MP8606

– 6A, 6.5V SYNCHRONOUS STEP-DOWN CONVERTER

MP8606 Rev. 1.03

www.MonolithicPower.com

12

8/2/2013

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© 2013 MPS. All Rights Reserved.

Figure 7 shows a simplified external ramp

compensation (R4 and C4) for PWM mode, with

HS-FET off. Chose R1, R2, and C4 of the

external ramp to meet the following condition:

12

SW

4

1

2

RR

11

2

F

C

5

R

R





 

 







(4)

Where:

R4

C4

FB

C4

I

I

I

I







(5)

estimated as:

REF

SLOPE1

44

V

V

RC







(6)

Reducing either R4 or C4, as seen from equation

(6), can control some of the instability in PWM

mode. If the condition from equation (4) prevents

has an expected range between 20V/ms to

40V/ms based on bench experiments.

The external ramp is not necessary for other

types of capacitors with higher ESR such as

POSCAPs.

Figure 8

—Simplified Circuit in PWM Mode

without External Ramp Compensation

Figure 8 shows the equivalent circuit in PWM

mode with the HS-FET off and without an

external ramp circuit. The ESR ripple dominates

ripple is:

REF

SLOPE1

ESR V

V

L





(7)

between 15V/ms to 30V/ms based on bench

experiments

—requires using the minimum ESR

value of the output capacitor with a small-value

inductor.

mode. Figure 9 shows an equivalent circuit with

HS-FET off and the current modulator regulating

the LS-FET. Instead, the downward slope of the





REF

SLOPE2

1

2

OUT

V

V

R

R

C







(8)

Figure 9

—Simplified Circuit in Skip Mode

To keep the system stable during light load

condition, the values of the FB resistors should

be better in

the range of 5kΩ to 50kΩ. It is

0.4mV/ms to 0.8mV/ms.

When using a large-ESR capacitor on the output,

add a ceramic capacitor with a value of 10uF or

less to in parallel to minimize the effect of ESL.

Soft Start/Stop

MP8606

employs

a

soft

start/stop

(SS)

mechanism to ensure smooth output during

power up and power shut-down. When the EN

pin goes high, the internal SS voltage slowly

ramps up. The output voltage smoothly ramps up

with the SS voltage. Once SS voltage rises

the FB voltage. At this point, the soft start

finishes and it enters steady state operation. The