Theory Of Operation - Analog Devices DC2990A Manual

DEMO MANUAL DC2990A
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THEORY OF OPERATION

Introduction to the DC2990A
The DC2990A features the LTC3307A, a low voltage syn-
chronous step-down Silent Switcher. The LTC3307A 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 LTC3307A 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 LTC3307A switching frequency will sync to
6
Figure 4. Optional Transient Response Circuit
the external clock while operating in forced continuous
mode. See the LTC3307A data sheet for more detailed
information.
The maximum allowable operating frequency is influenced
by the minimum on-time of the top switch, the ratio of
V
OUT
mum allowable operating frequency may be calculated
using a minimum TON of 42ns in Equation 1.
Select an operating switching frequency below f
The recommended ripple current in the output inductor is
0.9A peak-to-peak for the LTC3307A. This determines the
recommended inductor value for the application.
to V and the available inductor values. The maxi-
IN
V
OUT
f
=
SWMAX
V
•TON
INMAX MIN
(1)
.
SWMAX
Rev. 0