Theory Of Operation - Analog Devices LTC3308A Demo Manual

DEMO MANUAL DC2991A
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THEORY OF OPERATION

Introduction to the DC2991A
The DC2991A features the LTC3308A, a low voltage syn-
chronous step-down Silent Switcher. The LTC3308A is a
monolithic, constant frequency, current mode step-down
DC/DC converter. An oscillator, with frequency set using
a resistor on the RT pin, turns on the internal top power
switch at the beginning of each clock cycle. Current in the
inductor then increases until the top switch comparator
trips and turns off the top power switch. If the EN pin is
low, the LTC3308A is in shutdown and in a low quiescent
current state. When the EN pin is above its threshold, the
switching regulator will be enabled.
The MODE/SYNC pin sets the switching mode to pulse-
skipping, forced continuous, or Burst Mode operation.
If an external 1MHz to 3MHz clock is connected to the
MODE/SYNC turret while the JP1 is set to the FC/SYNC
position, the LTC3308A switching frequency will sync to
6
Figure 4. Optional Transient Response Circuit
the external clock while operating in forced continuous
mode. See the LTC3308A data sheet for more detailed
information.
The maximum allowable operating frequency is influenced
by the minimum on-time of the top switch and the ratio
of V
quency may be calculated using a minimum TON of 42ns
in Equation 1.
f
SWMAX
Select an operating switching frequency below f
The recommended ripple current in the output inductor
is 1.2A peak-to-peak for the LTC3308A. This determines
the recommended inductor value for the inductor.
to V . The maximum allowable operating fre-
OUT IN
V
OUT
=
V
•TON
INMAX MIN
(1)
.
SWMAX
Rev. 0