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LTC3783

3783f

maximum total gate charge of 35nC (the temperature

coefficient of the gate charge is low).

IQ(TOT) = 1.2mA + 35nC • 300kHz = 12mA

PIC = 12V • 12mA = 144mW

TJ = 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 VIN. 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 QG, low

RDS(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:

OUT

FBP

⎛

⎝⎜

⎞

⎠⎟

•1

where 0

≤ VFBP ≤ 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 VFBP = 1.23V. For

lower FBP voltages, R1 must be reduced accordingly to

maintain accuracy, e.g., R1 < 2k for 1% accuracy when

VFBP = 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 VFBP resistive divider, if

one is used:

FBP

REF

•

where R3 is subject to a similar 500nA bias current.

OPERATIO

LTC3783

RUN

PWMIN

ITH

VREF

FBP

FBN

FREQ

SYNC

VIN

OV/FB

PWMOUT

ILIM

GATE

SENSE

INTVCC

GND

VIN

3V TO 36V

GND

VOUT

3783 F04

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:

IN OFF

IN ON

()

.•

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

1 248

1 348

The resistor R1 is typically chosen to be less than 1M.

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