Datasheet

LT3958
10
3958f
APPLICATIONS INFORMATION
INTV
CC
Regulator Bypassing and Operation
An internal, low dropout (LDO) voltage regulator produces
the 7.2V INTV
CC
supply which powers the gate driver, as
shown in Figure 1. The LT3958 contains an undervoltage
lockout comparator A8 and an overvoltage lockout com-
parator A9 for the INTV
CC
supply. The INTV
CC
undervoltage
(UV) threshold is 3.75V (typical), with 0.15V hysteresis,
to ensure that the internal MOSFET has suffi cient gate
drive voltage before turning on. The logic circuitry within
the LT3958 is also powered from the internal INTV
CC
supply.
The INTV
CC
overvoltage threshold is set to be 12.8V
(typical) to protect the gate of the power MOSFET. When
INTV
CC
is below the UV threshold, or above the overvoltage
threshold, the internal power switch will be turned off and
the soft-start operation will be triggered.
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.
If the input voltage V
IN
does not exceed the INTV
CC
overvoltage lockout threshold voltage (12.8V), the INTV
CC
pin can be shorted directly to the V
IN
pin. In this condi-
tion, the internal LDO will be turned off and the gate driver
will be powered directly from the input voltage V
IN
. With
the INTV
CC
pin shorted to V
IN
, however, a small current
(around 16µA) will load the INTV
CC
in shutdown mode. For
applications that require the lowest shutdown mode input
supply current, do not connect the INTV
CC
pin to V
IN
.
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
< 12.8V (typical)
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.
Figure 2. Connecting INTV
CC
to V
OUT
C
VCC
4.7µF
V
OUT
3958 F02
INTV
CC
SGND
LT3958
R
VCC
D
VCC