LTC3829_15 Datasheet, PDF (28/40 Pages) Linear Technology – 3-Phase, Single Output Synchronous Step-Down DC/DC Controller with Diffamp

LTC3829
APPLICATIONS INFORMATION
will be affected as well. For better output regulation, use
the coincident tracking mode instead of ratiometric.
INTV_CC (LDO) and EXTV_CC
The LTC3829 features a true PMOS LDO that supplies power
to INTV_CC from the V_IN supply. INTV_CC powers the gate
drivers and much of the LTC3829’s internal circuitry. The
LDO regulates the voltage at the INTV_CC pin to 5V when V_IN
is greater than 5.5V. EXTV_CC connects to INTV_CC through
a P-channel MOSFET and can supply the needed power
when its voltage is higher than 4.7V. Each of these can
supply a peak current of 100mA and must be bypassed
to ground with a minimum of 4.7µF ceramic capacitor or
low ESR electrolytic capacitor. No matter what type of bulk
capacitor is used, an additional 0.1µF ceramic capacitor
placed directly adjacent to the INTV_CC and PGND pins is
highly recommended. Good bypassing is needed to supply
the high transient currents required by the MOSFET gate
drivers and to prevent interaction between the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3829 to be
exceeded. The INTV_CC current, which is dominated by the
gate charge current, may be supplied by either the 5V LDO
or EXTV_CC. When the voltage on the EXTV_CC pin is less
than 4.7V, the LDO is enabled. 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
as discussed in the Efficiency Considerations section.
The junction temperature can be estimated by using the
equations given in Note 3 of the Electrical Characteristics
tables. For example, the LTC3829 INTV_CC current is limited
to less than 42mA from a 38V supply in the UHF package
and not using the EXTV_CC supply:
T_J = 70°C + (42mA)(38V)(34°C/W) = 125°C
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current must be checked
while operating in continuous conduction mode (MODE
= SGND) at maximum V_IN. When the voltage applied to
EXTV_CC rises above 4.7V, the INTV_CC LDO is turned off
and the EXTV_CC is connected to the INTV_CC. The EXTV_CC
remains on as long as the voltage applied to EXTV_CC remains
above 4.5V. Using the EXTV_CC allows the MOSFET driver
and control power to be derived from one of switching
regulator outputs during normal operation and from the
INTV_CC when the output is out of regulation (e.g., start-
up, short circuit). If more current is required through the
EXTV_CC than is specified, 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
EXTV_CC < V_IN.
Significant efficiency and thermal gains can be realized
by powering INTV_CC from the output, since the V_IN cur-
rent resulting from the driver and control currents will be
scaled by a factor of (duty cycle)/(switcher efficiency).
Tying the EXTV_CC pin to a 5V supply reduces the junction
temperature in the previous example from 125°C to:
T_J = 70°C + (42mA)(5V)(34°C/W) = 77°C
However, for low voltage outputs, additional circuitry is
required to derive INTV_CC power from the output.
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 5V LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTV_CC connected directly to V_OUT . This is the normal
connection for a 5V regulator and provides the highest
efficiency.
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