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LT1374

1374fb

FB/SENSE: The feedback pin is the input to the error

amplifier which is referenced to an internal 2.42V source.

An external resistive divider is used to set the output

voltage. The fixed voltage (-5) parts have the divider

included on-chip and the FB pin is used as a SENSE pin,

connected directly to the 5V output. Three additional

functions are performed by the FB pin. When the pin

voltage drops below 1.7V, switch current limit is reduced.

Below 1.5V the external sync function is disabled. Below

1V, switching frequency is also reduced. See Feedback Pin

Function section in Applications Information for details.

BOOST: The BOOST pin is used to provide a drive voltage,

higher than the input voltage, to the internal bipolar NPN

power switch. Without this added voltage, the typical

switch voltage loss would be about 1.5V. The additional

boost voltage allows the switch to saturate and voltage

loss approximates that of a 0.07

Ω FET structure. Effi-

ciency improves from 75% for conventional bipolar de-

signs to > 89% for these new parts.

VIN: This is the collector of the on-chip power NPN switch.

This pin powers the internal circuitry and internal regulator

when the BIAS pin is not present. At NPN switch on and off,

high dI/dt edges occur on this pin. Keep the external

bypass and catch diode close to this pin. All trace induc-

tance on this path will create a voltage spike at switch off,

adding to the VCE voltage across the internal NPN. Both VIN

pins of the 16-lead TSSOP package must be shorted

together on the PC board.

GND: The GND pin connection needs consideration for

two reasons. First, it acts as the reference for the regulated

output, so load regulation will suffer if the “ground” end of

the load is not at the same voltage as the GND pin of the

IC. This condition will occur when load current or other

currents flow through metal paths between the GND pin

and the load ground point. Keep the ground path short

between the GND pin and the load and use a ground plane

when possible. The second consideration is EMI caused

by GND pin current spikes. Internal capacitance between

the VSW pin and the GND pin creates very narrow (<10ns)

current spikes in the GND pin. If the GND pin is connected

to system ground with a long metal trace, this trace may

radiate excess EMI. Keep the path between the input

bypass and the GND pin short.

VSW: The switch pin is the emitter of the on-chip power

NPN switch. This pin is driven up to the input pin voltage

during switch on time. Inductor current drives the switch

pin negative during switch off time. Negative voltage is

clamped with the external catch diode. Maximum negative

switch voltage allowed is – 0.8V. Both VSW pins of the

16-lead TSSOP package must be shorted together on the

PC board.

BOOST Pin Current

SWITCH CURRENT (A)

1374 G16

100

DUTY CYCLE = 100%

Inductor Core Loss

INDUCTANCE (

µH)

1.0

0.1

0.01

0.001

1.2

0.8

0.4

0.2

0.12

0.08

0.04

0.02

1374 G01

TYPE 52

POWDERED IRON

Kool M

µ®

PERMALLOY

µ = 125

VOUT = 5V, VIN = 10V, IOUT = 1A

CORE LOSS IS

INDEPENDENT OF LOAD

CURRENT UNTIL LOAD CURRENT FALLS

LOW ENOUGH FOR CIRCUIT TO GO INTO

DISCONTINUOUS MODE

VC Pin Shutdown Threshold

JUNCTION TEMPERATURE (

°C)

–50

1.4

1.2

1.0

0.8

0.6

0.4

100

1374 G11

–25

125

SHUTDOWN

TYPICAL PERFOR A CE CHARACTERISTICS

PI FU CTIO S

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