LTC3703_15 Datasheet, PDF (12/34 Pages) Linear Technology – 100V Synchronous Switching Regulator Controller

LTC3703
Operation (Refer to Functional Diagram)
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 regu-
lated. With the INV pin grounded, the LTC3703 operates
in buck mode with TG driving the main (topside) 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 (topside) 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 disabled
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 cur-
rent by looking at the drain voltage of the main (bottom
side) MOSFET. In boost mode, however, the peak MOS-
FET current does not equal the load current but instead
I_D = I_LOAD/(1 – D). This factor needs to be taken into ac-
count when programming the I_MAX voltage.
Applications Information
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 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 efficiency and component size. Next,
the power MOSFETs and D1 are selected based on voltage,
load and efficiency requirements. 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 is that in noise-sensitive communica-
tions systems, it is often desirable 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_SET pin to ground, as shown
in Figure 1. 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 7 or from the following equation:
R_SET
(kΩ)
=
7100
f(kHz) –
25
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 MOS-
FET switching losses and gate charge losses. However,
lower frequency operation requires more inductance for a
given amount of ripple current, resulting in a larger induc-
tor 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 converters with high
step-down V_IN to V_OUT ratios, another consideration is
the minimum on-time of the LTC3703 (see the Minimum
On-Time Considerations section). A final consideration for
1000
100
10
1
0
200 400 600 800 1000
FREQUENCY (kHz)
3703 F07
Figure 7. Timing Resistor (R_SET) Value
3703fc
12