LTC3863_15 Datasheet, PDF (15/38 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller

LTC3863
Applications Information
For an inverting buck-boost converter operating in con-
tinuous conduction mode (CCM), given the desired input,
output voltages and switching frequency, the peak-to-peak
inductor ripple current is determined by the inductor value:
( ^{( )} ) ^{∆IL(CCM)}
=
V_IN •D
L•f
=
L
•
V_IN
f•
• | V_OUT | +V_D
V_IN + | V_OUT | +V_D
where V_D is the diode forward conduction voltage. In cases
where V_OUT >> V_D, V_D can be ignored. D is the duty factor
and is given as:
D=
| V_OUT | +V_D
V_IN + | V_OUT | +V_D
(0 <D < 1)
The duty factor increases with increasing V_OUT and de-
creasing V_IN. For a given V_OUT, the maximum duty factor
occurs at minimum V_IN.
A typical starting point for selecting an inductor is to choose
the inductance such that the maximum peak-to-peak in-
ductor ripple current, ∆I_L(MAX), is set to 40% ~ 50% of the
inductor average current, I_L(AVG), at maximum load current.
Since ∆I_L(MAX) occurs at maximum V_IN in continuous mode,
the inductance is calculated at maximum V_IN:
( ) _{L =}
V_IN(MAX)2 • | V_OUT | +V_D
( ) ^{0.4 •I}_OUT(MAX) ^{• f • V}IN(MAX)^{+ | V}_OUT ^{| +V}D
2
The inductance can be further adjusted to achieve specific
design optimization of efficiency, output ripple, component
size and loop response.
Once the inductance value has been determined, the type
of inductor must be selected. Core loss is independent of
core size for a given inductor value, but it is very depen-
dent on the inductance selected. As inductance increases,
core losses decrease. Unfortunately, increased inductance
requires more turns of wire and therefore, copper losses
will increase.
High efficiency converters generally cannot tolerate the
core loss of low cost powdered iron cores, forcing the use
of more expensive ferrite materials. Ferrite designs have
very low core loss and are preferred at high switching
frequencies, so design goals can concentrate on cop-
per loss and preventing saturation. Ferrite core material
saturates hard, which means that inductance collapses
abruptly when the peak design current is exceeded. This
will result in an abrupt increase in inductor ripple current
and output voltage ripple. Do not allow the core to saturate!
A variety of inductors are available from manufacturers
such as Sumida, Panasonic, Coiltronics, Coilcraft, Toko,
Vishay, Pulse and Würth.
Current Sensing and Current Limit Programming
The LTC3863 senses the inductor current through a cur-
rent sense resistor, R_SENSE, placed across the V_IN and
SENSE pins. The voltage across the resistor, V_SENSE, is
proportional to inductor current and in normal operation is
compared to the peak inductor current setpoint. An inductor
current limit condition is detected when V_SENSE exceeds
95mV. When the current limit threshold is exceeded, the
P-channel MOSFET is immediately turned off by pulling
the GATE voltage to V_IN regardless of the controller input.
The peak inductor current limit is equal to:
I_{L(PEAK )}
≅


95mV
R_SENSE


This inductor current limit would translate to an output
current limit based on the inductor ripple and duty factor:
I_{OUT(LIM IT )}

= _
95mV
R_SENSE
–
∆I_L
2


• (1– D)
For more information www.linear.com/3863
3863fa
15