Datasheet

extra windings coupled to the step-down converter induc-
tor. The negative gain block (MAX1531) can be used in
conjunction with a charge pump or coupled winding to gen-
erate 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 ampli-
fier establishes an integrated error voltage at the COMP
pin. The heart of the current-mode PWM controller 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 current 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 voltage error signal. Since the
average inductor current is nearly the same as the peak
inductor current (assuming that the inductor value is rela-
tively 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
slopecompensation ramp is summed into the main PWM
comparator.
During the second half of the cycle, the high-side
MOSFET 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 cur-
rent 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 lowside 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 Amplier
The MAX1530/MAX1531s’ current-sense circuit ampli-
fies the current-sense voltage generated by the highside
MOSFET’s on-resistance. This amplified current-sense
signal and the internal slope compensation signal are
summed together and fed into the PWM comparator’s
inverting input. Place the high-side MOSFET 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 circuits
that use the two MOSFETs’ on-resistances as current-
sensing elements (Figure 4). The high-side MOSFET’s
voltage is used with a fixed 400mV (typ) current- limit
threshold during the high-side on-times. The low-side
MOSFET’s 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 cur-
rent limit. If the voltage across the high-side MOSFET,
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
“valley” 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 MOSFET
stays off for the entire next cycle.
Figure 4. Step-Down Controller Block Diagram
SLOPE
CURRENT
SENSE
AND
CURRENT
LIMIT
GM
FLTM
0.9VREF
SOFT-
START
V
REF
DC-DC EN
SS DONE
CURRENT
LIMIT
COMP
FB
PGND
DL
LX
DH
ILIM
R
S
Q
IN
CLOCK
FAULT COMPARATOR
PWM COMP
Q
MAX1530/MAX1531 Multiple-Output Power-Supply
Controllers for LCD Monitors
www.maximintegrated.com
Maxim Integrated │ 18