���������������������������������������������������������������� Maxim Integrated Products 20

MAX16929

Automotive TFT-LCD Power Supply with Boost

Converter and Gate Voltage Regulators

Stability Requirements

The positive-gate voltage regulator (GH) requires a mini-

mum output capacitance for stability. For an output volt-

15mA, use a minimum capacitance of 0.47FF.

Negative-Gate Voltage Regulator

Output-Voltage Selection

The output voltage of the negative-gate voltage regula-

tor can be adjusted by using a resistive voltage-divider

and the collector of the external npn transistor. Select

equation:

FBGL

GL

BOTTOM

TOP

REF

FBGL

V

V

R

R

V

V

−

=

×

−

the regulator).

Pass Transistor Selection

The pass transistor must meet specifications for current

voltage, and power dissipation. The transistor’s current

gain limits the guaranteed maximum output current to:

BE

LOAD(MAX)

DRVN

FE(MIN)

BE

V

I

(I

) h

R

=

−

×

between the transistor’s base and emitter. Furthermore,

the transistor’s current gain increases the regulator’s DC

loop gain (see the Stability Requirements section), so

excessive gain destabilizes the output.

The transistor’s saturation voltage at the maximum output

current determines the minimum input-to-output volt-

age differential that the regulator can support. Also, the

package’s power dissipation limits the usable maximum

input-to-output voltage differential. The maximum power-

dissipation capability of the transistor’s package and

mounting must exceed the actual power dissipated in

the device. The power dissipated equals the maximum

input-to-output voltage differential:

output voltage applied to the emitter of the transistor,

that the external transistor is not short-circuit protected.

Stability Requirements

The device’s negative-gate voltage regulator uses an

internal transconductance amplifier to drive an external

pass transistor. The transconductance amplifier, the

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

capacitor determine the loop stability.

The transconductance amplifier regulates the output volt-

age by controlling the pass transistor’s base current. The

total DC loop gain is approximately:

BIAS

FE

V_GL

REF

T

LOAD

I

h

4

A

(

) (1

) V

V

I

×

≅

× +

×

the device, the bias current for the negative-gate voltage

regulator is 0.1mA. Therefore, the base-to-emitter resistor

should be chosen to set 0.1mA bias current:

BE

BE

V

0.7V

R

7k

0.1mA

0.1mA

=

=

=

Ω

Use the closest standard resistor value of 6.8kI. The

output capacitor and the load resistance create the

dominant pole in the system. However, the internal

amplifier delay, 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 equations to verify that the regulator is properly

compensated:

1) First, determine the dominant pole set by the regula-

tor’s output capacitor and the load resistor:

LOAD(MAX)_GL

POLE_GL

OUT_GL

OUT_GL

I

f

2

C

V

=

π ×

×

The unity-gain crossover frequency of the regulator is:

2) The pole created by the internal amplifier delay is

approximately 1MHz: