LTC3703-5
12
37035fa
The basic LTC3703-5 application circuit is shown on the first
page of this data sheet. External component selection is de-
termined by the input voltage and load requirements as
explained in the following sections. After the operating
frequency is selected, R
SET
and L can be chosen. The
operating frequency and the inductor are chosen for a
desired amount of ripple current and also to optimize ef-
ficiency and component size. Next, the power MOSFETs and
D1 are selected based on voltage, load and efficiency re-
quirements. C
IN
is selected for its ability to handle the large
RMS currents in the converter and C
OUT
is chosen with low
enough ESR to meet the output voltage ripple and transient
specifications. Finally, the loop compensation components
are chosen to meet the desired transient specifications.
Operating Frequency
The choice of operating frequency and inductor value is a
trade off between efficiency and component size. Low
frequency operation improves efficiency by reducing
MOSFET switching losses and gate charge losses. How-
ever, lower frequency operation requires more induc-
tance for a given amount of ripple current, resulting in a
larger inductor size and higher cost. If the ripple current
is allowed to increase, larger output capacitors may be
required to maintain the same output ripple. For convert-
ers with high step-down V
IN
to V
OUT
ratios, another
consideration is the minimum on-time of the LTC3703-5
(see the Minimum On-time Considerations section). A
final consideration for operating frequency is that in
APPLICATIO S I FOR ATIO
WUUU
noise-
sensitive communications systems, it is often de-
sirable to keep the switching noise out of a sensitive
frequency band.
The LTC3703-5 uses a constant frequency architecture
that can be programmed over a 100kHz to 600kHz range
with a single resistor from the f
SET
pin to ground, as shown
in the circuit on the first page of this data sheet. The
nominal voltage on the f
SET
pin is 1.2V, and the current that
flows from this pin is used to charge and discharge an
internal oscillator capacitor. The value of R
SET
for a given
operating frequency can be chosen from Figure 6 or from
the following equation:
Rk
f kHz
SET
()
()–
Ω =
7100
25
Buck or Boost Mode Operation
The LTC3703-5 has the capability of operating both as a
step-down (buck) and step-up (boost) controller. In boost
mode, output voltages as high as 60V can be tightly
regulated. With the INV pin grounded, the LTC3703-5
operates in buck mode with TG driving the main (top side)
switch and BG driving the synchronous (bottom side)
switch. If the INV pin is pulled above 2V, the LTC3703-5
operates in boost mode with BG driving the main (bottom
side) switch and TG driving the synchronous (top side)
switch. Internal circuit operation is very similar regardless
of the operating mode with the following exceptions: In
boost mode, Pulse Skip Mode operation is always dis-
abled regardless of the level of the MODE/SYNC pin and
the line feedforward compensation is also disabled. The
overcurrent circuitry continues to monitor the load current
by looking at the drain voltage of the main (bottom side)
MOSFET. In boost mode, however, the peak MOSFET
current does not equal the load current but instead
I
D
= I
LOAD
/(1 – D). This factor needs to be taken into
account when programming the I
MAX
voltage.
FREQUENCY (kHz)
R
SET
(kΩ)
1000
37035 F06
10
1
100
200 1000800600400
0
Figure 6. Timing Resistor (R
SET
) Value
OPERATIO
U
(Refer to Functional Diagram)
