Datasheet

LTC3610
15
3610ff
3.3V OR 5V RUN/SS
V
IN
INTV
CC
RUN/SS
D1
(5a) (5b)
D2*
C
SS
R
SS
*
C
SS
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
R
SS
*
3610 F05
2N7002
applications inForMation
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated
When the voltage on RUN/SS reaches 1.5V, the LTC3610
begins operating with a clamp on I
TH
of approximately
0.9V. As the RUN/SS voltage rises to 3V, the clamp on I
TH
is raised until its full 2.4V range is available. This takes an
additional 1.3s/µF, during which the load current is folded
back until the output reaches 75% of its final value.
After the controller has been started and given adequate
time to charge up the output capacitor, C
SS
is used as a
short-circuit timer. After the RUN/SS pin charges above 4V,
if the output voltage falls below 75% of its regulated value,
then a short-circuit fault is assumed. A 1.8µA current then
begins discharging C
SS
. If the fault condition persists until
the RUN/SS pin drops to 3.5V, then the controller turns
off both power MOSFETs, shutting down the converter
permanently. The RUN/SS pin must be actively pulled
down to ground in order to restart operation.
T
h
e overcurrent protection timer requires that the soft-start
timing capacitor, C
SS
, be made large enough to guarantee
that the output is in regulation by the time C
SS
has reached
the 4V threshold. In general, this will depend upon the
size of the output capacitance, output voltage and load
current characteristic. A minimum soft-start capacitor
can be estimated from:
C
SS
> C
OUT
V
OUT
R
SENSE
(10
–4
[F/V s])
Generally 0.1µF is more than sufficient.
Overcurrent latchoff operation is not always needed or de-
sired. 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 Fig-
ure 5a is simple, but slightly increases shutdown current.
Connecting a resistor to INTV
CC
as shown in Figure 5b
eliminates the additional shutdown current, but requires
a diode to isolate C
SS
. Any pull-up network must be able
to pull RUN/SS above the 4.2V maximum threshold of the
latchoff circuit and overcome the 4µA maximum discharge
current.
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 LTC3610 circuits:
1.
DC
I
2
R losses. These arise from the resistance of the
internal resistance of the 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. The DC I
2
R loss for one MOSFET
can simply be determined by [R
DS(ON)
+ R
L
] • I
O
.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated re-
gion 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:
T
ransition 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 sup-
plying INTV
CC
current through the EXTV
CC
pin from a
high efficiency source, such as an output derived boost
network or alternate supply if available.