Datasheet

LT3957
10
3957f
APPLICATIONS INFORMATION
INTV
CC
Regulator Bypassing and Operation
An internal, low dropout (LDO) voltage regulator produces
the 5.2V INTV
CC
supply which powers the gate driver, as
shown in Figure 1. The LT3957 contains an undervoltage
lockout comparator A8 for the INTV
CC
supply. The INTV
CC
undervoltage (UV) threshold is 2.7V (typical), with 0.1V
hysteresis, to ensure that the internal MOSFET has suf-
fi cient gate drive voltage before turning on. When INTV
CC
is below the UV threshold, the internal power switch will
be turned off and the soft-start operation will be triggered.
The logic circuitry within the LT3957 is also powered from
the internal INTV
CC
supply.
The INTV
CC
regulator must be bypassed to SGND imme-
diately adjacent to the IC pins with a minimum of 4.7µF
ceramic capacitor. Good bypassing is necessary to supply
the high transient currents required by the MOSFET gate
driver.
In an actual application, most of the IC supply current is
used to drive the gate capacitance of the internal power
MOSFET. The on-chip power dissipation can be signifi cant
when the internal power MOSFET is being driven at a high
frequency and the V
IN
voltage is high.
An effective approach to reduce the power consumption of
the internal LDO for gate drive and to improve the effi ciency
is to tie the INTV
CC
pin to an external voltage source high
enough to turn off the internal LDO regulator.
In SEPIC or fl yback applications, the INTV
CC
pin can be
connected to the output voltage V
OUT
through a blocking
diode, as shown in Figure 2, if V
OUT
meets the following
conditions:
1. V
OUT
< V
IN
(pin voltage)
2. V
OUT
< 8V
A resistor R
VCC
can be connected, as shown in Figure 2, to
limit the inrush current from V
OUT
. Regardless of whether
or not the INTV
CC
pin is connected to an external voltage
source, it is always necessary to have the driver circuitry
bypassed with a 4.7µF low ESR ceramic capacitor to ground
immediately adjacent to the INTV
CC
and SGND pins.
If LT3957 operates at a low V
IN
and high switching fre-
quency, the voltage drop across the drain and the source of
the LDO PMOS (M2 in Figure 1) could push INTV
CC
to be
below the UV threshold. To prevent this from happening,
the INTV
CC
pin can be shorted directly to the V
IN
pin. V
IN
must not exceed the INTV
CC
Absolute Maximum Rating
(8V). In this condition, the internal LDO will be turned off
and the gate driver will be powered directly from V
IN
. It is
recommended that INTV
CC
pin be shorted to the V
IN
pin if
V
IN
is lower than 3.5V at 1MHz switching frequency, or V
IN
is lower than 3.2V at 100kHz switching frequency. With
the INTV
CC
pin shorted to V
IN
, however, a small current
(around 16µA) will load the INTV
CC
in shutdown mode.
Figure 2. Connecting INTV
CC
to V
OUT
C
VCC
4.7µF
V
OUT
3957 F02
INTV
CC
SGND
LT3957
R
VCC
D
VCC