LTC3703 datasheet(10/32 Pages) LINER

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LTC3703

3703f

OPERATIO

MOSFETs can occur. To prevent this from occuring, the

bottom driver return is brought out as a separate pin

(BGRTN) so that a negative supply can be used to reduce

the effect of the Miller pull-up. For example, if a –2V supply

is used on BGRTN, the switch node dV/dt could pull the

gate up 2V before the VGS of the bottom MOSFET has more

than 0V across it.

cycle control set to 0%. As CSS continues to charge, the

duty cycle is gradually increased, allowing the output

voltage to rise. This soft-start scheme smoothly ramps the

output voltage to its regulated value, with no overshoot.

The RUN/SS voltage will continue ramping until it reaches

an internal 4V clamp. Then the MIN feedback comparator

is enabled and the LTC3703 is in full operation. When the

RUN/SS is low, the supply current is reduced to 50

µA.

CURRENT

LIMIT

NORMAL OPERATION

START-UP

1.4V

POWER

DOWN MODE

MINIMUM

DUTY CYCLE

OUTPUT VOLTAGE

IN REGULATION

LTC3703

ENABLE

MIN COMPARATOR ENABLED

3703 F04

RUN/SS SOFT-STARTS

OUTPUT VOLTAGE AND

INDUCTOR CURRENT

SHUTDOWN

VOUT

VRUN/SS

(Refer to Functional Diagram)

BOOST

BGRTN

DRVCC

LTC3703

VIN

CIN

VOUT

COUT

3703 F03

0V TO –5V

Figure 4. Soft-Start Operation in Start Up and Current Limit

Figure 3. Floating TG Driver Supply and Negative BG Return

Constant Frequency

The internal oscillator can be programmed with an exter-

nal resistor connected from fSET to ground to run between

100kHz and 600kHz, thereby optimizing component size,

efficiency, and noise for the specific application. The

internal oscillator can also be synchronized to an external

clock applied to the MODE/SYNC pin and can lock to a

frequency in the 100kHz to 600kHz range. When locked to

an external clock, Pulse Skip Mode operation is automati-

cally disabled. Constant frequency operation brings with it

a number of benefits: Inductor and capacitor values can be

chosen for a precise operating frequency and the feedback

loop can be similarly tightly specified. Noise generated by

the circuit will always be at known frequencies.

Subharmonic oscillation and slope compensation, com-

mon headaches with constant frequency current mode

switchers, are absent in voltage mode designs like the

LTC3703.

Shutdown/Soft-Start

The main control loop is shut down by pulling RUN/SS pin

low. Releasing RUN/SS allows an internal 4

µA current

source to charge the soft-start capacitor CSS. When CSS

reaches 1V, the main control loop is enabled with the duty

Current Limit

The LTC3703 includes an onboard current limit circuit that

limits the maximum output current to a user-programmed

level. It works by sensing the voltage drop across the

bottom MOSFET and comparing that voltage to a user-

programmed voltage at the IMAX pin. Since the bottom

MOSFET looks like a low value resistor during its on-time,

the voltage drop across it is proportional to the current

flowing in it. In a buck converter, the average current in the

inductor is equal to the output current. This current also

flows through the bottom MOSFET during its on-time.

Thus by watching the drain-to-source voltage when the

bottom MOSFET is on, the LTC3703 can monitor the

output current. The LTC3703 senses this voltage and

inverts it to allow it to compare the sensed voltage (which

becomes more negative as peak current increases) with a

positive voltage at the IMAX pin. The IMAX pin includes a

µA pull-up, enabling the user to set the voltage at IMAX

with a single resistor (RIMAX) to ground. See the Current

Limit Programming section for RIMAX selection.

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