LTC3608 LINER [Linear Technology], LTC3608 Datasheet - Page 16

• Manufacturer Part Number: LTC3608
• Description: 18V, 8A Monolithic Synchronous Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3608
When the voltage on RUN/SS reaches 1.5V, the LTC3608
begins operating with a clamp on I
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 fi nal 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 suffi cient.
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
soft-start period. Using a resistor to V
ure 5a is simple, but slightly increases shutdown current.
Connecting a resistor to INTV
eliminates the additional shutdown current, but requires
a diode to isolate C
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
SS
R
SS
during a fault and also shortens the
be made large enough to guarantee
. Any pull-up network must be able
SENSE
. If the fault condition persists until
(10
CC
–4
as shown in Figure 5b
[F/V s])
TH
IN
of approximately
as shown in Fig-
SS
SS
is used as a
has reached
TH
Effi ciency Considerations
The percent effi ciency 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 effi ciency 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 LTC3608 circuits:
1. DC I
2. Transition loss. This loss arises from the brief amount
3. INTV
internal resistance of the MOSFETs, inductor and PC
board traces and cause the effi ciency to drop at high
output currents. In continuous mode the average output
current fl ows through L, but is chopped between the top
and bottom MOSFETs. The DC I
can simply be determined by [R
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 signifi cant at input voltages above 20V and can be
estimated from:
Transition Loss ≅ (1.7A
and control currents. This loss can be reduced by sup-
plying INTV
high effi ciency source, such as an output derived boost
network or alternate supply if available.
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated
3.3V OR 5V
2
CC
R losses. These arise from the resistance of the
current. This is the sum of the MOSFET driver
D1
(5a)
CC
V
current through the EXTV
IN
R
SS
*
RUN/SS
–1
C
SS
) V
IN
2
2
R loss for one MOSFET
I
DS(ON)
OUT
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
INTV
C
RSS
2N7002
CC
R
+ R
D2*
SS
(5b)
CC
*
f
L
RUN/SS
] • I
pin from a
O
3608 F05
C
.
SS
3608fa