LTC3859AL Datasheet, PDF (27/44 Pages) Linear Technology – Triple Output, Buck/Buck/Boost Synchronous Controller with 28μA Burst Mode IQ

LTC3859AL
applications information
INTV_CC Regulators
The LTC3859AL features two separate internal P-channel
low dropout linear regulators (LDO) that supply power
at the INTV_CC pin from either the V_BIAS supply pin or the
EXTV_CC pin depending on the connection of the EXTV_CC
pin. INTV_CC powers the gate drivers and much of the
LTC3859AL’s internal circuitry. The V_BIAS LDO and the
EXTV_CC LDO regulate INTV_CC to 5.4V. Each of these must
be bypassed to ground with a minimum of 4.7µF ceramic
capacitor. No matter what type of bulk capacitor is used, an
additional 1µF ceramic capacitor placed directly adjacent
to the INTV_CC and PGND IC 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 LTC3859AL to be
exceeded. The INTV_CC current, which is dominated by the
gate charge current, may be supplied by either the V_BIAS
LDO or the EXTV_CC LDO. When the voltage on the EXTV_CC
pin is less than 4.7V, the V_BIAS LDO is enabled. Power
dissipation for the IC in this case is highest and is equal to
V_BIAS • 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 2 of the
Electrical Characteristics. For example, the LTC3859AL
INTV_CC current is limited to less than 40mA from a 40V
supply when not using the EXTV_CC supply at a 70°C ambi-
ent temperature in the QFN package:
T_J = 70°C + (40mA)(40V)(34°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV_CC) at maximum V_IN.
When the voltage applied to EXTV_CC rises above 4.7V, the
V_BIAS LDO is turned off and the EXTV_CC LDO is enabled.
The EXTV_CC LDO remains on as long as the voltage applied
to EXTV_CC remains above 4.5V. The EXTV_CC LDO attempts
to regulate the INTV_CC voltage to 5.4V, so while EXTV_CC
is less than 5.4V, the LDO is in dropout and the INTV_CC
voltage is approximately equal to EXTV_CC. When EXTV_CC
is greater than 5.4V, up to an absolute maximum of 14V,
INTV_CC is regulated to 5.4V.
Using the EXTV_CC LDO allows the MOSFET driver and
control power to be derived from one of the LTC3859AL’s
switching regulator outputs (4.7V ≤ V_OUT ≤ 14V) dur-
ing normal operation and from the V_BIAS LDO when the
output is out of regulation (e.g., startup, short-circuit). If
more current is required through the EXTV_CC LDO than
is specified, an external Schottky diode can be added
between the EXTV_CC and INTV_CC pins. In this case, do
not apply more than 6V to the EXTV_CC pin and make sure
that EXTV_CC ≤ V_BIAS.
Significant efficiency and thermal gains can be realized
by powering INTV_CC from the buck output, since the V_IN
current resulting from the driver and control currents will
be scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
For 5V to 14V regulator outputs, this means connecting
the EXTV_CC pin directly to V_OUT. Tying the EXTV_CC pin to
a 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T_J = 70°C + (40mA)(8.5V)(34°C/W) = 82°C
However, for 3.3V and other low voltage outputs, additional
circuitry is required to derive INTV_CC power from the output.
For more information www.linear.com/3859AL
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