LTC3609 Datasheet, PDF (16/24 Pages) Linear Technology – 32V, 6A Monolithic Synchronous Step-Down DC/DC Converter

LTC3609
APPLICATIONS INFORMATION
When the voltage on RUN/SS reaches 1.5V, the LTC3609
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.
The 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.
3.3V OR 5V
D1
V_IN
RUN/SS
R_SS*
C_SS
(5a)
INTV_CC
R_SS*
D2* ^RUN/SS
2N7002
C_SS
3609 F05
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
(5b)
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated
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 LTC3609 circuits:
1. DC I²R 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 I²R 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 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:
Transition Loss ≅ (1.7A^–1) V_IN2 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 network
or alternate supply if available.
3609f
16