Datasheet

LTC3783
12
3783fb
OPERATION
a 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 reli-
able IC junction temperature. Prior to lowering the operat-
ing 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 technologies 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 ac-
cording 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.
LTC3783
R4
R3
RUN
PWMIN
I
TH
SS
V
REF
FBP
FBN
FREQ
SYNC
V
IN
OV/FB
PWMOUT
I
LIM
GATE
SENSE
INTV
CC
GND
V
IN
3V TO 36V
GND
V
OUT
R2
R1
3783 F04
Figure 4. LTC3783 Boost Application
Programming Turn-On and Turn-Off Thresholds
with the RUN Pin
The LTC3783 contains an independent, micropower voltage
reference and comparator detection circuit that remains
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+
R2
R1






V
IN(ON)
= 1.348V • 1+
R2
R1






The resistor R1 is typically chosen to be less than 1M.