Datasheet

LTC3783
11
3783fb
OPERATION
INTV
CC
Regulator Bypassing and Operation
An internal, P-channel low dropout voltage regulator pro-
duces the 7V supply which powers the gate drivers and
logic circuitry within the LTC3783 as shown in Figure 3.
The INTV
CC
regulator can supply up to 50mA and must be
bypassed to ground immediately adjacent to the IC pins
with a minimum of 4.7µF low ESR or ceramic capacitor.
Good bypassing is necessary to supply the high transient
currents required by the MOSFET gate driver.
For input voltages that don’t exceed 8V (the absolute
maximum rating for INTV
CC
is 9V), the internal low dropout
regulator in the LTC3783 is redundant and the INTV
CC
pin
can be shorted directly to the V
IN
pin. With the INTV
CC
pin shorted to V
IN
, however, the divider that programs the
regulated INTV
CC
voltage will draw 15µA from the input sup-
ply, even in shutdown mode. For applications that require
the lowest shutdown mode input supply current, do not
connect the INTV
CC
pin to V
IN
. Regardless of whether the
INTV
CC
pin is shorted to V
IN
or not, 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 GND pins.
In an actual application, most of the IC supply current is
used to drive the gate capacitance of the power MOSFET.
As a result, high input voltage applications in which a
large power MOSFET is being driven at high frequencies
can cause the LTC3783 to exceed its maximum junction
temperature rating. The junction temperature can be
estimated using the following equations:
I
Q(TOT)
= I
Q
+ f • Q
G
P
IC
= V
IN
• (I
Q
+ f • Q
G
)
T
J
= T
A
+ P
IC
• θ
JA
The total quiescent current I
Q(TOT)
consists of the static
supply current (I
Q
) and the current required to charge and
discharge the gate of the power MOSFET. The 16-lead FE
package has a thermal resistance of
θ
JA
= 38°C/W and
the DHD package has an
θ
JA
= 43°C/W
As an example, consider a power supply with V
IN
= 12V
and V
OUT
= 25V at I
OUT
= 1A. The switching frequency is
300kHz, and the maximum ambient temperature is 70°C.
The power MOSFET chosen is the Si7884DP, which has a
maximum R
DS(ON)
of 10mΩ (at room temperature) and
–
+
1.230V
R2 R1
P-CH
7V
DRIVER
GATE
C
VCC
4.7µF
X5R
C
IN
INPUT
SUPPLY
6V TO 36V
GND
PLACE AS CLOSE AS
POSSIBLE TO DEVICE PINS
M1
3783 F03
INTV
CC
V
IN
GND
LOGIC
6V-RATED
POWER
MOSFET
Figure 3. Bypassing the LDO Regulator and Gate Driver Supply