LTC3613 Datasheet, PDF (19/36 Pages) Linear Technology – 24V, 15A Monolithic Step Down Regulator

LTC3613
APPLICATIONS INFORMATION
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 0.1μF to 0.47μF, 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
5Ω to 10Ω 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 4.7μF
ceramic capacitor. Good bypassing is needed to supply
the high transient currents required by the power MOSFET
gate drivers.
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.
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