LTC3863_15 Datasheet, PDF (13/38 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller

LTC3863
Applications Information
The LTC3863 is a nonsynchronous inverting, current mode,
constant frequency PWM controller. It drives an external
P-channel power MOSFET which connects to a Schottky
power diode acting as the commutating catch diode. The
input range extends from 3.5V to 60V. The output range
has no theoretical minimum or maximum, but the duty
factor and external components practically limit the out-
put to one-tenth and ten times the input voltage. Higher
output ratios can be obtained with transformers and more
efficient external components.
The LTC3863 offers a highly efficient Burst Mode operation
with 70µA quiescent current, which delivers outstanding
efficiency in light load operation. The LTC3863 is a low
pin count, robust and easy-to-use inverting power supply
solution in applications which require high efficiency and
operate with widely varying input and output voltages.
The typical application on the front page is a basic LTC3863
application circuit. The LTC3863 can sense the inductor
current through a high side series sense resistor, R_SENSE,
placed between V_IN and the source of the external P-channel
MOSFET. Once the required output voltage and operating
frequency have been determined, external component
selection is driven by load requirements, and begins with
the selection of inductor and R_SENSE. Next, the power
MOSFET and catch diode are selected. Finally, input and
output capacitors are selected.
Output Voltage Programming
The output voltage is programmed by connecting a
feedback resistor divider from the output to the V_FB pin
as shown in Figure 1. The output voltage in steady-state
operation is set by the feedback resistors according to
the equation:
V_OUT
=
–0.8V
•
R_FB1
R_FB2
LTC3863
V_FB
V_FBN
R_FB2
C_FB2
3863 F01
R_FB1
V_OUT
Figure 1. Setting the Output Voltage
Great care should be taken to route the V_FB and V_FBN
lines away from noise sources, such as the inductor or
SW node or the GATE signal that drives the external P-
channel MOSFET.
The integrator capacitor, C_FB2, should be sized to ensure
the negative sense amplifier gain rolls off and limits high
frequency gain peaking in the DC/DC control loop. The
integrator capacitor pole can be safely set to be two times
the switching frequency without affecting the DC/DC phase
margin according to the following equation. It is highly
recommended that C_FB2 be used in most applications.
C_FB2
=
2
•
π
•
2
1
• R_FB2
• FREQ_SW
The feedback resistor R_FB2 placed from the V_FBN pin to
the V_FB pin is driven from V_FB by an internal amplifier with
limited V_FB output current capability. For Operation at V_IN
down to 3.5V, it is recommended to limit the V_FB output
current to 100µA, and R_FB2 should be no smaller than
8K. For V_IN always greater than 6V, it is recommended to
limit the current to 250µA, and R_FB2 should be no smaller
than 3.2K. For minimum V_IN between 3.5V and 6V, add
75µA for each volt V_IN-min is above 3.5V to the maximum
current until reaching a maximum of 250µA at 6V_IN, and
minimum R_FB2 would be 0.8V/lo-max.
For more information www.linear.com/3863
3863fa
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