LTC3637
9
3637fa
For more information www.linear.com/LTC3637
OPERATION
The LTC3637 is a step-down DC/DC regulator with an
internal high side power switch that uses Burst Mode
control. The low quiescent current and high switching
frequency results in high efficiency across a wide range
of load currents. Burst Mode operation functions by us-
ing short “burst” cycles to switch the inductor current
through the internal power MOSFET, followed by a sleep
cycle where the power switch is off and the load current
is supplied by the output capacitor. During the sleep cycle,
the LTC3637 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 efficiency. Figure 1 shows an example
of Burst Mode operation. The switching frequency and the
number of switching cycles during Burst Mode operation
are dependent on the inductor value, peak current, load
current, input voltage and output voltage.
reducing the V
IN
pin supply current to only 12µA. As the
load current discharges the output capacitor, the voltage
on the V
FB
pin decreases. 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 current
comparator threshold or the voltage on the V
FB
pin exceeds
800mV, at which time the high side power switch is turned
off and the external catch diode turns on. The inductor
current ramps down until the reverse current compara-
tor trips, signaling that the current is close to zero. If the
voltage on the V
FB
pin is still less than the 800mV refer-
ence, the high side power switch is turned on again and
another cycle commences. The average current during a
burst cycle will normally be 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 LTC3637 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 LTC3637
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 1.21V, normal operation
of the main control loop is enabled. The RUN pin com-
parator has 110mV of internal hysteresis, and therefore
must fall below 1.1V to stop switching and disable the
main control loop.
An internal 0.8ms 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
BURST
FREQUENCY
CURRENT
OUTPUT
VOLTAGE
∆V
BURST
CYCLE
CYCLE
SWITCHING
FREQUENCY
Figure 1. Burst Mode Operation
Main Control Loop
The LTC3637 uses the V
PRG1
and V
PRG2
control pins to
connect internal feedback resistors to the V
FB
pin. This
enables fixed outputs of 1.8V, 3.3V or 5V without increas-
ing component count, input supply current or exposure to
noise on the sensitive input to the feedback comparator.
External feedback resistors (adjustable mode) can still
be used by connecting both V
PRG1
and V
PRG2
to ground.
In adjustable mode the feedback comparator monitors
the voltage on the V
FB
pin and compares it to an inter-
nal 800mV reference. If this voltage is greater than the
reference, the comparator activates a sleep mode in which
the power switch and current comparators are disabled,
(Refer to Block Diagram)
