MAX1889 Просмотр технического описания (PDF) - Maxim Integrated

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MAX1889

Triple-Output TFT LCD Power Supply with Fault Protection

Maxim Integrated 

Triple-Output TFT LCD Power Supply

with Fault Protection

Therefore, the dominant pole and the RHP zero deter-

mine the loop response of the step-up regulator. The fre-

quency of the dominant pole is:

fP _DOMINANT

=

1

2πRLC

where RL is the load resistance and C is the output

capacitor. The frequency of the RHP zero is:

( ) fZ_RHP

=

1-D

2

RL

2πL

where D is the duty cycle, L is the inductance, and the

DC gain is given by:

ADC

=

20log







R1

R1+ R2

×

(1− D)

RCS

×

RL







where RCS is the internal current-sense resistor, and R1

and R2 are the feedback divider resistors in Figure 5.

However, adding lead or lag compensation (Figure 5)

can be useful to adjust the trade-off between stability

and transient response. If greater phase margin is need-

ed for stability, and lower bandwidth is acceptable, add

a pole-zero pair by connecting an RC network from the

FB pin to ground (lag compensation). Conversely, if

higher bandwidth is required for faster transient

response, and lower phase margin is acceptable, add a

L

D

VIN

LX

R2

FB

R3 D1

C2

C

MAX1889

R4

R1

C1

GND

PGND

VMAIN

RL

Figure 5. External Compensation

zero-pole pair to the loop by connecting an RC network

from the FB pin to the main output (lead compensation).

The frequencies of the pole and zero for the lag com-

pensation are:

fP _ FB

=



2πR4



1

R1×

R1+

R2

R2







C1

fZ _ FB

=

1

2πR4 × C1

The frequencies of the zero and pole for the lead com-

pensation are:

( ) fZ _ FF =

1

2π R2 + R3 × C2

fP _ FF

=



2πR3 +



1

R1×

R1+

R2

R2







C2

The compensation resistors R3 and R4 change the AC

gain affecting the loop bandwidth and phase margin at

crossover. Reducing the bandwidth too much (FB com-

pensation) harms the transient response, while increas-

ing it too much harms phase margin and stability. As a

rule, start with R3 (or R4) approximately equal to half of

R1 (or R2). In a typical application, the compensation

capacitors C1 and C2 can be in the range between

100pF to 1000pF. Then, check the stability by monitoring

the transient response waveform when a pulsed load is

applied to the output.

Using Compensation for Improved Soft-Start

The digital soft-start of the main step-up regulator limits

the average input current during startup. In order to

smooth out each step of the digital soft-start, add a low-

frequency lead compensation network (Figure 5). The

network effectively spreads out the switching pulses

and lowers the peak inductor currents.

The smoothing network is active only during soft-start

when the output voltage rises. Positive changes in the

output are instantaneously coupled to the FB pin

through D1 and feed-forward capacitor C2. This

arrangement generates a smoothly rising output volt-

age. When the output voltage reaches regulation, C2

charges up through R3 and D1 turns off. In most appli-

cations, the lead compensation is not needed and can

be disabled by making R3 large. With R3 > R2, the

pole and the zero in the compensation network are very

close to one another and cancel out.

18 ______________________________________________________________________________________

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