LTC3863_15 Datasheet, PDF (14/38 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller

LTC3863
Applications Information
Switching Frequency and Clock Synchronization
The choice of operating frequency is a trade-off between
efficiency and component size. Lowering the operating fre-
quency improves efficiency by reducing MOSFET switching
losses but requires larger inductance and/or capacitance
to maintain low output ripple voltage. Conversely, raising
the operating frequency degrades efficiency but reduces
component size.
The LTC3863 can free-run at a user programmed switch-
ing frequency, or it can synchronize with an external
clock to run at the clock frequency. When the LTC3863 is
synchronized, the GATE pin will synchronize in phase with
the rising edge of the applied clock in order to turn the
external P-channel MOSFET on. The switching frequency
of the LTC3863 is programmed with the FREQ pin, and the
external clock is applied at the PLLIN/MODE pin. Table 1
highlights the different states in which the FREQ pin can
be used in conjunction with the PLLIN/MODE pin.
Table 1
FREQ PIN
OV
Floating
Resistor to GND
Either of the Above
PLLIN/MODE PIN
DC Voltage
DC Voltage
DC Voltage
External Clock
FREQUENCY
350kHz
535kHz
50kHz to 850kHz
Phase Locked to
External Clock
The free-running switching frequency can be programmed
from 50kHz to 850kHz by connecting a resistor from FREQ
pin to signal ground. The resulting switching frequency
as a function of resistance on the FREQ pin is shown in
Figure 2.
Set the free-running frequency to the desired synchroni-
zation frequency using the FREQ pin so that the internal
oscillator is prebiased approximately to the synchronization
frequency. While it is not required that the free-running
frequency be near the external clock frequency, doing so
will minimize synchronization time.
1000
900
800
700
600
500
400
300
200
100
0
15 25 35 45 55 65 75 85 95 105 115 125
FREQ PIN RESISTOR (kΩ)
3863 F02
Figure 2. Switching Frequency vs Resistor on FREQ pin
Inductor Selection
Operating frequency, inductor selection, capacitor selection
and efficiency are interrelated. Higher operating frequen-
cies allow the use of smaller inductors, smaller capacitors,
but result in lower efficiency because of higher MOSFET
gate charge and transition losses. In addition to this basic
trade-off, the selection of inductor value is also influenced
by other factors.
Small inductor values result in large inductor ripple cur-
rents, large output voltage ripples and low efficiency due
to higher core and conduction loss. Large inductor ripple
currents result in high inductor peak currents, which re-
quire physically large inductors with large magnetic cross
sections and higher saturation current ratings.
The value of the inductor can also impact the stability of
the feedback loop. In continuous mode, the buck-boost
converter transfer function has a right-half plane zero at
a frequency that is inversely proportional to the value of
the inductor. As a result, large inductor values can move
this zero to a frequency that is low enough to degrade the
phase margin of the feedback loop. Large inductor values
also tend to degrade stability due to low noise margin
caused from low ripple current. Additionally, large value
inductors can lead to slow transient response due to slow
inductor current ramping time.
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