MAX1951A Просмотр технического описания (PDF) - Maxim Integrated

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MAX1951A

1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable

Maxim Integrated 

1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC

Step-Down Regulator with Enable

nominal state. The controller response time depends on

the closed-loop bandwidth. A higher bandwidth yields

a faster response time, thus preventing the output from

deviating further from its regulating value.

Compensation Design

The double pole formed by the inductor and output

capacitor of most voltage-mode controllers introduces a

large phase shift that requires an elaborate compensation

network to stabilize the control loop. The MAX1951A uti-

lizes a current-mode control scheme that regulates the

output voltage by forcing the required current through the

external inductor, eliminating the double pole caused by

the inductor and output capacitor, and greatly simplifying

the compensation network. A simple type 1 compensa-

tion with single compensation resistor (R1) and compen-

sation capacitor (C2) in Figure 2 creates a stable and

high-bandwidth loop.

An internal transconductance error amplifier compen-

sates the control loop. Connect a series resistor and

capacitor between COMP (the output of the error ampli-

fier) and GND to form a pole-zero pair. The external

inductor, internal current-sensing circuitry, output

capacitor, and the external compensation circuit deter-

mine the loop system stability. Choose the inductor and

output capacitor based on performance, size, and cost.

Additionally, select the compensation resistor and

capacitor to optimize control-loop stability. The compo-

nent values shown in the typical application circuit

(Figure 2) yield stable operation over a broad range of

input-to-output voltages.

The basic regulator loop consists of a power modulator,

an output feedback divider, and an error amplifier. The

power modulator has DC gain set by gmc x RLOAD, with

a pole-zero pair set by RLOAD, the output capacitor

(COUT), and its ESR. The following equations define the

power modulator:

Modulator gain:

GMOD = ∆VOUT/∆VCOMP = gmc x RLOAD

Modulator pole frequency:

fpMOD = 1/(2 x π x COUT x (RLOAD + ESR))

Modulator zero frequency:

fzESR = 1/(2 x π x COUT x ESR)

where RLOAD = VOUT/IOUT(MAX) and gmc = 4.2S.

The feedback divider has a gain of GFB = VFB/VOUT,

where VFB is equal to 0.8V. The transconductance error

amplifier has a DC gain, GEA(DC), of 70dB. The com-

pensation capacitor, C2, and the output resistance of

the error amplifier, ROEA (20MΩ), set the dominant

pole. C2 and R1 set a compensation zero. Calculate the

dominant pole frequency as:

fpEA = 1/(2π x C2 x ROEA)

Determine the compensation zero frequency as:

fzEA = 1/(2π x C2 x R1)

For best stability and response performance, set the

closed-loop unity-gain frequency much higher than the

modulator pole frequency. In addition, set the closed-

loop crossover unity-gain frequency less than, or equal

to 1/5 of the switching frequency. However, set the

maximum zero crossing frequency to less than 1/3 of

the zero frequency set by the output capacitance and

its ESR when using POSCAP, SPCAP, OSCON, or other

electrolytic capacitors. The loop-gain equation at the

unity-gain frequency is:

GEA(fc) x GMOD(fc) x VFB/VOUT = 1

where GEA(fc) = gmEA x R1, and GMOD(fc) = gmc x

RLOAD x fpMOD/fC, where gmEA = 60µS.

R1 calculated as:

R1 = VOUT x K/(gmEA x VFB x GMOD(fc))

where K is the correction factor due to the extra phase

introduced by the current loop at high frequencies

(>100kHz). K is related to the value of the output

capacitance (see Table 1 for values of K vs. C). Set the

error-amplifier compensation zero formed by R1 and C2

at the modulator pole frequency at maximum load. C2

is calculated as follows:

C2 = (2 x VOUT x COUT/(R1 x IOUT(MAX))

As the load current decreases, the modulator pole also

decreases; however, the modulator gain increases

accordingly, resulting in a constant closed-loop unity-

gain frequency. Use the following numerical example to

calculate R1 and C2 values of the typical application

circuit of Figure 2.

VOUT = 1.5V

IOUT(MAX) = 2A

Table 1. K Value

DESCRIPTION

COUT (µF) K Values are for output inductance from 1.2µH

10 0.55 to 2.2µH. Do not use output inductors larger

22

0.47 than 2.2µH. Use fC = 200kHz to calculate R1.

COUT = 10µF

RESR = 0.010Ω

gmEA = 60µS

gmc = 4.2S

fSWITCH = 1MHz

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