MAX1533 Datasheet, PDF (27/38 Pages) Maxim Integrated Products – High-Efficiency, 5x Output, Main Power-Supply Controllers for Notebook Computers

High-Efficiency, 5x Output, Main Power-Supply
Controllers for Notebook Computers
Thermal Fault Protection
The MAX1533/MAX1537 feature a thermal fault-protec-
tion circuit. When the junction temperature rises above
+160°C, a thermal sensor activates the fault latch, pulls
PGOOD low, and shuts down both SMPS controllers
using discharge mode (see the Output Discharge (Soft-
Shutdown) section). When an SMPS output voltage
drops to 0.3V, its synchronous rectifier turns on, clamp-
ing the discharged output to GND. Cycle V_CC below 1V
or toggle either ON3, ON5, or SHDN to clear the fault
latch and restart the controllers after the junction tem-
perature cools by 15°C.
Auxiliary LDO Detailed
Description (MAX1537 Only)
The MAX1537 includes an auxiliary linear regulator that
delivers up to 150mA of load current. The output
(LDOA) can be preset to 12V, ideal for PCMCIA power
requirements, and for biasing the gates of load switch-
es in a portable device. In adjustable mode, LDOA can
be set to anywhere from 5V to 23V. The auxiliary regu-
lator has an independent ON/OFF control, allowing it to
be shut down when not needed, reducing power con-
sumption when the system is in a low-power state.
A flyback-winding control loop regulates a secondary
winding output, improving cross-regulation when the pri-
mary output is lightly loaded or when there is a low
input-output differential voltage. If V_INA - V_LDOA falls
below 0.8V, the low-side switch is turned on for a time
equal to 33% of the switching period. This reverses the
inductor (primary) current, pulling current from the out-
put filter capacitor and causing the flyback transformer
to operate in forward mode. The low impedance pre-
sented by the transformer secondary in forward mode
dumps current into the secondary output, charging up
the secondary capacitor and bringing V_INA - V_LDOA
back into regulation. The secondary feedback loop does
not improve secondary output accuracy in normal fly-
back mode, where the main (primary) output is heavily
loaded. In this condition, secondary output accuracy is
determined by the secondary rectifier drop, transformer
turns ratio, and accuracy of the main output voltage.
Adjustable LDOA Voltage
(Dual-Mode Feedback)
Connect ADJA to GND to enable the fixed, preset 12V
auxiliary output. Connect a resistive voltage-divider at
ADJA between LDOA and GND to adjust the respective
output voltage between 5V and 23V (Figure 8). Choose
R2 (resistance from ADJA to GND) to be approximately
SECONDARY
FEEDBACK
INA
LDOA
ONA
REF (2.0V)
FIXED 12V
5R
R
ADJA
0.15V
Figure 8. Linear-Regulator Functional Diagram
100kΩ and solve for R1 (resistance from LDOA to
ADJA) using the following equation:
R1
=
R2
⎛
⎝⎜
V_LDOA
V_ADJA
⎞
- 1_⎠⎟
where V_ADJA = 2V nominal.
Design Procedure
Firmly establish the input voltage range and maximum
load current before choosing a switching frequency
and inductor operating point (ripple-current ratio). The
primary design trade-off lies in choosing a good switch-
ing frequency and inductor operating point, and the fol-
lowing four factors dictate the rest of the design:
• Input Voltage Range. The maximum value
(V_IN(MAX)) must accommodate the worst-case, high
AC-adapter voltage. The minimum value (V_IN(MIN))
must account for the lowest battery voltage after
drops due to connectors, fuses, and battery-selector
switches. If there is a choice at all, lower input volt-
ages result in better efficiency.
______________________________________________________________________________________ 27