MP4470-Z データシートの表示（PDF） - Monolithic Power Systems

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MP4470-Z

High-Efficiency, Fast-Transient, 5A, 36V Synchronous, Step-Down Converter

Monolithic Power Systems 

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

FSW

(kHz)

=

106

[ 96

×

RFREQ (kΩ)

VIN

+

tDELAY

(ns)]

×

VIN

VOUT

(3)

Where tDELAY is the comparator delay (~20ns).

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

both PWM mode and skip mode. Noise on VFB’s

downward slope causes the HS-FET ON time to

deviate from its intended position and produces

jitter. There is a relationship between system

stability and the steepness of the VFB ripple: The

slope steepness of the VFB ripple dominates

noise immunity. The magnitude of the VFB ripple

doesn’t affect the noise immunity directly.

Figure 4: Jitter in PWM Mode

IR4 IC4

IFB

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

compensation circuit (R4, C4), shown as a

simplified circuit in Figure 6. Derive the external

ramp from the inductor-ripple current. Choose C4,

R1, and R2 to meet the following condition:

1

2π × FSW

× C4

<

1

5

×

⎛

⎜

⎝

R1

R1

×

+

R2

R2

⎞

⎟

⎠

(4)

Then:

IR4 = IC4 + IFB ≈ IC4

(5)

The VFB downward slope ripple is then estimated

as:

VSLOPE1

=

−VOUT

R4 × C4

(6)

From equation 6, reduce R4 or C4 to reduce

instability in PWM mode. If C4 cannot be reduced

further due to equation 4’s limitations, then only

reduce R4. Based on bench experiments, VSLOPE1

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 3: Jitter in Skip Mode

Ceramic output capacitors lack enough ESR

ripple to stabilize the system, and requires an

external compensation ramp.

Figure 7: Simplified Circuit in PWM Mode without

External Ramp Compensation

MP4470/4470A Rev. 1.1

www.MonolithicPower.com

10

8/26/2013

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

© 2013 MPS. All Rights Reserved.

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