LTC3704 datasheet(8/28 Pages) LINER

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LTC3704

sn3704 3704fs

Main Control Loop

The LTC3704 is a constant frequency, current mode

controller for DC/DC positive-to-negative converter appli-

cations. The LTC3704 is distinguished from conventional

current mode controllers because the current control loop

can be closed by sensing the voltage drop across the

power MOSFET switch instead of across a discrete sense

resistor, as shown in Figure 2. This sensing technique

improves efficiency, increases power density, and re-

duces the cost of the overall solution.

OPERATIO

The nominal operating frequency of the LTC3704 is pro-

grammed using a resistor from the FREQ pin to ground

and can be controlled over a 50kHz to 1000kHz range. In

addition, the internal oscillator can be synchronized to an

external clock applied to the MODE/SYNC pin and can be

locked to a frequency between 100% and 130% of its

nominal value. When the MODE/SYNC pin is left open, it is

pulled low by an internal 50k resistor and Burst Mode

operation is enabled. If this pin is taken above 2V or an

external clock is applied, Burst Mode operation is disabled

and the IC operates in continuous mode. With no load (or

an extremely light load), the controller will skip pulses in

order to maintain regulation and prevent excessive output

ripple.

The RUN pin controls whether the IC is enabled or is in a

low current shutdown state. A micropower 1.248V refer-

ence and comparator C2 allow the user to program the

supply voltage at which the IC turns on and off (compara-

tor C2 has 100mV of hysteresis for noise immunity). With

the RUN pin below 1.248V, the chip is off and the input

supply current is typically only 10

µA.

The LTC3704 can be used either by sensing the voltage

drop across the power MOSFET or by connecting the

SENSE pin to a conventional shunt resistor in the source

of the power MOSFET, as shown in Figure 2. Sensing the

voltage across the power MOSFET maximizes converter

efficiency and minimizes the component count, but limits

the output voltage to the maximum rating for this pin

(36V). By connecting the SENSE pin to a resistor in the

source of the power MOSFET, the user is able to program

output voltages significantly greater than the 36V maxi-

mum input voltage rating for the IC.

Programming the Operating Mode

For applications where maximizing the efficiency at very

light loads (e.g., <100

µA) is a high priority, Burst Mode

operation should be applied (i.e., the MODE/SYNC pin

should be connected to ground). In applications where

fixed frequency operation is more critical than low cur-

rent efficiency, or where the lowest output ripple is

desired, pulse-skip mode operation should be used and

the MODE/SYNC pin should be connected to the INTVCC

pin. This allows discontinuous conduction mode (DCM)

operation down to near the limit defined by the chip’s

2a. SENSE Pin Connection for

Maximum Efficiency (VSW < 36V)

VSW

VIN

GND

RSENSE

3704 F02

2b. SENSE Pin Connection for Precise

Control of Peak IIN/IOUT or for VSW > 36V

VSW

VIN

GND

GATE

GND

VIN

SENSE

GATE

GND

VIN

SENSE

Figure 2. Using the SENSE Pin On the LTC3704

For circuit operation, please refer to the Block Diagram of

the IC and Figure 1. In normal operation, the power

MOSFET is turned on when the oscillator sets the PWM

latch and is turned off when the current comparator C1

resets the latch. The divided-down output voltage is com-

pared to an internal 1.230V reference by the error amplifier

EA, which outputs an error signal at the ITH pin. The voltage

on the ITH pin sets the current comparator C1 input

threshold. When the load current increases, a fall in the

NFB voltage relative to the reference voltage causes the ITH

pin to rise, which causes the current comparator C1 to trip

at a higher peak inductor current value. The average

inductor current will therefore rise until it equals the load

current, thereby maintaining output regulation.

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