MPQ4458 –INDUSTRIAL GRADE,1A, 4MHz, 36V STEP-DOWN CONVERTER

MPQ4458 Rev. 1.0

www.MonolithicPower.com

13

12/5/2012

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2. Choose the compensation capacitor (C3) to

achieve

the

desired

phase

margin.

For

applications with typical inductor values, setting

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

×

=

High Frequency Operation

The switching frequency of MPQ4458 can be

programmed up to 4MHz by an external resistor.

Please pay attention to the following if the

switching frequency is above 2MHz.

The minimum on time of MPQ4458 is about

80ns (typ). Pulse skipping operation can be

seen more easily at higher switching frequency

due to the minimum on time. Recommended

operating voltage at 4MHz is 12V or below, and

24V or below at 2MHz.

30

25

20

15

10

5

1.5

2.0

2.5

3.0

3.5

4.0

Input Max vs

Switching Frequency

Since the internal bootstrap circuitry has higher

impedance, which may not be adequate to

charge the bootstrap capacitor during each

charging period, an external bootstrap charging

diode is strongly recommended if the switching

frequency is above 2MHz (see External

Bootstrap

Diode

section

for

detailed

implementation information).

With higher switching frequencies, the inductive

reactance (XL) of a capacitor dominates, such

that the ESL of the input/output capacitor

determines the input/output ripple voltage at

higher switching frequencies. As a result, high

frequency ceramic capacitors are strongly

recommended as input decoupling capacitors

and output filtering capacitors.

Layout becomes more important when the

device switches at higher frequency. It is

essential

to

place

the

input

decoupling

capacitor, catch diode and the MPQ4458 as

close together as possible, with traces that are

very short and fairly wide. This can help to

greatly reduce the voltage spikes on SW and

also lower the EMI noise level.