LT8650SPJVPBF datasheet(12/34 Pages) AD

12 / 34 page

LT8650SP

Rev. B

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PIN FUNCTIONS

BST2 (Pin 18): This pin is used to provide a drive volt-

age, higher than the input voltage, to the top side power

switch of channel 2.

SW2 (Pins 19, 20): The SW2 pin is the output of the chan-

nel 2 internal power switches. Tie these pins together and

connectthemtotheinductorandboostcapacitor.Thisnode

should be kept small on the PCB for good performance.

SW1 (Pins 22, 23): The SW1 pin is the output of the chan-

nel 1 internal power switches. Tie these pins together and

connectthemtotheinductorandboostcapacitor.Thisnode

should be kept small on the PCB for good performance.

BST1 (Pin 24): This pin is used to provide a drive volt-

age, higher than the input voltage, to the top side power

switch of channel 1.

VCC (Pin 25): Internal Regulator Bypass Pin. The internal

power drivers and control circuits are powered from this

voltage. VCC current will be supplied from BIAS if VBIAS >

3.1V, otherwise current will be drawn from VIN1. Voltage

on VCC will vary between 2.8V and 3.3V when VBIAS is

between 3.0V and 3.5V. Decouple this pin to ground with

at least a 1μF low ESR ceramic capacitor. Do not load the

VCC pin with external circuitry.

BIAS(Pin26):Theinternalregulatorwilldrawcurrentfrom

BIAS instead of VIN1 when BIAS is tied to a voltage higher

than 3.1V. For output voltages of 3.3V and above this pin

should be tied to VOUT. If this pin is tied to a supply other

than VOUT use a 1µF local bypass capacitor on this pin.

VC1 (Pin 27): Channel 1 Error Amplifier Output and

Switching Regulator Compensation Pin. Connect this pin

to appropriate external components to compensate the

regulator loop frequency response. Connect this pin to

VCC to use the default internal compensation. If internal

compensation is used, the burst mode quiescent current

is only 2.5µA for channel 1. If external compensation is

used, the burst mode quiescent current is increased to

about 50µA for channel 1.

FB1 (Pin 28): The LT8650SP regulates the FB1 pin to

800mV.Connectthefeedbackresistordividertaptothispin.

SS1 (Pin 29): Channel 1 Output Tracking and Soft-Start

Pin. This pin allows user control of output voltage ramp

rate during startup. A SS1 voltage below 0.8V forces the

LT8650SP to regulate the FB1 pin to equal the SS1 pin

voltage. When SS1 is above 0.8V, the tracking function is

disabled and the internal reference resumes control of the

error amplifier. An internal 2μA pull-up current from VCC

on this pin allows a capacitor to program output voltage

slew rate. This pin is pulled to ground with an internal

200Ω MOSFET during shutdown and fault conditions; use

a series resistor if driving from a low impedance output.

This pin may be left floating if the soft-start feature is not

being used.

SS2 (Pin 30): Channel 2 Output Tracking and Soft-Start

Pin. This pin allows user control of output voltage ramp

rate during startup. A SS2 voltage below 0.8V forces the

LT8650SP to regulate the FB2 pin to equal the SS2 pin

voltage. When SS2 is above 0.8V, the tracking function is

disabled and the internal reference resumes control of the

error amplifier. An internal 2μA pull-up current from VCC

on this pin allows a capacitor to program output voltage

slew rate. This pin is pulled to ground with an internal

200Ω MOSFET during shutdown and fault conditions; use

a series resistor if driving from a low impedance output.

This pin may be left floating if the soft-start feature is not

being used.

FB2 (Pin 31): The LT8650SP regulates the FB2 pin to

800mV.Connectthefeedbackresistordividertaptothispin.

VC2 (Pin 32): Channel 2 Error Amplifier Output and

Switching Regulator Compensation Pin. Connect this pin

to appropriate external components to compensate the

regulator loop frequency response. Connect this pin to

VCC to use the default internal compensation. If internal

compensation is used, the burst mode quiescent current

is only 2.5µA for channel 2. If external compensation is

used, the burst mode quiescent current is increased to

about 50µA for channel 2.

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