Auto PWM / PFM Mode of S6BP501A and S6BP502A – KBA224577

Author:  yukinorim_01  Version: **

 

Question:

What is Auto PWM / PFM mode of S6BP501A and S6BP502A?

 

Answer:

Auto PWM / PFM mode is an operation mechanism where S6BP501A and S6BP502A automatically switch between pulse width modulation (PWM) operation and pulse frequency modulation (PFM) operation depending on the load condition. PWM operation is used for working with a heavy load current condition, PFM operation is for working with a light load current condition.

 

Figure1 shows how PWM operation and PFM operation are based on the primary DC/DC converter (DD3V) schematics of S6BP501A and S6BP502A.

 

Figure1. Schematic of Primary DC/DC converter (DD3V)

1.png

PWM operation:

The switching period is always constant, and the output voltage is controlled by the ON duty cycle of the High-side FET (MH). Figure 2 shows the timing chart of the PWM operation.

 

Figure 2. Timing Chart of Fixed PWM mode

2.png

Constant and continuous switching period

PFM operation:

In PFM operation, the switching period is not constant. The ON duty cycle of the High-side FET (MH) changes by changing the switching period, and the output voltage is controlled. The timing chart of PFM operation is shown in Figure 3.

 

Figure3. Timing Chart of Auto PWM/PFM mode

3.png

              Not constant and dis-continuous switching period

 

The features of each operation are as follows:

 

PWM operation

PFM operation

Conversion efficiency *1

High

Low

Switching ripple voltage

Small

Large

Transient response

Fast

Slow

EMI base frequency

Fixed

Varying*2

*1: Conversion efficiency example of each operation

*2: EMI Countermeasure is difficult because the EMI base frequency varies depending on the load current.

 

Figure4. Conversion Efficiency Example (VOUT3 Output from DD3V)

4.png

The following practical use case with S6BP501A and S6BP502A demonstrates these.

S6BP501A and S6BP502A can change the switching mode between the Fixed PWM mode and Auto PWM / PFM mode by setting the SYNC pin to H or L.

§  During normal operation of the application, by setting the fixed PWM mode of SYNC terminal to H, the accuracy of the output power supply voltage is improved with excellent load transient response and small ripple voltage.

§  During standby operation of the application, by setting the Auto PWM / PFM mode of SYNC terminal to L, current consumption of the application is suppressed.

By doing these, S6BP501A and S6BP502A can work with the best performance by each mode’s advantage according to the application.