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LTC1871 Datasheet(PDF) 11 Page - Linear Technology

Part # LTC1871
Description  PWM LED Driver and Boost, Flyback and SEPIC Controller
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

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11
LTC3783
3783f
INTVCC 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 INTVCC regulator can supply up to 50mA and must be
bypassed to ground immediately adjacent to the IC pins
with a minimum of 4.7
µF 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 INTVCC is 9V), the internal low drop-
out regulator in the LTC3783 is redundant and the INTVCC
pin can be shorted directly to the VIN pin. With the INTVCC
pin shorted to VIN, however, the divider that programs the
regulated INTVCC 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 INTVCC pin to VIN. Regardless of
whether the INTVCC pin is shorted to VIN or not, it is always
necessary to have the driver circuitry bypassed with a
4.7
µF low ESR ceramic capacitor to ground immediately
adjacent to the INTVCC 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 tem-
perature rating. The junction temperature can be esti-
mated using the following equations:
IQ(TOT) = IQ + f • QG
PIC = VIN • (IQ + f • QG)
TJ = TA + PIC • θJA
The total quiescent current IQ(TOT) consists of the static
supply current (IQ) and the current required to charge and
discharge the gate of the power MOSFET. The 16-lead FE
package has a thermal resistance of
θJA = 38°C/W and the
DHD package has an
θJA = 43°C/W
As an example, consider a power supply with VIN = 12V
and VOUT = 25V at IOUT = 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 RDS(ON) of 10mΩ (at room temperature) and a
OPERATIO
1.230V
R2
R1
P-CH
7V
DRIVER
GATE
CVCC
4.7
µF
X5R
CIN
INPUT
SUPPLY
6V TO 36V
GND
PLACE AS CLOSE AS
POSSIBLE TO DEVICE PINS
M1
3783 F03
INTVCC
VIN
GND
LOGIC
6V-RATED
POWER
MOSFET
Figure 3. Bypassing the LDO Regulator and Gate Driver Supply


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