MAX1530 Datasheet, PDF(18/33 Page) Maxim Integrated Products – Multiple-Output Power-Supply Controllers for LCD Monitors

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MAX1530 Datasheet, PDF (18/33 Pages) Maxim Integrated Products – Multiple-Output Power-Supply Controllers for LCD Monitors

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Multiple-Output Power-Supply Controllers for

LCD Monitors

SLOPE

SS DONE

DC-DC EN

VREF

COMP

CLOCK

SOFT-

START

CURRENT

SENSE

AND

CURRENT

LIMIT

PWM COMP

ILIM

CURRENT

LIMIT

0.9VREF

FAULT COMPARATOR

PGND

FLTM

Figure 4. Step-Down Controller Block Diagram

pumps attached to the switching node or extra wind-

ings coupled to the step-down converter inductor. The

negative gain block (MAX1531) can be used in con-

junction with a charge pump or coupled winding to

generate the LCD gate-off voltage or other negative

supplies.

Step-Down Controller

The MAX1530/MAX1531 include step-down controllers

that use a fixed-frequency current-mode PWM control

scheme (Figure 4). An internal transconductance

amplifier establishes an integrated error voltage at the

COMP pin. The heart of the current-mode PWM con-

troller is an open-loop comparator that compares an

integrated voltage-feedback signal with an amplified

current-sense signal plus a slope-compensation ramp.

At each rising edge of the internal clock, the high-side

MOSFET turns on until the PWM comparator trips or the

maximum duty cycle is reached. During this on-time,

current ramps up through the inductor, sourcing cur-

rent to the output and storing energy in a magnetic

field. The current-mode feedback system regulates the

peak inductor current as a function of the output volt-

age error signal. Since the average inductor current is

nearly the same as the peak inductor current (assum-

ing that the inductor value is relatively high to minimize

ripple current), the circuit acts as a switch-mode

transconductance amplifier. That pushes the output LC

filter pole, normally found in a voltage-mode PWM, to a

higher frequency. To preserve loop stability, the slope-

compensation ramp is summed into the main PWM

comparator.

During the second half of the cycle, the high-side MOS-

FET turns off and the low-side N-channel MOSFET turns

on. Now the inductor releases the stored energy as its

current ramps down, providing current to the output.

The output capacitor stores charge when the inductor

current exceeds the load current and discharges when

the inductor current is lower, smoothing the voltage

across the load. Under overload conditions, when the

inductor current exceeds the selected current limit (see

Current Limit Circuit), the high-side MOSFET is not

turned on at the rising edge of the clock and the low-

side MOSFET remains on to let the inductor current

ramp down.

Under light-load conditions, the MAX1530/MAX1531

maintain a constant switching frequency to minimize

cross-regulation errors in applications that use a trans-

former. The low-side gate-drive waveform is the comple-

ment of the high-side gate-drive waveform, which

causes the inductor current to reverse under light loads.

Current-Sense Amplifier

The MAX1530/MAX1531sâ current-sense circuit ampli-

fies the current-sense voltage generated by the high-

side MOSFETâs on-resistance. This amplified

current-sense signal and the internal slope compensa-

tion signal are summed together and fed into the PWM

comparatorâs inverting input. Place the high-side MOS-

FET near the controller, and connect IN and LX to the

MOSFET using Kelvin-sense connections to guarantee

current-sense accuracy and improve stability.

Current-Limit Circuit

The MAX1530/MAX1531 include two current-limit cir-

cuits that use the two MOSFETsâ on-resistances as cur-

rent-sensing elements (Figure 4). The high-side

MOSFETâs voltage is used with a fixed 400mV (typ) cur-

rent-limit threshold during the high-side on-times. The

low-side MOSFETâs voltage is used with an adjustable

current-limit threshold during the low-side on-times.

Using both circuits together ensures that the current is

always measured and controlled.

The high-side MOSFET current limit employs a peak

current limit. If the voltage across the high-side MOS-

FET, measured from IN to LX, exceeds the 400mV

threshold during an on-time, the high-side MOSFET

turns off and the low-side MOSFET turns on.

The low-side MOSFET current-limit circuit employs a

âvalleyâ current limit. If the voltage across the low-side

MOSFET, measured from LX to PGND, exceeds the

low-side threshold at the end of a low-side on-time, the

low-side MOSFET remains on and the high-side MOS-

FET stays off for the entire next cycle.

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