Rainbow-electronics MAX1531 Uživatelský manuál stáhnout pdf (Strana 30)

MAX1530/MAX1531

Multiple-Output Power-Supply Controllers for

LCD Monitors

30 ______________________________________________________________________________________

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 transistor’s base cur-

rent. The total DC loop gain is approximately:

where V

is 26mV at room temperature, I

LOAD

is the

output current of the linear regulator, V

REF

is the linear

regulator’s internal reference voltage, and I

BIAS

is the

current through the base-to-emitter resistor (R

). 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 (I

BIAS

Each controller’s 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 controller’s bias current.

The base-to-emitter resistor for each controller should

be chosen to set the correct I

BIAS

The output capacitor and the load resistance create the

dominant pole in the system. However, the internal

amplifier delay, the pass transistor’s input capacitance,

and the stray capacitance at the feedback node create

additional poles in the system, and the output capacitor’s

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

lowing steps to ensure the linear regulator’s stability:

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

regulator’s output capacitor and the load resistor:

where C

is the output capacitance of the linear

regulator and R

LOAD

is the load resistance corre-

sponding to the maximum load current.

The unity-gain crossover of the linear regulator is:

2) The pole created by the internal amplifier delay is

about 1MHz:

3) Next, calculate the pole set by the transistor’s input

capacitance, the transistor’s input resistance, and

the base-to-emitter pullup resistor:

transconductance of the pass transistor, and f

is the

transition frequency. Both parameters can be found

in the transistor’s data sheet.

Because R

is much greater than R

, the above

equation can be simplified:

The equation can be further simplified:

POLE C

()

POLE C

IN IN

()

≈

2π

where C

g is the

, , ===

2π

CR R

POLE C

IN BE IN

()

(||)

2π

f MHz

POLE AMP()

≅1

fAf

CROSSOVER V LDO POLE LDO

() ()

POLE LR

LR LOAD

()

2π

BIAS

VLR

BIAS FE

LOAD

REF()

≈

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