LTC3703IGN-5 Linear Technology, LTC3703IGN-5 Datasheet - Page 12

• Manufacturer Part Number: LTC3703IGN-5
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,CMOS,SSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

LTC3703
OPERATIO
Buck or Boost Mode Operation
The LTC3703 has the capability of operating both as a
step-down (buck) and step-up (boost) controller. In boost
mode, output voltages as high as 80V can be tightly
regulated. With the INV pin grounded, the LTC3703 oper-
ates 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
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
APPLICATIO S I FOR ATIO
The basic LTC3703 application circuit is shown in
Figure 1. External component selection is determined by
the input voltage and load requirements as explained in the
following sections. After the operating frequency is se-
lected, R
and the inductor are chosen for a desired amount of ripple
current and also to optimize efficiency and component
size. Next, the power MOSFETs and D1 are selected based
on voltage, load and efficiency requirements. C
lected for its ability to handle the large RMS currents in the
converter and C
meet the output voltage ripple and transient specifica-
tions. 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
consideration is the minimum on-time of the LTC3703
(see the Minimum On-time Considerations section). A
final consideration for operating frequency is that in
12
SET
and L can be chosen. The operating frequency
OUT
U
U
is chosen with low enough ESR to
U
IN
to V
W
OUT
ratios, another
U
IN
is se-
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
account when programming the I
noise-sensitive communications systems, it is often de-
sirable to keep the switching noise out of a sensitive
frequency band.
The LTC3703 uses a constant frequency architecture that
can be programmed over a 100kHz to 600kHz range with
a single resistor from the f
Figure 1. The nominal voltage on the f
the current that flows from this pin is used to charge and
discharge an internal oscillator capacitor. The value of
R
Figure 7 or from the following equation:
D
SET
= I
R
SET
for a given operating frequency can be chosen from
LOAD
(
k
Ω =
/(1 – D). This factor needs to be taken into
1000
100
Figure 7. Timing Resistor (R
)
10
1
0
f kHz
(
7100
200
) –
FREQUENCY (kHz)
25
400
SET
pin to ground, as shown in
600
MAX
SET
800
SET
voltage.
) Value
3703 F07
pin is 1.2V, and
1000
3703fa