LTC3703-5_15 Datasheet, PDF (13/32 Pages) Linear Technology – 60V Synchronous Switching Regulator Controller

LTC3703-5
APPLICATIO S I FOR ATIO
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 7, 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
t_MIN = 25ns
2V TO 10V
0.8T T T = 1/f_O
TG
D = 40%
0.1T
I_L
37035 F07
Figure 7. MODE/SYNC Clock Input and Switching
Waveforms for Synchronous Operation
Inductor
The inductor in a typical LTC3703-5 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, how-
ever, 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 fre-
quency begins to decrease when the output current drops
below the average inductor current at which the LTC3703-5
is operating at its t_ON(MIN) in discontinuous mode (see
Figure 5). 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 LTC3703-5 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 capaci-
tance MOSFET should be used for the top MOSFET in
applications 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 impor-
tant for overall efficiency than its input capacitance at
operating frequencies above 300kHz. MOSFET manufac-
turers have designed special purpose devices that provide
reasonably low “on” resistance with significantly reduced
input capacitance for the main switch application in switch-
ing 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 applica-
tions 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 during its off-time and must be chosen with the
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