MAX16928_12 Datasheet, PDF(11/20 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

Positive-Gate Voltage Regulator (GH)

The positive-gate voltage regulator includes a p-channel

FET output stage to generate a regulated output between

5V and (VCP - 2V). The regulator maintains accuracy over

wide line and load conditions. It is capable of at least

20mA of output current and includes current-limit protec-

tion. VGH is typically used to provide the TFT-LCD gate

driversâ gate-on voltage.

The regulator derives its positive supply voltage from a

noninverting charge pump, a single-stage example of

which is shown in the Typical Operating Circuit. A higher

voltage using a multistage charge pump is possible, as

described in the Charge Pumps section.

Negative-Gate Voltage Regulator (GL)

The negative-gate voltage regulator is an analog gain

block with an open-drain p-channel output. It drives an

external npn pass transistor with a 6.8kI base-to-emitter

resistor (see the Pass Transistor Selection section). Its

guaranteed base drive source current is at least 2mA.

VGL is typically used to provide the TFT-LCD gate driv-

ersâ gate-off voltage.

The output of the negative-gate voltage regulator (i.e.,

the collector of the external npn pass transistor) has load-

dependent bypassing requirements. Connect a ceramic

capacitor between the collector and ground with the

value shown in Table 3.

The regulator derives its negative supply voltage from an

inverting charge pump, a single-stage example of which

is shown in the Typical Operating Circuit. A more negative

voltage using a multistage charge pump is possible, as

described in the Charge Pumps section.

The external npn transistor is not short-circuit protected.

To maintain proper pulldown capability of the external npn

transistor and optimal regulation, a minimum load of at least

500FA is recommended on the output of the GL regulator.

Table 1. Supply Sequencing

CONTROL INPUTS

ENP

SEQ

FIRST

VSH

Enable (ENP)

Use the enable input (ENP) to enable and disable the

boost section of the device. Connect ENP to INA for

normal operation and to GND to place the device in shut-

down. In shutdown, the INA supply current is reduced to

0.5FA.

Soft-Start and Supply Sequencing (SEQ)

When enabled, the boost output ramps up from VINA to

its set voltage. Once the boost output reaches 85% of the

set voltage and the soft-start timer expires, the gate volt-

age regulators turn on in the order shown in Table 1. The

1.8V/3.3V regulator controller is enabled at the beginning

of the boost converterâs soft-start.

Both gate voltage regulators have a 7.45ms soft-start

time. The second one turns on as soon as the output of

the first reaches 85% of its set voltage.

Thermal Shutdown

Internal thermal shutdown circuitry shuts down the

device immediately when the die temperature exceeds

+165NC. A 15NC thermal shutdown hysteresis prevents

the device from resuming normal operation until the die

temperature falls below +150NC.

Design Procedure

Boost Converter

Inductor Selection

Three key inductor parameters must be specified for

operation with the device: inductance value (L), inductor

saturation current (ISAT), and DC resistance (RDC). To

determine the inductance value, select the ratio of inductor

peak-to-peak ripple current to average output current (LIR)

first. For LIR values that are too high, the RMS currents are

high, and therefore, I2R losses are high. Use high-valued

inductors to achieve low LIR values. Typically, inductance

is proportional to resistance for a given package type,

SUPPLY SEQUENCING

SECOND

Device is in shutdown

VGH

VGL

THIRD

VGL

VGH

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