing this function can supply the battery voltage to the system side by setting it to standby status when MCU is in sleep status, for example. For RF that requires high supply voltage or at high-speed operation, voltage such as 3.3 V can be supplied to the system side by implementing the step-up operation.
The power consumption of whole system can be reduced by this voltage switching.

4. Vout "OR" type (product equipped with built-in Low Side/High Side driver FET)

This is Vout "OR" type enabling OR connection that applies external power supply to the output side.
With Vout "OR" type, input voltage and output voltage are monitored for both of step-up operation and standby status.

It determines which is higher, input voltage or output voltage, and controls the polarity of parasitic diode of High Side driver FET to prevent backward flow via parasitic diode.

Output voltage fixed products (Vout products) and output voltage externally adjusted products (FB products) show the output voltage setting method for DC/DC and voltage regulators.

An output voltage fixed product (Vout product) is a product in which the output voltage is fixed inside the IC, and an output voltage externally adjusted product (FB product) is a product that can adjust the output voltage using an externally connected resistance, etc.

(a) Output voltage fixed products (Vout products)

An output voltage fixed product has a high-accuracy feedback resistance for adjusting the output voltage built into the IC. An externally connected resistance to adjust the output voltage is not required, making miniaturization and component reduction possible.

In addition, since the feedback resistance is built into the IC, the product is resistant to external noise and a higher feedback resistance value than with output voltage externally adjusted products can be used.

This makes it possible to hold down the quiescent current consumed by the feedback resistance, making it optimal for low consumption applications.

(b) Output voltage externally adjusted products (FB products)

An output voltage externally adjusted product requires the feedback resistance for adjusting the output voltage to be external to the IC. Since the output voltage can be changed by just changing the externally connected resistance, this type of product can be used to build multiple power supply lines using the same IC, which is advantageous for components control and inventory control.

A disadvantage is that an externally connected feedback resistance and a phase compensation capacitor are required, so there is a greater number of components compared to an output voltage fixed product.

In addition, since there is a feedback resistance external to the IC, the product is more easily affected by external noise, and if the feedback resistance value is too high, the IC could malfunction, so caution is required.

A function that operates when the input signal is High is said to operate at High Active or Active High.
Conversely, a function that operates when the input signal is Low is said to operate at Low Active or Active Low.

Load SW (XC8107): CE logical table

Nch open drain output and CMOS output show the type of output configuration of the output terminal.

Nch open drain output

CMOS output

Nch open drain output: output voltage

CMOS output: output voltage

The Nch open drain output is only input by the Nch FET between the output terminal and the ground terminal.
FET is only input to the ground side, so pull-up resistance and pull-up voltage needs to be provided externally. Providing pull-up voltage to match the output destination microcomputer, etc., makes it possible to change the level of the output voltage to match the output destination device.

In the CMOS output, a CMOS inverter is built into the output terminal.
Unlike the Nch open drain output, there is no need to provide a pull-up resistance or pull-up voltage. For this reason, a quiescent current that flows in the pull-up resistance does not exist in the CMOS output, so this is optimum for low consumption applications.

In addition to PWM control that prevents reverse flow of the coil current, PWM control that allows reverse flow of the coil current is called forced PWM control.

Condition : Vin=5V, Vout=3.3V/0mA

Unlike PWM control, pulse skip does not occur with forced PWM control even at light load conditions and it is easy to take noise countermeasures.

The characteristics of PMW control and forced PWM control are as follows.

PWM Control

This becomes DCM in light load states, and the duty increases as the output current increases.

In the light load state, the ON duty required to maintain the output voltage becomes very small and may become even lower than the minimum duty that can be controlled by the IC.

Under these conditions, the oscillation frequency cannot be maintained and a pulse skip that lowers the oscillation frequency occurs.

In addition, switching node ringing occurs in DCM. Switching node ringing is a cause of output voltage noise interference and radiation noise.

Forced PWM Control

With forced PWM control, it comes CCM even at light loads to reverse the coil current, so the duty is fixed from light loads to heavy loads.

Unlike regular PWM control, the duty does not become small during light loads, so a pulse skip does not occur during light loads.

Further, it does not become DCM, so switching node ringing does not occur and thus ringing is not a cause of noise.

A disadvantage is that the efficiency at light loads is lower than that of regular PWM control.

Further, with forced PWM control, the coil current must be reversed, so forced PWM control with an asynchronous rectifier type DC/DC that requires and externally attached diode cannot be conducted.

CL discharge (discharge function) is a function that rapidly discharges to capacitance charge connected to the output line during standby. TOREX these are called the CL discharge and CL discharge function, and they are also called by other names, such as discharge function and auto discharge function.

Without a CL discharge function, it is now possible to draw out the output capacitance load during standby, which causes the output voltage to gradually decrease. If the output voltage gradually decreases, it could cause downstream devices to malfunction.

In addition, a power-down sequence may be required by FPGA and other devices. In this case, a CL discharge function is used to quickly power down the output voltage when the power supply sequence is satisfied, so the product must be designed to follow the power-down sequence.

