MAX16928_12 Datasheet, PDF(10/20 Page) Maxim Integrated Products – Automotive TFT-LCD Power Supply with Boost Converter and Gate Voltage Regulators

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MAX16928

Automotive TFT-LCD Power Supply with Boost

Converter and Gate Voltage Regulators

Spread-Spectrum Modulation

The high-frequency 2.2MHz operation of the boost con-

verter moves switching noise outside of the AM band.

The device achieves enhanced EMI performance by

modulating the switching frequency by Q4%. The modu-

lating signal is pseudorandom and changes each switch-

ing period (i.e., fSS = 2.2MHz).

Startup

Immediately after power-up, coming out of shutdown,

or going into autoretry, the boost converter performs a

short-circuit detection test on the output by connecting

the input (INA) to the switching node (LXP) through an

internal 50I resistor.

If the resulting voltage on LXP exceeds 1.2V, the device

turns on the external pMOS switch by pulling GATE low.

The boost output ramps to its final value in 15ms.

An overloaded or shorted output is detected if the result-

ing voltage on LXP is below 1.2V. The external pMOS

switch remains off and the converter does not switch.

After the fault blanking period of 238ms, the device pulls

PGOOD low and starts the autoretry timer.

The short-circuit detection feature places a lower limit

on the output load of approximately 46I when the input

voltage is 3V.

Fault Conditions and PGOOD

PGOOD signals whether all the regulators and the boost

converter are operating normally. PGOOD is an open-

drain output that pulls low if any of the following faults

occur:

1) The boost output voltage falls below 85% of its set

value.

2) The positive-gate voltage regulator output (VGH) falls

below 85% of its set value.

3) The negative-gate voltage regulator output (VGL) falls

below 85% of its set value.

4) The LXP voltage is greater than 21V (typ).

5) The positive charge-pump voltage (VCP) is greater

than 30.5V (typ).

6) The 1.8V/3.3V regulator output voltage falls below

85% of its nominal value.

If any of the first three fault conditions persists for longer

than the 238ms fault blanking period, the device pulls

PGOOD low, turns off all outputs, and starts the autoretry

timer.

If either condition 4 or 5 occurs, the device pulls PGOOD

low and turns off all outputs immediately. The device initi-

ates startup only after the fault has cleared.

If the last condition occurs, the device pulls PGOOD low,

but does not turn off any of the outputs.

During startup, PGOOD is masked and goes high as

soon as the 1.8V/3.3V regulator controller turns on. This

regulator turns on as soon as VINA exceeds the INA

undervoltage lockout threshold.

Autoretry

When the autoretry counter finishes incrementing after

1.9s, the device attempts to turn on the boost con-

verter and gate voltage regulators in the order shown

in Table 1. The device continues to autoretry as long as

the fault condition persists. A fault on the 1.8V/3.3V regu-

lator output causes PGOOD to go low, but does not result

in the device shutting down and going into autoretry.

Current Limit

The effective current limit of the boost converter is

reduced by the internally injected slope compensation by

an amount dependent on the duty cycle of the converter.

The effective current limit is given by:

ILIM(EFF)=192

-12

ILIM_DC_0

COMPI

where ILIM(EFF) is the effective current limit, ILIM_DC_0 =

1.1A or 2.2A, depending on the boost converter current-

limit option, D is the duty cycle of the boost converter,

and CCOMPI is the value of the capacitor at the COMPI

input. Estimate the duty cycle of the converter using the

formulas shown in the Design Procedure section.

1.8V/3.3V Regulator Controller

The 1.8V/3.3V regulator controller delivers 4.5mA (min)

to an external load. Connect FB to DR for a regulated

1.8V/3.3V output.

For higher output capability, use an external npn transis-

tor as shown in the Typical Operating Circuit. The drive

capability of the regulator is then increased by the cur-

rent gain of the transistor (hFE). When using an external

transistor, use DR as the base drive and connect FB to

the transistorâs emitter. Bypass the base to ground with a

0.1FF ceramic capacitor.

If the boost output current is greater than 300mA, con-

nect a 30kI resistor between DR and GND.

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