LTC3632 LINER [Linear Technology], LTC3632 Datasheet - Page 8

• Manufacturer Part Number: LTC3632
• Description: High Effi ciency, High Voltage 20mA Synchronous Step-Down Converter
• Manufacturer: LINER [Linear Technology]

OPERATION
LTC3632
The LTC3632 is a step-down DC/DC converter with internal
power switches that uses Burst Mode control, combin-
ing low quiescent current with high switching frequency,
which results in high effi ciency across a wide range of
load currents. Burst Mode operation functions by using
short “burst” cycles to ramp the inductor current through
the internal power switches, followed by a sleep cycle
where the power switches are off and the load current is
supplied by the output capacitor. During the sleep cycle,
the LTC3632 draws only 12μA of supply current. At light
loads, the burst cycles are a small percentage of the total
cycle time which minimizes the average supply current,
greatly improving effi ciency.
Main Control Loop
The feedback comparator monitors the voltage on the V
pin and compares it to an internal 800mV reference. If
this voltage is greater than the reference, the comparator
activates a sleep mode in which the power switches and
current comparators are disabled, reducing the V
supply current to only 12μA. As the load current discharges
the output capacitor, the voltage on the V
When this voltage falls 5mV below the 800mV reference,
the feedback comparator trips and enables burst cycles.
At the beginning of the burst cycle, the internal high side
power switch (P-channel MOSFET) is turned on and the
inductor current begins to ramp up. The inductor current
increases until either the current exceeds the peak cur-
rent comparator threshold or the voltage on the V
exceeds 800mV, at which time the high side power switch
is turned off and the low side power switch (N-channel
MOSFET) turns on. The inductor current ramps down until
the reverse-current comparator trips, signaling that the
current is close to zero. If the voltage on the V
still less than the 800mV reference, the high side power
switch is turned on again and another cycle commences.
The average current during a burst cycle will normally be
8
(Refer to Block Diagram)
FB
pin decreases.
FB
FB
IN
pin is
pin
pin
FB
greater than the average load current. For this architecture,
the maximum average output current is equal to half of
the peak current.
The hysteretic nature of this control architecture results
in a switching frequency that is a function of the input
voltage, output voltage and inductor value. This behavior
provides inherent short-circuit protection. If the output
is shorted to ground, the inductor current will decay very
slowly during a single switching cycle. Since the high side
switch turns on only when the inductor current is near
zero, the LTC3632 inherently switches at a lower frequency
during start-up or short-circuit conditions.
Start-Up and Shutdown
If the voltage on the RUN pin is less than 0.7V, the LTC3632
enters a shutdown mode in which all internal circuitry is
disabled, reducing the DC supply current to 3μA. When the
voltage on the RUN pin exceeds 1.2V, normal operation of
the main control loop is enabled. The RUN pin comparator
has 110mV of internal hysteresis, and therefore must fall
below 1.1V to disable the main control loop.
The HYST pin provides an added degree of fl exibility for
the RUN pin operation. This open-drain output is pulled
to ground whenever the RUN comparator is not tripped,
signaling that the LTC3632 is not in normal operation. In
applications where the RUN pin is used to monitor the
V
pin can be used to increase the effective RUN comparator
hysteresis.
An internal 0.75ms soft-start function limits the ramp rate
of the output voltage on start-up to prevent excessive input
supply droop. If a longer ramp time and consequently less
supply droop is desired, a capacitor can be placed from the
SS pin to ground. The 5μA current that is sourced out of this
pin will create a smooth voltage ramp on the capacitor. If
this ramp rate is slower than the internal 0.75ms soft-start,
then the output voltage will be limited by the ramp rate
IN
voltage through an external resistive divider, the HYST
3632fb