MAX1530_09 Datasheet, PDF (22/33 Pages) Maxim Integrated Products – Multiple-Output Power-Supply Controllers for LCD Monitors

Multiple-Output Power-Supply
Controllers for LCD Monitors
total delay. Because of the 6µA current flowing through
R1 (51kΩ), the voltage on ONLc is 0.31V greater than
the voltage on ONLd and it crosses the 1.238V thresh-
old and enables its LR_ controller about 4ms before
ONLd’s controller. Similarly, the 4µA current through R2
(75kΩ) and the 2µA current through R3 (150kΩ) cause
their LR_ controllers to each start about 4ms before the
next one. Any desired sequence and delay can be pro-
grammed by calculating the charge rate of C1 and volt-
age drops across R1 through R3.
Soft-Start
The soft-start function controls the slew rate of the out-
put voltages and reduces inrush currents during start-
up. Each regulator (step-down, LR1 to LR5) goes
through a soft-start routine after it is enabled. During
soft-start, the reference voltage for each positive regu-
lator gradually ramps up from 0V to the internal refer-
ence in 32 steps. The reference voltage of the negative
regulator ramps down from VL to 125mV in 32 steps.
The total soft-start period for each regulator is 1024
clock cycles for 250kHz switching frequency and 2048
clock cycles for 500kHz switching frequency.
Reset
The MAX1530/MAX1531 include an open-drain timed
microprocessor supervisor function to ensure proper
startup of digital circuits. The RESET output asserts low
whenever RSTIN is less than the RSTIN trip threshold.
RESET also asserts low when VL is less than the VL
UVLO threshold, EN is low, or the thermal, undervolt-
age or overcurrent fault latches are set. RESET enters
the high-impedance state only after RSTIN remains
above the trip threshold for the duration of the reset
timeout period. The state of RESET has no effect on
other portions of the IC.
The RSTIN threshold (1.114V typ) is designed to allow
RSTIN to directly connect to any of the MAX1530/
MAX1531s’ feedback input pins, eliminating the need
for an additional resistive divider. Typically, RSTIN is
connected to FB or FBL1 to monitor the supply voltage
for digital logic ICs, but it can be used to monitor any
desired output voltage or it can even be used as a gen-
eral-purpose comparator.
Fault Protection
Undervoltage Protection
After its soft-start is done, if the output of the main step-
down regulator or any of the linear-regulator outputs
(LR1 to LR5) are below 90% of their normal regulation
point, the MAX1530/MAX1531 activate an internal fault
timer. If the fault condition remains continuously for the
entire fault timer duration, the MAX1530/MAX1531 set
the fault latch, shutting down all the regulator outputs.
Undervoltage faults do not turn off VL. Once the fault
condition is removed, cycling the input voltage or
applying a rising edge on SEQ or EN clears the fault
latch and reactivates the device.
Thermal Protection
The thermal protection limits total power dissipation in
the MAX1530/MAX1531. If the junction temperature
exceeds +160°C, a thermal sensor immediately sets the
thermal fault latch, shutting off all the IC’s outputs
including VL, allowing the device to cool down. The only
way to clear the thermal fault latch is to cycle the input
voltage after the device cools down by at least 15°C.
Overcurrent Protection Block (CSH, CSL)
(MAX1531 Only)
The MAX1531 includes an uncommitted overcurrent
protection block that can be used to measure any input
or output current, using a current-sense resistor or
other sense element. If the measured current exceeds
the overcurrent protection threshold (300mV typ), the
MAX1531 immediately sets the undervoltage fault latch,
shutting down all the regulator outputs. Overcurrent
faults do not turn off VL. An internal lowpass filter pre-
vents large current transients of short duration (less
than 50µs) from setting the latch. Once the overcurrent
condition is removed, cycling the input voltage clears
the fault latch and reactivates the device. A rising edge
on SEQ or EN also clears the fault latch.
In Figure 1’s circuit, the overcurrent protection is used
with the LR4 source driver regulator since that regulator
is powered directly from the input supply and has no
current limit of its own. The current-sense resistor is
placed in series with the input supply, before the linear
regulator’s external PNP pass transistor. CSH and CSL
are connected to the positive and negative sides of the
sense resistor.
Design Procedures
Main Step-Down Regulator
Inductor Selection
Three key inductor parameters must be specified:
inductance value (L), peak current (I_PEAK), and DC
resistance (R_DC). The following equation includes a
constant, LIR, which is the ratio of peak-to-peak induc-
tor ripple current to DC load current. A higher LIR value
allows smaller inductance, but results in higher losses
and higher ripple. A good compromise between size
and losses is typically found at a 30% ripple current to
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