MP4470/4470A–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4470/4470A Rev. 1.1

www.MonolithicPower.com

10

8/26/2013

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6

SW

FREQ

IN

DELAY

IN

OUT

10

F(kHz)

96 R

(k )

V

[t

(ns)]

VV

=

×Ω

+×

(3)

The MP4470/4470A is optimized for 200kHz-to-

1MHz applications to operate at high switching

frequencies with high efficiency. The high-

switching frequency allows for smaller LC-filter

components to reduce PCB space requirements.

Ramp Compensation

Figure 4 and Figure 5 show jitter occurring in

downward slope causes the HS-FET ON time to

deviate from its intended position and produces

jitter. There is a relationship between system

doesn’t affect the noise immunity directly.

Figure 4: Jitter in PWM Mode

Figure 3: Jitter in Skip Mode

Ceramic output capacitors lack enough ESR

ripple to stabilize the system, and requires an

external compensation ramp.

I

C4

I

FB

I

I

R4

I

FB

Figure 6: Simplified Circuit in PWM Mode with

External Ramp Compensation

In PWM mode has an equivalent circuit with HS-

FET

OFF

and

uses

a

external

ramp

simplified circuit in Figure 6. Derive the external

12

SW

4

1

2

RR

11

2F

C

5

R

R

⎛⎞

×

<× ⎜⎟

π×

×

+

⎝⎠

(4)

Then:

R4

C4

FB

C4

II

I

I

=

+≈

(5)

as:

OUT

SLOPE1

44

V

V

RC

−

=

×

(6)

instability in PWM mode. If C4 cannot be reduced

further due to equation 4’s limitations, then only

is around 20V/ms-40V/ms.

In the case of POSCAP or other types of

capacitors with higher ESR, an external ramp is

not necessary.

Figure 7: Simplified Circuit in PWM Mode without

External Ramp Compensation