LTC3703IGN Datasheet, PDF (13/34 Pages) Linear Integrated Systems – 100V Synchronous Switching Regulator

LTC3703
Applications Information
The oscillator can also be synchronized to an external
clock applied to the MODE/SYNC pin with a frequency in
the range of 100kHz to 600kHz (refer to the MODE/SYNC
Pin section for more details). In this synchronized mode,
pulse-skip mode operation is disabled. The clock high level
must exceed 2V for at least 25ns. As shown in Figure 8,
the top MOSFET turn-on will follow the rising edge of the
external clock by a constant delay equal to one-tenth of
the cycle period.
MODE/
SYNC
TG
t_MIN = 25ns
2V TO 10V
0.8T T T = 1/f_O
D = 40%
0.1T
I_L
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Figure 8. MODE/SYNC Clock Input and Switching
Waveforms for Synchronous Operation
Inductor
The inductor in a typical LTC3703 circuit is chosen for a
specific ripple current and saturation current. Given an
input voltage range and an output voltage, the inductor
value and operating frequency directly determine the ripple
current. The inductor ripple current in the buck mode is:
∆I_L
=
V_OUT
(f)(L)


1–
V_OUT
V_IN


Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Thus highest efficiency operation is obtained at
low frequency with small ripple current. To achieve this
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
between 20% and 40% of I_O(MAX). Note that the largest
ripple current occurs at the highest V_IN. To guarantee that
ripple current does not exceed a specified maximum, the
inductor in buck mode should be chosen according to:
L
≥
f
V_OUT
∆ I_{L(MAX )}

 ^1–

V_OUT
V_{IN(MAX )}



The inductor also has an affect on low current operation
when pulse-skip mode operation is enabled. The frequency
begins to decrease when the output current drops below
the average inductor current at which the LTC3703 is
operating at its t_ON(MIN) in discontinuous mode (see
Figure 6). Lower inductance increases the peak inductor
current that occurs in each minimum on-time pulse and
thus increases the output current at which the frequency
starts decreasing.
Power MOSFET Selection
The LTC3703 requires at least two external N-channel
power MOSFETs, one for the top (main) switch and one or
more for the bottom (synchronous) switch. The number,
type and “on” resistance of all MOSFETs selected take into
account the voltage step-down ratio as well as the actual
position (main or synchronous) in which the MOSFET will
be used. A much smaller and much lower input capacitance
MOSFET should be used for the top MOSFET in applica-
tions that have an output voltage that is less than 1/3 of
the input voltage. In applications where V_IN >> V_OUT, the
top MOSFETs’ “on” resistance is normally less important
for overall efficiency than its input capacitance at operating
frequencies above 300kHz. MOSFET manufacturers have
designed special purpose devices that provide reason-
ably low “on” resistance with significantly reduced input
capacitance for the main switch application in switching
regulators.
Selection criteria for the power MOSFETs include the “on”
resistance R_DS(ON), input capacitance, breakdown voltage
and maximum output current.
The most important parameter in high voltage applications
is breakdown voltage BV_DSS. Both the top and bottom
MOSFETs will see full input voltage plus any additional
ringing on the switch node across its drain-to-source dur-
ing its off-time and must be chosen with the appropriate
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