LTC3611EWP LINER [Linear Technology], LTC3611EWP Datasheet - Page 16

• Manufacturer Part Number: LTC3611EWP
• Description: 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

LTC3611
APPLICATIONS INFORMATION
0.9V. As the RUN/SS voltage rises to 3V, the clamp on I
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
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
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.
The overcurrent protection timer requires that the soft-start
timing capacitor C
that the output is in regulation by the time C
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:
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
the soft-start period. Using a resistor to V
in Figure 5a is simple, but slightly increases shutdown
current. Connecting a resistor to INTV
Figure 5b eliminates the additional shutdown current,
but requires a diode to isolate C
must be able to pull RUN/SS above the 4.2V maximum
threshold of the latchoff circuit and overcome the 4μA
maximum discharge current.
16
C
SS
> C
OUT
V
OUT
SS
SS
R
SS
be made large enough to guarantee
SENSE
during a fault and also shortens
. If the fault condition persists until
(10
–4
SS
[F/V s])
. Any pull-up network
CC
SS
SS
as shown in
IN
is used as a
has reached
as shown
TH
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 LTC3611 circuits:
1. DC I
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 bot-
tom MOSFETs. If the two MOSFETs have approximately
the same R
can simply be determined by [R
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
capacitance, among other factors. The loss is significant
at input voltages above 20V and can be estimated from:
3. INTV
and control currents. This loss can be reduced by sup-
plying INTV
high efficiency source, such as an output derived boost
network or alternate supply if available.
3.3V OR 5V
Transition Loss ≅ (1.7A
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated
2
CC
R losses. These arise from the resistance of the
current. This is the sum of the MOSFET driver
D1
DS(ON)
CC
(5a)
V
current through the EXTV
IN
R
, then the DC I
SS
*
RUN/SS
C
SS
–1
) V
IN
2
DS(ON)
2
R loss for one MOSFET
*OPTIONAL TO OVERRIDE
I
OUT
OVERCURRENT LATCHOFF
INTV
+ R
2N7002
C
CC
R
D2*
RSS
SS
(5b)
*
L
CC
] • I
RUN/SS
f
pin from a
O
.
C
3611 F05
SS
3611fb