LTC1159CS datasheet(5/20 Pages) LINER

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LTC1159/LTC1159-3.3/LTC1159-5

PI FU CTIO S

Sense+: The (+) Input for the Current Comparator. A built-

in offset between the Sense+ and Sense– pins, in conjunc-

tion with RSENSE, sets the current trip threshold.

N-Gate: High Current Drive for the Bottom N-Channel

MOSFET. The N-Gate pin swings from ground to VCC.

P-Gate: Level-Shifted Gate Drive Signal for the Top

P-Channel MOSFET. The voltage swing at the P-gate pin is

from VIN to VIN – VCC.

P-Drive: High Current Gate Drive for the Top P-Channel

MOSFET. The P-drive pin(s) swing(s) from VCC to ground.

CAP: Charge Compensation Pin. A capacitor to VCC pro-

vides charge required by the P-gate level-shift capacitor

during supply transitions.

The charge compensation ca-

pacitor must be larger than the gate drive capacitor.

SHDN1: This pin shuts down the control circuitry only (VCC

is not affected). Taking SHDN1 pin high turns off the

control circuitry and holds both MOSFETs off. This pin

must be at ground potential for normal operation.

SHDN2: Master Shutdown Pin. Taking SHDN2 high shuts

down VCC and all control circuitry.

The LTC1159 uses a current mode, constant off-time

architecture to synchronously switch an external pair of

complementary power MOSFETs. Operating frequency is

set by an external capacitor at the CT pin.

The output voltage is sensed either by an internal voltage

divider connected to the Sense – pin (LTC1159-3.3 and

LTC1159-5) or an external divider returned to the VFB pin

(LTC1159). A voltage comparator V, and a gain block G,

compare the divided output voltage with a reference

voltage of 1.25V. To optimize efficiency, the LTC1159

automatically switches between two modes of operation,

burst and continuous.

A low dropout 4.5V regulator provides the operating volt-

age VCC for the MOSFET drivers and control circuitry

during start-up. During normal operation, the LTC1159

family powers the drivers and control from the output via

the EXTVCC pin to improve efficiency. The N-gate pin is

referenced to ground and drives the N-channel MOSFET

(Refer to Functional Diagram)

gate directly. The P-channel gate drive must be referenced

to the main supply input VIN, which is accomplished by

level-shifting the P-drive signal via an internal 550k resis-

tor and external capacitor.

During the switch “ON” cycle in continuous mode,

current comparator C monitors the voltage between the

Sense+ and Sense– pins connected across an external

shunt in series with the inductor. When the voltage

across the shunt reaches its threshold value, the P-gate

output is switched to VIN, turning off the P-channel

MOSFET. The timing capacitor CT is now allowed to

discharge at a rate determined by the off-time controller.

The discharge current is made proportional to the

output voltage to model the inductor current, which

decays at a rate which is also proportional to the output

voltage. While the timing capacitor is discharging, the

N-gate output is high, turning on the N-channel

MOSFET.

OPERATIO

VIN: Main Supply Input Pin.

S-GND: Small Signal Ground. Must be routed separately

from other grounds to the (–) terminal of COUT.

P-GND: Driver Power Grounds. Connect to source of N-

channel MOSFET and the (–) terminal of CIN.

VCC: Outputs of internal 4.5V linear regulator, EXTVCC

switch, and supply inputs for driver and control circuits.

The driver and control circuits are powered from the higher

of the 4.5V regulator or EXTVCC voltage. Must be closely

decoupled to power ground.

CT: External capacitor CT from this pin to ground sets the

operating frequency. (The frequency is also dependent on

the ratio VOUT/VIN.)

ITH: Gain Amplifier Decoupling Point. The current com-

parator threshold increases with the ITH pin voltage.

VFB: For the LTC1159 adjustable version, the VFB pin

receives the feedback voltage from an external resistive

divider used to set the output voltage.

Sense–: Connects to internal resistive divider which sets

the output voltage in fixed output versions. The Sense– pin

is also the (–) input of the current comparator.

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