LTC1704B Просмотр технического описания (PDF) - Linear Technology

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LTC1704B

550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller

Linear Technology 

TYPICAL PERFOR A CE CHARACTERISTICS

LTC1704/LTC1704B

Supply Current vs Temperature

6.0

VCC = PVCC = BOOST = 5V

TG, BG FLOAT

IVCC

4.5

IPVCC

3.0

IBOOST

1.5

0

–50 –25

0 25 50 75

TEMPERATURE (°C)

100 125

1704 G26

IPVCC, IBOOST vs Driver Load

35

TA = 25°C

30 PVCC = BOOST = 5V

25

20

15

10

5

0

0 2000 4000 6000 8000 10000

TG, BG LOAD (pF)

1704 G27

PI FU CTIO S

TG (Pin 1): Switcher Controller Top Gate Drive. The TG pin

drives the gate of the top N-channel MOSFET, QT. The TG

driver draws power from the BOOST pin and returns it to

the SW pin, providing true floating drive to QT. TG is de-

signed to typically drive up to 10,000pF of gate capacitance.

SW (Pin 2): Switcher Controller Switching Node. Connect

SW to the switching node of the main converter. The TG

driver ground returns to SW, providing floating gate drive

to the top N-channel MOSFET, QT. The voltage at SW is

compared to IMAX by the current limit comparator while

the bottom MOSFET, QB is on. The Burst comparator

(BURST, see Block Diagram) monitors the potential at SW

and switches to Burst Mode operation under light load

conditions.

IMAX (Pin 3): Switcher Controller Current Limit Set. The

IMAX pin sets the current limit comparator threshold for

the switcher controller. If the voltage drop across the

bottom MOSFET, QB, exceeds the magnitude of the volt-

age at IMAX, the switcher controller enters current limit.

The IMAX pin has an internal 10µA current source pull-up,

allowing the current threshold to be set with a single

external resistor to PGND. Kelvin connect this current

setting resistor to the source of QB. Refer to the Current

Limit Programming section for more information on choos-

ing RIMAX.

RUN/SS (Pin 4): Switcher Controller Soft-Start. A capaci-

tor from RUN/SS to GND controls the turn-on time and

rate of rise of the switcher output voltage at power up. An

internal 3µA current source pull-up at RUN/SS sets the

turn-on time at approximately 300ms/µF. If both RUN/SS

and REGILM are pulled low, the LTC1704 enters shutdown

mode.

COMP (Pin 5): Switcher Controller Loop Compensation.

The COMP pin is connected directly to the output of the

switcher controller’s error amplifier and the input to the

PWM comparator. Use an RC network between the COMP

pin and the FB pin to compensate the feedback loop for

optimum transient response.

FB (Pin 6): Switcher Controller Feedback Input. FB should

be connected through a resistor divider network to VOUTSW

to set the switcher output voltage. Also, connect the

switcher loop compensation network to FB.

REGDR (Pin 7): Linear Regulator Controller Driver Output.

Connect REGDR to the base of the external NPN Pass

transistor. The REGILM pin input current controls the

linear regulator controller maximum driving capability.

GND (Pin 8): Signal Ground. All internal low power cir-

cuitry returns to the GND pin. Connect to a low impedance

ground, separated from the PGND node. All feedback,

1704bfa

7

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