LTC3785 LINER [Linear Technology], LTC3785 Datasheet - Page 21

• Manufacturer Part Number: LTC3785
• Description: High Efficiency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
to ground. The soft-start range is defined to be the voltage
range from 0V to 0.8V on the SS pin. The total soft-start
time can be calculated as:
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator will always start in pulse-skipping mode up
to SS = 0.8V.
Fault Conditions: Current Limit and Current Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage. In
the boost region, maximum sense voltage and the sense
resistance determine the maximum allowed inductor peak
current, which is:
In the buck region, maximum sense voltage and the sense
resistance determine the maximum allowed inductor valley
current, which is:
To further limit current in the event of a short circuit to
ground, the LTC3789 includes foldback current limiting.
If the output falls by more than 50%, then the maximum
sense voltage is progressively lowered to about one-third
of its full value.
Efficiency Considerations
The percent efficiency of a switching regulator is equal
to the output power divided by the input power times
100%. It is often useful to analyze individual losses
to determine what is limiting the efficiency and which
change would produce the most improvement. Although
all dissipative elements in circuit produce losses, four
I
I
t
L(MAX,BOOST)
L(MAX,BUCK)
SOFTSTART
= 0.8 •
=
=
R
90mV
140mV
R
SENSE
SENSE
3µA
C
SS
main sources account for most of the losses in LTC3789
circuits:
1. DC I
2. Transition loss. This loss arises from the brief amount
3. INTV
4. C
5. Other losses. Schottky diodes D1 and D2 are respon-
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If one
makes a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
MOSFETs, sensing resistor, inductor and PC board traces
and cause the efficiency to drop at high output currents.
of time switch A or switch C spends in the saturated
region during switch node transitions. It depends upon
the input voltage, load current, driver strength and
MOSFET capacitance, among other factors. The loss
is significant at input voltages above 20V and can be
estimated from:
where C
and control currents. This loss can be reduced by sup-
plying INTV
high efficiency source, such as the output (if 4.7V <
V
job of filtering the large RMS input current to the regula-
tor in buck mode. The output capacitor has the more
difficult job of filtering the large RMS output current in
boost mode. Both C
low ESR to minimize the AC I
capacitance to prevent the RMS current from causing
additional upstream losses in fuses or batteries.
sible for conduction losses during dead time and light
load conduction periods. Inductor core loss occurs
predominately at light loads. Switch C causes reverse
recovery current loss in boost mode.
OUT
IN
Transition Loss ≈ 1.7A
and C
2
CC
R losses. These arise from the resistances of the
< 14V) or alternate supply if available.
current. This is the sum of the MOSFET driver
RSS
OUT
CC
is the reverse transfer capacitance.
loss. The input capacitor has the difficult
current through the EXTV
IN
and C
–1
• V
OUT
IN
2
R loss and sufficient
2
are required to have
• I
OUT
LTC3789
CC
• C
RSS
pin from a
21
• f
3789f