NB650/NB650H – 6A, 28V, FAST-TRANSIENT, SYNCHRONOUS STEP-DOWN CONVERTERS

NB650/NB650H Rev. 1.12

www.MonolithicPower.com

11

10/11/2012

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© 2012 MPS. All Rights Reserved.

Figure 7: Simplified Circuit in PWM Mode without

External Ramp Compensation

To realize the stability without the use of an

external ramp, select an ESR value as follows:

SW

ON

ESR

OUT

tt

0.7

2

R

C

+

×π

≥

(4)

Ramp with Small ESR Capacitor

The ESR ripple when using ceramic output

capacitors is not high enough to stabilize the

system and requires an external compensation

ramp. The application section includes a

description

of

designing

with

small

ESR

capacitors.

Figure 8: Simplified Circuit in PWM Mode with

External Ramp Compensation

Figure 7 shows a simplified equivalent circuit in

PWM mode with the HS-FET OFF and an

external ramp compensation circuit (R4, C4). The

external ramp is derived from the inductor ripple

current. If one chooses C4, R9, R1 and R2 to

meet the following condition:

12

9

SW

4

1

2

RR

11

R

2f

C

5

R

R

⎛⎞

×

<×

+

⎜⎟

π×

×

+

⎝⎠

(5)

Where:

R4

C4

FB

C4

II

II

=+≈

(6)

And R2 is the equivalent resistor from FB to GND

can then be estimated as:

IN

O

12

RAMP

ON

44

1

2

9

VV

R// R

Vt

RC

R //R

R

−

=×

×

×+

(7)

Usually R9 is set to 0Ω, then equation 7 can be

simplified as:

4

4 C

R

)

V

V

(

V

ON

O

IN

RAMP

×

τ

×

−

=

(8)

follows

OUT

RAMP

SLOPE1

off

4

4

V

V

V

tR

C

−

−

==

×

(9)

As shown in equation 8, if there is instability in

PWM mode, we can reduce either R4 or C4. If

C4 can not be reduced further due to limitations

from equation 5, then we can only reduce R4.

designed as follows.

3

10

SW

ON

ESR

OUT

slope1

OUT

OUT

SW

on

tt

+-R

C

Io

0.7 π

2

-V

V

+

2L C

t

-t

−

×

×

≥

××

(10)

ripple is almost the same with or without the

external ramp. Figure 9 shows the simplified

circuit of the skip mode when both HS-FET and

LS-FET are off.

Figure 9: Simplified Circuit in Skip Mode

mode can be determined as:

()

REF

SLOPE2

12

OUT

V

V

(R

R // Ro) C

−

=

+×

(11)