LTC3783 Datasheet, PDF (11/24 Pages) Linear Technology – PWM LED Driver and Boost, Flyback and SEPIC Controller

U
OPERATIO
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 drop-
out 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
supply, 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.
LTC3783
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 tem-
perature rating. The junction temperature can be esti-
mated 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 a
–
1.230V
+
R2
V_IN
P-CH
R1
7V INTVCC
LOGIC
DRIVER
GATE
INPUT
SUPPLY
6V TO 36V
C_IN
C_VCC
4.7µF
X5R
6V-RATED
M1
POWER
MOSFET
GND
3783 F03
GND
PLACE AS CLOSE AS
POSSIBLE TO DEVICE PINS
Figure 3. Bypassing the LDO Regulator and Gate Driver Supply
3783f
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