6
Rev. 0
QUICK START PROCEDURE
Figure 3. Optional Transient Response Circuit
THEORY OF OPERATION
Introduction to the DC3123A
The DC3123A demonstration circuit features the
LTC3315A, a 5V Dual 2A Low voltage Synchronous Step-
Down regulator. The LTC3315A is a monolithic, constant
frequency, current mode step-down DC/DC converter.
An internal oscillator 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 LTC3315A is in shutdown and in a low quiescent
current state. When the EN pin is above its threshold, the
switching regulator will be enabled. Buck 1 and Buck 2
are 180° out of phase with each other.
The MODE/SYNC pin sets the switching mode to pulse
skip, forced continuous, or burst mode. 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
LTC3315A switching frequency will sync to the external
clock while operating in forced continuous mode. See
the LTC3315A 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
to V
IN
and the available inductor values. The maximum
allowable operating frequency may be calculated using a
minimum Ton of 45ns in the formula below.
f
SWMAX
=
V
OUT
V
INMAX
•
Ton
MIN
Select an operating switching frequency below f
SWMAX
.
The LTC3315A has a requirement that the magnitude of
the ripple current in the output inductor be greater than
0.6A peak to peak. This will determine the maximum
inductor value for the application.
Accurately Measuring Output Ripple of the LTC3315A
With the fast edge rates of the circuit, high frequency
noise can be observed when measuring the output voltage
with 1MΩ terminated oscilloscope probes. To better view
the output ripple with oscilloscopes of 400MHz bandwidth
and above a 50Ω coax cable connected as close to
the output caps as possible should be used with the
