LTC3899_15 Datasheet, PDF (23/38 Pages) Linear Technology – 60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3899
Applications Information
Both MOSFETs have I²R losses while the main N-channel
equations for the buck and boost controllers include an
additional term for transition losses, which are highest at
high input voltages for the bucks and low input voltages
for the boost. For V_IN < 20V (higher V_IN for the boost)
the high current efficiency generally improves with larger
MOSFETs, while for V_IN > 20V (lower V_IN for the boost)
the transition losses rapidly increase to the point that the
use of a higher R_DS(ON) device with lower C_MILLER actu-
ally provides higher efficiency. The synchronous MOSFET
losses for the buck controllers are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R_DS(ON) vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
Optional Schottky diodes placed across the synchronous
MOSFET conduct during the dead-time between the con-
duction of the two power MOSFETs. This prevents the
body diode of the synchronous MOSFET from turning
on, storing charge during the dead-time and requiring a
reverse recovery period that could cost as much as 3%
in efficiency at high V_IN. A 1A to 3A Schottky is generally
a good compromise for both regions of operation due to
the relatively small average current. Larger diodes result
in additional transition losses due to their larger junction
capacitance.
Boost C_IN, C_OUT Selection
The input ripple current in a boost converter is relatively
low (compared with the output ripple current), because
this current is continuous. The boost input capacitor C_IN
voltage rating should comfortably exceed the maximum
input voltage. Although ceramic capacitors can be relatively
tolerant of overvoltage conditions, aluminum electrolytic
capacitors are not. Be sure to characterize the input voltage
for any possible overvoltage transients that could apply
excess stress to the input capacitors.
The value of C_IN is a function of the source impedance, and
in general, the higher the source impedance, the higher the
required input capacitance. The required amount of input
capacitance is also greatly affected by the duty cycle. High
output current applications that also experience high duty
cycles can place great demands on the input supply, both
in terms of DC current and ripple current.
In a boost converter, the output has a discontinuous current,
so C_OUT must be capable of reducing the output voltage
ripple. The effects of ESR (equivalent series resistance)
and the bulk capacitance must be considered when choos-
ing the right capacitor for a given output ripple voltage.
The steady ripple due to charging and discharging the
bulk capacitance is given by:
( ) _Ripple
=
I_OUT(MAX) •
C_OUT
V_OUT −
• V_OUT
V_IN(MIN)
•f
V
where C_OUT is the output filter capacitor.
The steady ripple due to the voltage drop across the ESR
is given by:
∆V_ESR = I_L(MAX) • ESR
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic
and ceramic capacitors are all available in surface mount
packages. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient.
Capacitors are now available with low ESR and high ripple
current ratings such as OS-CON and POSCAP.
Buck C_IN, C_OUT Selection
The selection of C_IN for the two buck controllers is simplified
by the 2-phase architecture and its impact on the worst-
case RMS current drawn through the input network (bat-
tery/fuse/capacitor). It can be shown that the worst-case
capacitor RMS current occurs when only one controller
is operating. The controller with the highest (V_OUT)(I_OUT)
product needs to be used in the formula shown in Equa-
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