MP9472–0.45A, 18V, NON-SYNCHRONOUS, RECTIFIED, STEP-DOWN CONVERTER

MP9472 Rev. 1.0

www.MonolithicPower.com

13

12/9/2015

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In this case, a third pole set by the compensation

compensation

resistor

(R3)

is

used

to

compensate for the effect of the ESR zero on the

loop gain. This pole can be determined with

Equation (17):

P3

POLE

1

f

2C

R3

=

π×

×

(17)

The goal of the compensation design is to shape

the converter transfer function to obtain a desired

loop gain. The system crossover frequency

where the feedback loop has unity gain is

important. Lower crossover frequencies result in

slower line and load transient responses, while

higher crossover frequencies can cause system

instability. It is recommended to set the crossover

frequency below one-tenth of the switching

frequency.

To optimize the compensation components, the

following procedure can be used.

1. Choose the compensation resistor (R3) to set

the desired crossover frequency. R3 can be

determined with Equation (18):

FB

OUT

CS

EA

S

FB

OUT

CS

EA

C

V

V

G

G

f

1

.

0

2

C

2

V

V

G

G

f

2

C

2

3

R

×

×

×

×

×

π

<

×

×

×

×

π

=

(18)

typically below one-tenth of the switching

frequency.

2. Choose the compensation capacitor (C3) to

achieve the desired phase margin. For

applications with typical inductor values,

one-fourth

of

the

crossover

frequency

provides sufficient phase margin.

Determine the C3 value with Equation (19):

C

f

3

R

2

4

3

C

×

×

π

>

(19)

Where R3 is the compensation resistor.

3. Determine

if

a

second

compensation

required. It is required if the frequency of the

zero created by the ESR of the output

capacitor is less than half of the switching

frequency. Otherwise, Equation (20) is valid:

S

ESR

f

1

2C2 R

2

<

π×

×

(20)

If this is the case, then add a second

ESR

POLE

C2 R

C

R3

×

=

(21)

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency of the regulator and is required under

the following conditions:

•

•

Duty cycle is high: D =

IN

OUT

V

V

> 65%

In these cases, an external BST diode is

recommended from the output of the voltage

regulator to BST (see Figure 2).

Figure 2: Optional External Bootstrap Diode

Added to Enhance Efficiency

The recommended external BST diode is IN4148,

and the recommended BST cap is 0.01µF.

The MP9472 charges the BST capacitor through

an internal 5V power supply. During each period,

it pulls SW to GND internally to charge the BST

capacitor. In light-load mode, if SW cannot be

pulled down to GND and charge the BST voltage

high enough, even when the duty cycle is low

and the external bootstrap diode is added, avoid

running the MP9472 in extremely light loads or

adding a dummy load to keep it in normal

operation.