LTC1553 Datasheet, PDF (10/24 Pages) Linear Technology – 5-Bit Programmable Synchronous Switching Regulator Controller for Pentium II Processor

LTC1553
APPLICATIONS INFORMATION
OVERVIEW
The LTC1553 is a voltage feedback, synchronous switch-
ing regulator controller (see Block Diagram) designed for
use in high power, low voltage step-down (buck) convert-
ers. It is designed to satisfy the requirements of the Intel
Pentium II power supply specification. It includes an
on-chip DAC to control the output voltage, a PWM genera-
tor, a precision reference trimmed to ±1%, two high power
MOSFET gate drivers and all the necessary feedback and
control circuitry to form a complete switching regulator
circuit.
The LTC1553 includes a current limit sensing circuit that
uses the upper external power MOSFET as a current
sensing element, eliminating the need for an external
sense resistor. Once the current comparator, CC, detects
an overcurrent condition, the duty cycle is reduced by
discharging the soft start capacitor through a voltage-
controlled current source. Under severe overloads or
output short circuit conditions, the chip will be repeatedly
forced into soft start until the short is removed, preventing
the external components from being damaged. Under
output overvoltage conditions, the MOSFET drivers will be
disabled permanently until the chip power supply is
recycled or the OUTEN pin is toggled.
OUTEN can optionally be connected to an external nega-
tive temperature coefficient (NTC) thermistor placed near
the external MOSFETs or the microprocessor. Three thresh-
old levels are provided internally. When OUTEN drops to
2V, OT will trip, issuing a warning to the external CPU. If
the temperature continues to rise and the OUTEN input
drops to 1.7V, the G1 and G2 pins will be forced low. If
OUTEN is pulled below 1.2V, the LTC1553 will go into
shutdown mode, cutting the supply current to a minimum.
If thermal shutdown is not required, OUTEN can be con-
nected to a conventional TTL enable signal. The free-
running 300kHz PWM frequency can be synchronized to
a faster external clock connected to OUTEN. Adjusting the
oscillator frequency can add flexibility in the external
component selection. See the Clock Synchronization
section.
Output regulation can be monitored with the PWRGD pin
which in turn monitors the internal MIN and MAX com-
parators. If the output is ±5% beyond the selected value
for more than 500µs, the PWRGD output will be pulled
low. Once the output has settled within ±5% of the
selected value for more than 1ms, PWRGD will return
high.
THEORY OF OPERATION
Primary Feedback Loop
The regulator output voltage at the SENSE pin is divided
down internally by a resistor divider with a total resistance
of approximately 120kΩ. This divided down voltage is
subtracted from a reference voltage supplied by the DAC
output. The resulting error voltage is amplified by the error
amplifier and the output is compared to the oscillator ramp
waveform by the PWM comparator. This PWM signal
controls the external MOSFETs through G1 and G2. The
resulting chopped waveform is filtered by L_O and C_OUT
closing the loop. Loop frequency compensation is achieved
with an external RC + C network at the COMP pin, which is
connected to the output node of the transconductance
amplifier.
MIN, MAX Feedback Loops
Two additional comparators in the feedback loop provide
high speed fault correction in situations where the ERR
amplifier may not respond quickly enough. MIN compares
the feedback signal FB to a voltage 60mV (5%) below the
internal reference. If FB is lower than the threshold of this
comparator, the MIN comparator overrides the ERR
amplifier and forces the loop to full duty cycle which is set
by the internal oscillator typically to 84%. Similarly, the
MAX comparator forces the output to 0% duty cycle if FB
is more than 5% above the internal reference. To prevent
these two comparators from triggering due to noise, the
MIN and MAX comparators’ response times are deliber-
ately controlled so that they take two to three microsec-
onds to respond. These two comparators help prevent
extreme output perturbations with fast output transients,
while allowing the main feedback loop to be optimally
compensated for stability.
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