MP8736– HIGHEFFICIENCY,FAST TRANSIENT,6A, 19V SYNCHRONOUSBUCKCONVERTERIN ATINY3X4mmPACKAGE

MP8736 Rev. 1.34

www.MonolithicPower.com

15

4/18/2012

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2012 MPS. All Rights Reserved.

Figure 7—Simplified Circuit in PWM Mode

with External Ramp Compensation

In PWM mode, an equivalent circuit with HS-FET

off

and

the

use

of

an

external

ramp

compensation circuit (R4, C4) is simplified in

Figure 7. 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 +I

I

=≈

(6)

as:

IN

O

12

RAMP

ON

44

1

2

9

VV

R//R

VT

RC

R // R

R

−

=×

×

×+

(7)

follows

−

−

==

×

OUT

RAMP

SLOPE1

off

4

4

V

V

V

TR

C

(8)

As can be seen from 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

limitation from equation 5, then we can only

reduce R4. For a stable PWM operation, the

SW

ON

-3

ESR

OUT

slope1

O

OUT

SW

on

TT

+-R

C

Io 10

0.7 π

2

-V

V +

2L C

T -T

×

×

≥

××

(9)

Io is the load current.

ripple is almost the same whether the external

ramp is used or not. Fig.9 shows the simplified

circuit of the skip mode when both the HS-FET

and LS-FET are off.

Figure 8—Simplified Circuit in skip Mode

mode can be determined as follow:

()

REF

SLOPE2

12

OUT

V

V

(R

R // Ro) C

−

=

+×

(10)

Where Ro is the equivalent load resistor.

lower than that is in the PWM mode, so it is

reasonable that the jitter in the skip mode is

larger. If one wants a system with less jitter

during ultra light load condition, the values of the

will decrease the light load efficiency.

Soft Start/Stop

The MP8736 employs soft start/stop (SS)

mechanism to ensure smooth output during

power-up and power shutdown. When the EN pin

charges up the SS CAP. The SS CAP voltage

takes over the REF voltage to the PWM

comparator. The output voltage smoothly ramps

up with the SS voltage. Once the SS voltage

reaches the same level as the REF voltage, it

PWM comparator. At this point, the soft start

finishes and it enters into steady state operation.

When the EN pin becomes low, the SS CAP

voltage is discharged through an 8.5uA internal

current source. Once the SS voltage reaches

REF voltage, it takes over the PWM comparator.

The output voltage will decrease smoothly with

SS voltage until zero level. The SS CAP value

can be determined as follows:

()

( )

( )

()

×μ

SS

SS

REF

Tms

I

A

CnF

VV

(11)

If the output capacitors have large capacitance

value, it’s not recommended to set the SS time