LTC3854_15 Datasheet, PDF (13/28 Pages) Linear Technology – Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller

LTC3854
APPLICATIONS INFORMATION
An optional Schottky diode connected from GND (anode)
to the SW node (cathode) conducts during the dead time
between the conduction of the two power MOSFETs. This
prevents the body diode of the bottom MOSFET from turn-
ing on, storing charge during the dead time and requiring
a reverse recovery period that could cost as much as 3%
in efficiency at high V_IN. A 1A to 3A Schottky is generally
a good size due to the relatively small average current.
Larger diodes result in additional transition losses due to
their larger junction capacitance.
Soft-Start
When the LTC3854 is configured to soft-start by itself,
a capacitor must be connected to the RUN/SS pin. The
LTC3854 is in the shutdown state if the RUN/SS pin volt-
age is below 1.2V. The RUN/SS pin has an internal 1.25µA
pull-up current and should be externally pulled low (<0.4V)
to keep IC in shutdown mode.
Once the RUN/SS pin voltage reaches 1.2V, the LTC3854
is enabled. As the RUN/SS pin moves from 1.2V to 2V the
LTC3854 operates in a forced discontinuous mode with
the bottom gate turning on only one time for every four
clock cycles to allow the output to come up to its required
value. During this time the error amp compares the FB
pin to a level shifted version of the RUN/SS pin allowing
the output to come up in a controlled fashion. Current
foldback is disabled during this phase to ensure smooth
soft-start or tracking. Once the RUN/SS pin is greater
than 2V the LTC3854 operates in forced continuous mode.
The LTC3854 output voltage is soft-start controlled when
RUN/SS is between 1.2V and 2V. The total soft-start time
can be calculated as:
t_SOFT-START
=
0.8
•
C_SS
1.25µA
If the RUN/SS pin is externally driven beyond 2V (5V
is recommended) the soft-start feature is disabled and
the LTC3854 will immediately go into forced continuous
mode. Care must be taken to insure the RUN/SS pin has
either a capacitor tied to it or is driven externally. Do not
let this pin float.
INTV_CC Regulator
The LTC3854 features a PMOS low dropout linear regulator
(LDO) that supplies power to INTV_CC from the V_IN supply.
INTV_CC powers the gate drivers and much of the LTC3854’s
internal circuitry. The LDO regulates the voltage at the
INTV_CC pin to 5V when V_IN is greater than 6V.
The LDO supplies a peak current of 40mA and must be
bypassed to ground with a minimum of 2.2μF low ESR
ceramic capacitor. Good bypassing is needed to supply
the high transient currents required by the MOSFET gate-
drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3854 to be
exceeded. The INTV_CC current, which is dominated by the
gate-charge current, is supplied by the 5V LDO.
Power dissipation for the IC in this case is highest and is
equal to V_IN•I_INTVCC. The gate-charge current is dependent
on operating frequency (400kHz), and the Q_G of the power
MOSFETs, as discussed in the Efficiency Considerations
section. The junction temperature can be estimated by
using the equations given in Note 3 of the Electrical Char-
acteristics section. For example, if the LTC3854 INTV_CC
current is limited to less than 17mA from a 36V supply in
the DFN package; then the junction temperature is:
T_J = 70°C + [(17mA•36V)•(76°C/W)] = 116.5°C
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current must be checked
during operation at maximum V_IN.
Topside MOSFET Driver Supply (C_B, DB)
An external bootstrap capacitor C_B connected from the
BOOST pin to the SW pin and supplies the gate drive volt-
age for the topside MOSFET. Capacitor C_B in the Functional
Diagram is charged though external diode DB from INTV_CC
when the SW pin is low. When the topside MOSFET is to
be turned on, the driver places the C_B voltage across the
gate source of the MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node volt-
age, SW, rises to V_IN and the BOOST pin follows. With the
topside MOSFET on, the boost voltage is above the input
supply: V_BOOST = V_IN + V_INTVCC. The value of the boost
3854fb
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