MAX1530_09 Datasheet, PDF(30/33 Page) Maxim Integrated Products – Multiple-Output Power-Supply Controllers for LCD Monitors

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MAX1530_09 Datasheet, PDF (30/33 Pages) Maxim Integrated Products – Multiple-Output Power-Supply Controllers for LCD Monitors

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Multiple-Output Power-Supply

Controllers for LCD Monitors

amount of ripple after the PC board layout has been

optimized, consider increasing output capacitance.

Adding more capacitance does not eliminate the ripple,

but proportionally reduces the amplitude of the ripple. If

increasing the output capacitance is not desirable

because of space or cost concerns, then consider

slowing the turn-on of the step-down DC-to-DC

MOSFETs. Slower turn-on leads to smoother LX rising

and falling edges and consequently reduces the

switching noise. When slowing down MOSFET turn-on,

ensure the turn-off time is not affected. Otherwise, the

adaptive dead-time circuitry may not work properly and

shoot-through may occur. See the MOSFET Gate

Drivers section for details on how to slow down the

turn-on of both DH and DL.

Stability Requirements

The MAX1530/MAX1531 linear-regulator controllers use

an internal transconductance amplifier to drive an

external pass transistor. The transconductance amplifi-

er, the pass transistor, the base-emitter resistor, and

the output capacitor determine loop stability. The fol-

lowing applies equally to all linear regulators in the

MAX1530 and MAX1531. Any differences are highlight-

ed where appropriate.

The transconductance amplifier regulates the output

voltage by controlling the pass transistorâs base cur-

rent. The total DC loop gain is approximately:

AV(LR)

ââ

â â

ââ1+

IBIAS Ã hFE

ILOAD

â â

VREF

where VT is 26mV at room temperature, ILOAD is the

output current of the linear regulator, VREF is the linear

regulatorâs internal reference voltage, and IBIAS is the

current through the base-to-emitter resistor (RBE). Each

of the linear regulator controllers is designed for a dif-

ferent maximum output current so they have different

output drive currents and different bias currents (IBIAS).

Each controllerâs bias current can be found in the

Electrical Characteristics. The current listed in the

Conditions column for the FBL_ regulation voltage

specification is the individual controllerâs bias current.

The base-to-emitter resistor for each controller should

be chosen to set the correct IBIAS:

RBE

VBE

IBIAS

The output capacitor and the load resistance create the

dominant pole in the system. However, the internal

amplifier delay, the pass transistorâs input capacitance,

and the stray capacitance at the feedback node create

additional poles in the system, and the output capacitorâs

ESR generates a zero. For proper operation, use the fol-

lowing steps to ensure the linear regulatorâs stability:

1) First, calculate the dominant pole set by the linear

regulatorâs output capacitor and the load resistor:

fPOLE(LR)

2ÏCLRRLOAD

where CLR is the output capacitance of the linear

regulator and RLOAD is the load resistance corre-

sponding to the maximum load current.

The unity-gain crossover of the linear regulator is:

fCROSSOVER = AV(LDO)f POLE(LDO)

2) The pole created by the internal amplifier delay is

about 1MHz:

fPOLE(AMP) â 1MHz

3) Next, calculate the pole set by the transistorâs input

capacitance, the transistorâs input resistance, and

the base-to-emitter pullup resistor:

fPOLE(CIN)

2ÏCIN(RBE

|| RIN)

where

CIN

2ÏfT

RIN

RÏ

hFE

the

transconductance of the pass transistor, and fT is the

transition frequency. Both parameters can be found

in the transistorâs data sheet.

Because RBE is much greater than RIN, the above

equation can be simplified:

fPOLE(CIN)

2ÏCINRIN

The equation can be further simplified:

fPOLE(CIN)

hFE

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