MAX8643AETG(2007) 查看數據表（PDF） - Maxim Integrated

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MAX8643AETG
(Rev.:2007)

3A, 2MHz Step-Down Regulator with Integrated Switches

Maxim Integrated 

3A, 2MHz Step-Down Regulator

with Integrated Switches

Input-Capacitor Selection

The input capacitor reduces the current peaks drawn

from the input power supply and reduces switching

noise in the IC. The total input capacitance must be

equal to or greater than the value given by the following

equation to keep the input ripple voltage within specs

and minimize the high-frequency ripple current being

fed back to the input source:

CIN_

MIN

=

D x tS x IOUT

VIN−RIPPLE

where VIN-RIPPLE is the maximum allowed input ripple

voltage across the input capacitors and is recommend-

ed to be less than 2% of the minimum input voltage. D

is the duty cycle (VOUT / VIN), and tS is the switching

period (1/fS).

The impedance of the input capacitor at the switching

frequency should be less than that of the input source so

high-frequency switching currents do not pass through

the input source but are instead shunted through the

input capacitor. High source impedance requires high

input capacitance. The input capacitor must meet the

ripple current requirement imposed by the switching cur-

rents. The RMS input ripple current is given by:

IRIPPLE = ILOAD ×

VOUT × (VIN − VOUT)

VIN

where IRIPPLE is the input RMS ripple current.

Compensation Design

The power transfer function consists of one double pole

and one zero. The double pole is introduced by the out-

put filtering inductor, L, and the output filtering capacitor,

CO. The ESR of the output filtering capacitor determines

the zero. The double pole and zero frequencies are

given as follows:

fP1_LC = fP2_LC =

1

2π

x

L

x

CO

x

⎛

⎝⎜

RO + ESR

RO + RL

⎞

⎠⎟

fZ_ESR = 2π

x

1

ESR

x

CO

where RL is equal to the sum of the output inductor’s

DCR and the internal switch resistance, RDSON. A typical

value for RDSON is 37mΩ. RO is the output load resis-

tance, which is equal to the rated output voltage divided

by the rated output current. ESR is the total equivalent

series resistance of the output filtering capacitor. If there

is more than one output capacitor of the same type in

parallel, the value of the ESR in the above equation is

equal to that of the ESR of a single output capacitor

divided by the total number of output capacitors.

The high switching frequency range of the MAX8643A

allows the use of ceramic output capacitors. Since the

ESR of ceramic capacitors is typically very low, the fre-

quency of the associated transfer function zero is higher

than the unity-gain crossover frequency, fC, and the zero

cannot be used to compensate for the double pole creat-

ed by the output filtering inductor and capacitor. The dou-

ble pole produces a gain drop of 40dB/decade and a

phase shift of 180°/decade. The error amplifier must com-

pensate for this gain drop and phase shift to achieve a

stable high-bandwidth closed-loop system. Therefore,

use type III compensation as shown in Figure 3 and

Figure 4. Type III compensation possesses three poles

and two zeros with the first pole, fP1_EA, located at zero

frequency (DC). Locations of other poles and zeros of the

type III compensation are given by:

fZ1_EA

=

2π

x

1

R1

x

C1

L

VOUT

LX

MAX8643A

OUT

COUT

R2

R3

C3

CTL1

FB

R1 C1

CTL2

COMP

R4

C2

a) EXTERNAL RESISTOR-DIVIDER

L

VOUT

LX

COUT

MAX8643A

R2

OUT

R3

8kΩ

C3

VOLTAGE

CTL1

SELECT

CTL2

FB

COMP

R1 C1

C2

b) INTERNAL PRESET VOLTAGE

Figure 3. Type III Compensation Network

12 ______________________________________________________________________________________

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