5
DEMO MANUAL DC275
DC/DC CONVERTER
OPERATIO
U
The circuit in Figure 1 highlights the capabilities of the
LTC1702. This design provides one fixed 3.3V output
(V
OUT2
) and one output (V
OUT1
) that is jumper selectable
from 1.6V to 2.5V. The LTC1702 is a voltage mode
controller, designed to drive a pair of external N-channel
MOSFETs using a fixed 550kHz switching frequency. The
synchronous buck architecture automatically shifts to
discontinuous operation and then to Burst Mode
TM
opera-
tion as the output load decreases, ensuring maximum
efficiency over a wide range of load currents. This mode is
recommended for load currents less than 1A and can be
implemented on the demo board by moving jumper JP4 to
the “Burst” position.
Theory of Operation
The LTC1702 has two independent switching regulators.
For the sake of simplicity and to minimize repetition, only
side “1” will be discussed. The divided output (V
OUT1
) is
compared to the 0.8V reference. The difference voltage is
multiplied by the error amplifier’s (FB) gain. The resulting
error signal is then compared to an internally generated,
fixed frequency sawtooth waveform by the PWM com-
parator, which generates a pulse width modulated signal.
This PWM signal drives the external MOSFETs through
TG1 and BG1. The output of this chopper circuit is then
filtered by L1 and C9 to C12 to produce the desired DC
output voltage.
2-Phase Operation
The LTC1702 dual switching regulator controller also
features the considerable benefits of 2-phase operation.
The LTC1702 includes a single master clock that drives the
two sides such that side 1 is 180
°
out of phase with side
2. This technique, known as 2-phase switching, has the
effect of doubling the frequency of the switching pulses
seen by the input capacitor and significantly reduces their
RMS value. With 2-phase switching, the input capacitor is
sized as required to support the larger of the two sides at
maximum load current. As the load current increases on
the lower current side, it tends to cancel, rather than add
to, the RMS current seen by the input capacitor; thus no
additional capacitance is needed.
Capacitor Considerations
The input capacitors are Kemet T510X337K010AS, 330
µ
F,
10V tantalums. The input capacitors must be rated for the
RMS input ripple. A good rule of thumb is that the input
ripple current will be 50% of the output current. Since the
LTC1702 uses 2-phase switching, the input bulk capaci-
tors should be able to fully handle the RMS ripple current
of just one load. As the load current increases on the other
side, it tends to cancel, rather than to add to, the ripple
current requirements for the input capacitors. For a con-
tinuous output current of 15A, the ripple current rating of
the input capacitors should be 7.5A. The capacitors cho-
sen are rated at 2.5A each, so three are adequate. Without
the 2-phase operation, six capacitors would be required to
handle two 15A loads.
Output capacitors need to have a ripple current rating
greater than the RMS value of the inductor ripple current.
This is a function of the operating frequency and inductor
value, as well as input and output voltages. Because the
ripple current is relatively small, the controlling parameter
is generally the capacitor’s ESR (equivalent series resis-
tance). The maximum allowable ESR is equal to the
maximum allowable peak-to-peak output ripple voltage
divided by the peak-to-peak inductor ripple current. In
general, if the ESR is low enough for the ripple voltage and
transient requirements, the capacitors will have more than
adequate ripple current capability.
Inductor Selection
Inductor selection is not extremely critical. The inductor
used here was chosen for fairly low cost and ready
availability. The main concerns in choosing an appropri-
ate inductor are the inductance value required, the satu-
ration current rating and the temperature rise. Most
manufacturers specify a DC current rating that produces
a temperature rise of 40
°
C. If a design will not see high
ambient temperatures, a larger temperature rise can
usually be tolerated. Another maximum current specifica-
tion is related to core saturation. A manufacturer may
specify that maximum rated current is the point at which
inductance is down by 10% (some specify 25%). Since
most core materials and structures will result in a gentle,
Burst Mode is a trademark of Linear Technology Corporation.
