LTC3789_15 Datasheet, PDF (22/30 Pages) Linear Technology – High Efficiency, Synchronous, 4-Switch Buck-Boost Controller

LTC3789
APPLICATIONS INFORMATION
Soft-Start Function
When a capacitor is connected to the SS pin, a soft-start
current of 3µA starts to charge the capacitor. A soft-start
function is achieved by controlling the output ramp volt-
age according to the ramp rate on the SS pin. Current
foldback is disabled during this phase to ensure smooth
soft-start. When the chip is in the shutdown state with its
RUN pin voltage below 1.22V, the SS pin is actively pulled
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:
t_SOFTSTART
=
0.8
•
C_SS
3µA
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:
I_{L(M AX ,BOOST )}
=
140mV
R_SENSE
In the buck region, maximum sense voltage and the sense
resistance determine the maximum allowed inductor valley
current, which is:
I_{L(MAX,BUCK )}
=
90mV
R_SENSE
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
main sources account for most of the losses in LTC3789
circuits:
1. DC I²R losses. These arise from the resistances of the
MOSFETs, sensing resistor, inductor and PC board traces
and cause the efficiency to drop at high output currents.
2. MOSFET Transition loss. This loss arises from the
brief amount 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.
3. INTV_CC current. This is the sum of the MOSFET driver
and control currents. This loss can be reduced by sup-
plying INTV_CC current through the EXTV_CC pin from a
high efficiency source, such as the output (if 4.7V <
V_OUT < 14V) or alternate supply if available.
4. C_IN and C_OUT loss. The input capacitor has the difficult
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_IN and C_OUT are required to have
low ESR to minimize the AC I²R loss and sufficient
capacitance to prevent the RMS current from causing
additional upstream losses in fuses or batteries.
5. Other losses. Schottky diodes D1 and D2 are responsible
for conduction losses during dead time and light load
conduction periods. Inductor core loss should also be
considered. Switch C causes reverse recovery current
loss in boost mode.
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.
3789fc
22
For more information www.linear.com/LTC3789