Datasheet

LTC3859A
27
3859af
applicaTions inForMaTion
INTV
CC
Regulators
The LTC3859A 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
LTC3859A’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 LTC3859A 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 dis-
sipation 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 LTC3859A
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 LTC3859A’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
than 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, addi-
tional circuitry is required to derive INTV
CC
power from
the output.