In addition, in some applications the CL discharge function is a demerit.
For applications where an external voltage is applied from the DC/DC or LDO output side using an OR connection, the CL discharge function activates to allow the current to be continuously drawn from the output side.

Caution is required because this not only uses the extra quiescent current from the external power supply but could also cause characteristic degradation to occur in the internal elements that conduct the CL discharge.

Continuous Current Mode (CCM) and Discontinuous Current Mode (DCM) are the names of operation modes of switching power supplies, such as DC/DC and AC/DC.

The CCM and DCM are divided by the flow direction of the coil current. The difference in these operation modes causes a difference in the output voltage stability (transfer function model), so it is important to understand switching power supply control and stability.

The definitions for CCM and DCM are as follows.

Step-down DC/DC CCM

Step-down DC/DC DCM

Generally, DCM is used under light load conditions and then changed to CCM when the output current becomes large. The output current at which the change from CCM to DCM occurs differs for each product and depends on the power supply specifications, inductance, IC control method, etc.

In addition, products that employ forced PWM control even during PWM control operate in CCM without changing to DCM even during light loads.

Condition: Vin=5V, Vout=3.3V/0mA

There are two types of power supplies: linear types and switching types.

Switching type

Switching type power supply outputs the voltage by turning the driver transistor/FET, etc., ON and OFF.Since perform switching, they have high efficiency and low loss.However, they generate switching noise, so measures might be necessary to counter the EMI. In addition, compared to linear types, they have a high degree of design difficulty and might not operate normally depending on the peripheral components or PCB layout.

Linear type

Linear type power supply outputs the voltage without turning the driver transistor/FET, etc., ON and OFF.Since do not perform switching, so they are low-noise power supplies. However, the loss is large, and their efficiency is power.A specific example is a voltage regulator that controls the output voltage by adjusting the driver FET’s ON resistance.

The load-transient response characteristic and the line-transient response characteristic are characteristics that show the amount of output voltage change and responsiveness of the DC/DC and voltage regulator when the output current or the input voltage suddenly change.

Load-transient response characteristic

This shows how much the output voltage changes and how long takes it to converge when the output current suddenly changes.
The output voltage change amount is affected by the IC control method, the output current conditions, and the output capacity, etc.

For the voltage regulator, high-speed type voltage regulators have better load-transient response characteristics, but the quiescent current becomes large.

For DC/DC, the control method causes a large change in the load-transient response characteristic, so a product that uses a control method that allows a fast transient response must be selected.

In addition, using the load transient change, it is possible to determine the product stability from the output voltage response wavelength.

Line-transient response characteristic

This shows how much the output voltage changes and how long it takes it to converge when the input voltage suddenly changes.
The line-transient response characteristic is also affected by the IC control method, the input voltage conditions, and the output capacity, etc., the same as is the load-transient response characteristic.

Generally, products with a good load-transient response characteristic tend to also have a good line-transient response characteristic.

Thermal shutdown is a function that prevents the IC from malfunctioning, suffering characteristic degradation, or being damaged when the IC junction temperature becomes excessively hot.

In the case of general voltage regulators and DC/DC, when the thermal shutdown function activates, the current supply to the output side is stopped, and output turns off.

When the current supply to the output side stops, self-heating stops, which decreases the junction temperature.

For ICs with a built-in self-reset type thermal shutdown function, when the junction temperature decreases to the thermal shutdown reset temperature, the IC restarts.

When the excessive load continues after restarting, the thermal shutdown activates again, and it will behave by repeatedly detection and reset.

DC/DC converter (XC6702): Thermal shutdown detection ⇔ Reset state
(Condition: Vin=11.8V, Vout=1.8V)

There are also products with a similar function where the charger IC monitors the battery temperature and performs optimum charging control based on the battery temperature state.

The chip enable function (CE function) and enable function (EN function) are functions that stop all IC operations or part of its functions.

When the IC and part of its functions are operating, it is called to be enabled.

Conversely, when the operation of the IC and part of its functions is stopped, it is called to be disabled.

Generally, in the disabled state where the IC and part of its functions are stopped, output stops and the quiescent current is reduced.

The detailed operation differs depending on the IC, so check the specifications of the IC to be used.

See here for the method of current limit characteristics (V_OUT vs I_OUT) of voltage regulators.

Current limit method of voltage regulator

A DC/DC converter is more efficient than a voltage regulator because the method for controlling the output voltage is different.

A voltage regulator controls the output voltage by adjusting the driver FET’s ON resistance. In other words, the dropout voltage, which is the difference between the input voltage and the output voltage, in the driver FET decreases the voltage in the IC.

This driver FET voltage decrease results in loss, so the ideal voltage regulator efficiency is “V_OUT/V_IN x 100”.

On the other hand, a DC/DC converter controls the output voltage by adjusting the ON/OFF time of the driver FET.

