Datasheet

LTC3613
19
3613fa
Top MOSFET Driver Supply (C
B
, D
B
)
An external bootstrap capacitor, C
B
, connected to the BOOST
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode D
B
from INTV
CC
when the switch node is low. When the top MOSFET turns
on, the switch node rises to V
IN
and the BOOST pin rises to
approximately PV
IN
+ INTV
CC
. The boost capacitor needs to
store approximately 100 times the gate charge required by
the top MOSFET. In most applications a 0.1F to 0.47F, X5R
or X7R dielectric capacitor is adequate. It is recommended
that the BOOST capacitor be no larger than 10% of the
INTV
CC
capacitor, C
VCC
, to ensure that the C
VCC
can supply
the upper MOSFET gate charge and BOOST capacitor under
all operating conditions. Variable frequency in response
to load steps offers superior transient performance but
requires higher instantaneous gate drive. Gate charge
demands are greatest in high frequency low duty factor
applications under high dI/dt load steps and at start-up.
In order to minimize SW node ringing and EMI, connect a
5 to 10 resistor in series with the BOOST pin. Make the
C
B
and D
B
connections on the other side of the resistor. This
series resistor helps to slow down the SW node rise time,
limiting the high dI/dt current through the top MOSFET that
causes SW node ringing.
INTV
CC
Regulator and EXTV
CC
Power
The LTC3613 features a PMOS low dropout linear regulator
(LDO) that supplies power to INTV
CC
from the SV
IN
supply.
INTV
CC
powers much of the LTC3613’s internal circuitry.
The LDO regulates the voltage at the INTV
CC
pin to 5.3V.
The LDO can supply a maximum current of 50mA
RMS
and
must be bypassed to ground with a minimum of 4.7F
ceramic capacitor. Good bypassing is needed to supply
the high transient currents required by the power MOSFET
gate drivers.
APPLICATIONS INFORMATION
When the voltage applied to EXTV
CC
pin rises above 4.6V,
the INTV
CC
LDO is turned off and the EXTV
CC
is connected
to INTV
CC
with an internal switch. This switch remains on
as long as the voltage applied to EXTV
CC
remains above
4.4V. Using the EXTV
CC
allows the MOSFET driver and
control power to be derived from the LTC3613’s switching
regulator output during normal operation and from the
LDO when the output is out of regulation (e.g., start-up,
short circuit). If more than 50mA
RMS
current is required
through EXTV
CC
, then an external Schottky diode can be
added between the EXTV
CC
and INTV
CC
pins. Do not apply
more than 6V to the EXTV
CC
pin and make sure that this
external voltage source is less than SV
IN
.
Significant efficiency and thermal gains can be realized
by powering INTV
CC
from the switching regulator output,
since the V
IN
current resulting from the driver and control
currents will be scaled by a factor of (Duty Cycle)/(Switcher
Efficiency).
The following list summarizes the four possible connec-
tions for EXTV
CC
:
1. EXTV
CC
left open (or grounded). This will cause INTV
CC
to be powered from the internal 5.3V LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTV
CC
connected directly to switching regulator output
V
OUT
> 4.6V. This provides the highest efficiency.
3. EXTV
CC
connected to an external supply. If a 4.6V or
greater external supply is available, it may be used to
power EXTV
CC
provided that the external supply is suf-
ficient enough for MOSFET gate drive requirements.
4. EXTV
CC
connected to an output-derived boost network.
For 3.3V and other low voltage converters, efficiency
gains can still be realized by connecting EXTV
CC
to an
output-derived voltage that has been boosted to greater
than 4.6V.