16
LTC3778
3778f
Overcurrent latchoff operation is not always needed or
desired. Load current is already limited during a short-
circuit by the current foldback circuitry and latchoff
operation can prove annoying during troubleshooting.
The feature can be overridden by adding a pull-up current
greater than 5µA to the RUN/SS pin. The additional
current prevents the discharge of C
SS
during a fault and
also shortens the soft-start period. Using a resistor to V
IN
as shown in Figure 6a is simple, but slightly increases
shutdown current. Connecting a resistor to INTV
CC
as
shown in Figure 6b eliminates the additional shutdown
current, but requires a diode to isolate C
SS
. Any pull-up
network must be able to maintain RUN/SS above the 4V
maximum latch-off threshold and overcome the 4µA
maximum discharge current.
with the resistances of L and the board traces to obtain the
DC I
2
R loss. For example, if R
DS(ON)
= 0.01Ω and
R
L
= 0.005Ω, the loss will range from 15mW to 1.5W
as the output current varies from 1A to 10A for a 1.5V
output.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region
during switch node transitions. It depends upon the input
voltage, load current, driver strength and MOSFET capaci-
tance, among other factors. The loss is significant at input
voltages above 20V and can be estimated from:
Transition Loss ≅ (1.7A
–1
) V
IN
2
I
OUT
C
RSS
f
3. INTV
CC
current. This is the sum of the MOSFET driver
and control currents. This loss can be reduced by supply-
ing INTV
CC
current through the EXTV
CC
pin from a high
efficiency source, such as an output derived boost net-
work or alternate supply if available.
4. C
IN
loss. The input capacitor has the difficult job of
filtering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
2
R loss and
sufficient capacitance to prevent the RMS current from
causing additional upstream losses in fuses or batteries.
Other losses, including C
OUT
ESR loss, Schottky diode D1
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss.
When making adjustments to improve efficiency, the
input current is the best indicator of changes in efficiency.
If you make 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.
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V
OUT
immediately shifts by an amount
equal to ∆I
LOAD
(ESR), where ESR is the effective series
resistance of C
OUT
. ∆I
LOAD
also begins to charge or
discharge C
OUT
generating a feedback error signal used by
the regulator to return V
OUT
to its steady-state value.
APPLICATIO S I FOR ATIO
WUUU
Figure 6. RUN/SS Pin Interfacing with Latchoff Defeated
3.3V OR 5V RUN/SS
V
IN
INTV
CC
RUN/SS
D1
(6a) (6b)
D2*
C
SS
R
SS
*
C
SS
*OPTIONAL TO OVERRIDE OVERCURRENT LATCHOFF
R
SS
*
3778 F06
2N7002
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 the circuit produce losses, four main sources
account for most of the losses in LTC3778 circuits:
1. DC I
2
R losses. These arise from the resistances of the
sense resistor, MOSFETs, inductor and PC board traces
and cause the efficiency to drop at high output currents. In
continuous mode the average output current flows through
L, but is chopped between the top and bottom MOSFETs.
If the two MOSFETs have approximately the same R
DS(ON)
,
then the resistance of one MOSFET can simply be summed