The ON resistance is low while the driver FET is ON, so the conduction loss is small while current is flowing in the ON resistor and driver FET. Switching loss and other losses in addition to the conduction loss also occur, but even when including these the loss is less than that of the conduction loss, which makes a DC/DC converter more efficient than a voltage regulator.

The above is a general explanation, and the efficiency will differ depending on the power supply specifications, IC quiescent current, or the ON resistance of the DC/DC converter.

There might be times when the voltage regulator is more efficient depending on these conditions, so it is important to select the optimum product based on the usage conditions.

A soft-start function is a function that controls the time from when an IC enters the startup start state until the output voltage reaches the set voltage. When the device contains a soft-start function, the output rise time is constant regardless of the output capacity.

A similar function is the inrush current prevention function. The inrush current prevention function only suppresses the inrush current without controlling the startup time.

Voltage regulator startup waveform

(a) Without soft-start XC6216 @V_IN=12V, V_OUT=5V

(b) With soft-start XC6702 @V_IN=12V, V_OUT=5V

Having a soft-start function has the following advantages.

(a) Suppresses inrush currents and input voltage decreases
(b) Controls the output voltage rise time and slew rate

(a) Suppresses inrush currents and input voltage decreases

ICs without a soft-start function are generally used in products where the output voltage slew rate during startup is not controlled. In this case, the IC supplies current to the output side until the output voltage reaches the set output voltage, so an excessive inrush current is generated.

 When an excessive inrush current is generated, the impedance on the input side reduces the input voltage. If this reduction in input voltage is large, it could an IC startup failure, system reset, etc.

Voltage regulator startup waveform : V_IN=4V, V_OUT=3V, CL=1μF, RL=20mA/3.0V

(b) Controls the output voltage rise time and slew rate

The soft-start function controls the time from when an IC enters the startup start state until the output voltage reaches the set voltage, so it can be used to adjust the output voltage rise time and slew rate for the startup sequence of downstream devices, etc.

 Some products have a fixed soft-start function inside the IC but depending on the product, an externally connected component, etc., can be used to adjust the soft-start time.

 Products that can adjust the soft-start time can adjust the output voltage slew rate and reduce the inrush current when a high-capacity capacitor is connected to the output.

 When a high-capacity capacitor is connected to the output, the flow of excessive inrush current activates the protective function of the power supply IC, which can cause startup to fail, so generally, this startup failure is prevented by adjusting the soft-start time.

Startup waveform: XC9271 500kHz : V_IN=12V/V_OUT=5V, CL=22μF x2 + 330μF x 3

A DC/DC with a built-in driver FET is called a DC/DC converter.

Conversely, a DC/DC without a built-in driver FET that controls an externally attached FET is called a DC/DC controller.

When a power supply is configured with a DC/DC converter, it has few externally connected components, which reduces the mounted surface area. However, the driver FET built-in the IC, which is a heat source, and the junction temperature to increase due to power dissipation caused by the driver FET. Therefore, great care with the thermal design is required to ensure the junction temperature stays within the operation range.

A DC/DC controller can be used to build optimum circuits fitting a variety of power supply specifications by selecting the driver FET. However, compared to a DC/DC converter, there are more externally attached components, which increases the mounted surface area. In addition, since increasing the number of components also increases the mounted surface area, the expertise for creating a low-noise design and selecting the optimum peripheral components is required.

The Under Voltage Lock Out (UVLO) function stops the internal circuits to prevent the IC from malfunctioning when the IC power supply is low.

The purpose of this product is not to stop the IC from malfunctioning, but rather is to suppress the quiescent current when the battery voltage drops, so there are also products with a built-in UVLO function.

Generally, for a switching power supply, like DC/DC, when the battery voltage drops causing the UVLO to activate, the switching operation stops. When the power supply voltage subsequently increases, the UVLO function is released, and the switching operation starts.

The detailed behavior differs depending on the product, so the data sheet of each product must be checked.

The frequency of switching of the switching power supply, such as DC/DC or AC/DC, is called the oscillation frequency.
Generally, the IC shows the switching frequency when switching is performed in a loaded state.

Generally, if the oscillation frequency is high, there are the advantages of component miniaturization and fast response speed, but the disadvantages are the efficiency decreases or the EMI noise increases. For this reason, the optimum oscillation frequency must be selected to match the application and purpose.

In addition, for PFM control and PFM/PWM control, the switching frequency during light loads becomes lower than the oscillation frequency, so care needs to be taken when implementing noise countermeasures.

Load regulation is the characteristic that shows how much the output voltage changes before and after the output current is increased.

This is expressed by the _mV change in the output voltage when the output current is changed from _mA to _mA. As the output current increases, the output voltage decreases, so generally the electrical characteristics of these products are fixed.

In the example below, we can see the voltage decreases by 50 mV when the current is changed from 1 mA to 50 mA.

Voltage regulator (XC6216 Vout=5.0V)

Voltage regulator (XC6216 Vout=5.0V): Output voltage vs. output current characteristic