MP8762 datasheet(14/39 Pages) MPS

14 / 39 page

MP8762 ― 10A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8762 Rev. 1.4

www.MonolithicPower.com

11/4/2013

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

ESR

POSCAP

VOUT

Figure 6—Simplified Circuit in PWM Mode

without External Ramp Compensation

To realize the stability when no external ramp is

applied, usually the ESR value should be chosen

as follow:

OUT

ESR

≥

(4)

Where TSW is the switching period.

Ramp with Small ESR Capacitor

When the output capacitors are ceramic ones,

the ESR ripple is not high enough to stabilize the

system, and external ramp compensation is

needed. Skip to application information section

for design steps with small ESR caps.

Ceramic

VOUT

IR4

IC4

IFB

Figure 7—Simplified Circuit in PWM Mode

with External Ramp Compensation

In PWM mode, an equivalent circuit with HS-FET

off

and

the

use

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:

⎟

⎠

⎞

⎜

⎝

⎛

(5)

Where:

≈

(6)

And the ramp on the VFB can then be estimated

as:

⎟

⎠

⎞

⎜

⎝

⎛

−

OUT

RAMP

(7)

The downward slope of the VFB ripple then

follows:

OUT

OFF

RAMP

SLOPE

−

(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

Vslope1 should be design follow equation 9.

ESR

OUT

SLOPE1

OUT

I10

0.7

2L C

−

+−

×π

−≥

××

−

(9)

Where IOUT is the load current.

In skip mode, the downward slope of the VFB

ripple is almost same whether the external ramp

is used or not. Fig.8 shows the simplified circuit

of the skip mode when both the HS-FET and LS-

FET are off.

COUT

VOUT

ROUT

Figure 8—Simplified Circuit in skip Mode

The downward slope of the VFB ripple in skip

mode can be determined as follows:

OUT

REF

SLOPE

]

)

[(

−

(10)

Where ROUT is the equivalent load resistor.

As described in Fig.5, VSLOPE2 in the skip mode is

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

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