MAX1530_09 Datasheet, PDF(19/33 Page) Maxim Integrated Products – Multiple-Output Power-Supply Controllers for LCD Monitors

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MAX1530_09 Datasheet, PDF (19/33 Pages) Maxim Integrated Products – Multiple-Output Power-Supply Controllers for LCD Monitors

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Multiple-Output Power-Supply Controllers for

LCD Monitors

The ILIM pin is a dual-mode input. When ILIM is con-

nected to VL, a default low-side current limit of 250mV

(typ) is used. If ILIM is connected to a voltage between

250mV and 3V, the low-side current limit is typically

1/5th the ILIM voltage.

The MAX1530/MAX1531sâ current limits are compara-

tively inaccurate, since the maximum load current is a

function of the MOSFETsâ on-resistances and the induc-

tor value, as well as the accuracy of the two thresholds.

However, using MOSFET current sensing reduces both

cost and circuit size and increases efficiency, since

sense resistors are not needed.

MOSFET Gate Drivers (DH, DL)

The DH and DL drivers are optimized for driving mod-

erate-size high-side and low-side MOSFETs. Adaptive

dead-time circuits monitor the DL and DH drivers and

prevent either FET from turning on until the other is fully

off. This algorithm allows operation without shoot-

through with a wide range of MOSFETs, minimizing

delays and maintaining efficiency. When the gates are

turning off, there must be low-resistance, low-induc-

tance paths from the gate drivers to the MOSFET gates

for the adaptive dead-time circuit to work properly.

Otherwise, the sense circuitry in the MAX1530/

MAX1531 interpret the MOSFET gate as "off" while gate

charge actually remains. Use short, wide traces mea-

suring less than 50 squares (at least 20 mil wide if the

MOSFET is 1in from the device).

It is advantageous to slow down the turn-on of both

gate drivers if there is noise coupling between the

switching regulator and the linear regulators. The noise

coupling can result in excessive switching ripple on the

linear regulator outputs. Slowing down the turn-on of

the gate drivers proves to be an effective way of reduc-

ing the output ripple. Take care to ensure that the turn-

off times are not affected at the same time. As

explained above, slowing down the turn-off times may

result in shoot-through problems. In Figure 1, a 10Î©

resistor (R5) is inserted in series with the BST pin to

slow down the turn-on of the high-side MOSFET (N1-B)

without affecting the turn-off. A 10Î© resistor (R6) is also

inserted between DL and the gate of the low-side MOS-

FET (N1-A) to slow its turn-on. Because the gate resis-

tor would slow down the turn-off time, connect a

switching diode (D2) (such as 1N4148) in parallel with

the gate resistor as shown in Figure 1 to prevent poten-

tial shoot-through.

High-Side Gate-Drive Supply (BST)

A flying-capacitor bootstrap circuit generates gate-

drive voltage for the high-side N-channel switch (Figure

1). The capacitor C5 between BST and LX is alternately

charged from the VL supply and placed parallel to the

high-side MOSFETâs gate-source terminals.

On startup, the synchronous rectifier (low-side MOS-

FET) forces LX to ground and charges the boost

capacitor from VL through diode D1. On the second

half-cycle, the switch-mode power supply turns on the

high-side MOSFET by closing an internal switch

between BST and DH. This provides the necessary

gate-to-source voltage to turn on the high-side switch,

an action that boosts the 5V gate-drive signal above

the input voltage.

Oscillator Frequency Selection (FREQ)

The FREQ pin can be used to select the switching fre-

quency of the step-down regulator. Connect FREQ to

VL for 500kHz operation. Connect FREQ to AGND for

250kHz operation. The 500kHz operation minimizes the

size of the inductor and capacitors. The 250kHz opera-

tion improves efficiency by 2% to 3%.

Linear Regulator Controllers

The MAX1530/MAX1531 include three positive linear

regulator controllers, LR1, LR2, and LR3. These linear

regulator controllers can be used with external pass

transistors to regulate supplies for TFT LCDs. The

MAX1531 includes an additional positive linear regula-

tor controller (LR4) and a negative linear regulator con-

troller (LR5).

Low-Voltage Logic Regulator Controller (LR1)

LR1 is an analog gain block with an open-drain N-

channel output. It drives an external PNP pass transis-

base drive sink current is at least 3mA. The regulator

including transistor Q1 in Figure 1 uses a 10ÂµF output

capacitor and is designed to deliver 500mA at 2.5V.

LR1 is typically used to generate low-voltage logic sup-

plies for the timing controller and the digital sections of

the TFT LCD source/gate driver ICs.

LR1 is enabled when the soft-start of the main step-

down regulator is complete. (See the Startup Sequence

(ONL_,SEQ) section.) Each time it is enabled, the con-

troller goes through a soft-start routine that ramps up its

internal reference DAC. (See the Soft-Start section.)

Gamma Regulator Controller (LR2)

LR2 is an analog gain block with an open-drain N-

channel output. It drives an external PNP pass transis-

base drive sink current is at least 2mA. The regulator

including transistor Q2 in Figure 1 uses a 0.47ÂµF output

capacitor and is designed to deliver 50mA at 9.7V.

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