MP28372 — DUAL 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER

MP28372 Rev. 1.4

www.MonolithicPower.com

9

12/10/2007

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TM

ESR

ESR

R

2

C

2

1

f

×

×

π

=

In this case (as shown in Figure 2), a third pole

set by the compensation capacitor (C6) and the

compensation

resistor

(R3)

is

used

to

compensate the effect of the ESR zero on the

loop gain. This pole is located at:

3

R

6

C

2

1

P

×

×

π

=

The goal of compensation design is to shape

the converter transfer function to get a desired

loop gain. The system crossover frequency

where the feedback loop has the unity gain is

important.

Lower crossover frequencies result in slower

line and load transient responses, while higher

crossover frequencies could cause system

unstable. A good rule of thumb is to set the

crossover frequency to below one-tenth of the

switching

frequency.

To

optimize

the

compensation components for conditions not

listed in Table 2, the following procedure can be

used:

1. Choose the compensation resistor (R3) to set

the desired crossover frequency. Determine the

R3 value by the following equation:

FB

OUT

CS

EA

C

V

V

G

G

f

2

C

2

3

R

×

×

×

×

π

=

which is typically less than one tenth of the

switching frequency.

2. Choose the compensation capacitor (C3) to

achieve

the

desired

phase

margin.

For

applications with typical inductor values, setting

of the crossover frequency provides sufficient

phase margin. Determine the C3 value by the

following equation:

C

f

3

R

2

4

3

C

×

×

π

>

Where R3 is the compensation resistor value.

3. Determine if the second compensation

capacitor (C6) is required. It is required if the

ESR zero of the output capacitor is located at

less than half of the switching frequency, or the

following relationship is valid:

2

f

R

2

C

2

1

S

ESR

<

×

×

π

If this is the case, then add the second

at the location of the ESR zero. Determine the

C6 value by the equation:

3

R

R

2

C

6

C

ESR

×

=

Soft-Start

Each channel is soft-start controlled with the

SSA and SSB pins. Use capacitors to control

the ramp time using the equation:

4

C

k

9

2

.

2

RampTime

×

Ω

×

=

External Bootstrap Diode

It is recommended that an external bootstrap

diode be added when the system has a 5V

fixed input or the power supply generates a 5V

output. This helps improve the efficiency of the

regulator. The bootstrap diode can be a low

cost one such as IN4148 or BAT54.

MP28372

SWA/B

BSA/B

10nF

5V

Figure 2—External Bootstrap Diode

This diode is also recommended for high duty

cycle operation (when

IN

OUT

V

V

>65%) and high