LTC3783 Datasheet, PDF (12/24 Pages) Linear Technology – PWM LED Driver and Boost, Flyback and SEPIC Controller

LTC3783
U
OPERATIO
maximum total gate charge of 35nC (the temperature
coefficient of the gate charge is low).
I_Q(TOT) = 1.2mA + 35nC • 300kHz = 12mA
P_IC = 12V • 12mA = 144mW
T_J = 70°C + 110°C/W • 144mW = 86°C
This demonstrates how significant the gate charge current
can be when compared to the static quiescent current in
the IC.
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked when
operating in a continuous mode at high V_IN. A tradeoff
between the operating frequency and the size of the power
MOSFET may need to be made in order to maintain a
reliable IC junction temperature. Prior to lowering the
operating frequency, however, be sure to check with the
power MOSFET manufacturers for the latest low Q_G, low
R_DS(ON) devices. Power MOSFET manufacturing tech-
nologies are continually improving, with newer and better-
performing devices being introduced almost monthly.
Output Voltage Programming
In constant voltage mode, in order to regulate the output
voltage, the output voltage is set by a resistor divider
according to the following formula:
V_OUT
=
V_FBP
•
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
where 0 ≤ V_FBP ≤ 1.23V. The external resistor divider is
connected to the output as shown in Figure 4, allowing
remote voltage sensing. The resistors R1 and R2 are
typically chosen so that the error caused by the 500nA
input bias current flowing out of the FBN pin during
normal operation is less than 1%, which translates to a
maximum R1 value of about 25k at V_FBP = 1.23V. For
lower FBP voltages, R1 must be reduced accordingly to
maintain accuracy, e.g., R1 < 2k for 1% accuracy when
V_FBP = 100mV. More accuracy can be achieved with lower
resistances, at the expense of increased dissipation and
decreased light load efficiency.
A similar analysis applies to the V_FBP resistive divider, if
one is used:
V_FBP
=
V_REF
•
R3
R3 + R4
where R3 is subject to a similar 500nA bias current.
R4
R3
LTC3783
RUN
V_IN
PWMIN OV/FB
I_TH PWMOUT
SS
I_LIM
V_REF GATE
FBP SENSE
FBN INTV_CC
FREQ GND
SYNC
V_IN
3V TO 36V
V_OUT
R2
R1
GND
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Figure 4. LTC3783 Boost Application
Programming Turn-On and Turn-Off Thresholds
with the RUN Pin
The LTC3783 contains an independent, micropower volt-
age reference and comparator detection circuit that re-
mains active even when the device is shut down, as shown
in Figure 5. This allows users to accurately program an
input voltage at which the converter will turn on and off.
The falling threshold on the RUN pin is equal to the internal
reference voltage of 1.248V. The comparator has 100mV
of hysteresis to increase noise immunity.
The turn-on and turn-off input voltage thresholds are
programed using a resistor divider according to the fol-
lowing formulas:
V_IN(OFF) = 1.248V • ^{⎛⎝⎜}1+ ^{RR21⎞⎠⎟}
V_IN(ON) = 1.348V • ^{⎛⎝⎜}1+ ^{RR21⎞⎠⎟}
The resistor R1 is typically chosen to be less than 1M.
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