LT1576CS8 datasheet(6/28 Pages) LINER

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LT1576/LT1576-5

PIN FUNCTIONS

VSW (Pin 1): 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 volt-

age is clamped with the external catch diode. Maximum

negative switch voltage allowed is – 0.8V.

VIN (Pin 2): 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

inductance on this path will create a voltage spike at switch

off, adding to the VCE voltage across the internal NPN.

BOOST (Pin 3): 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 0.2

Ω FET structure.

Efficiency improves from 75% for conventional bipolar

designs to > 88% for these new parts.

GND (Pin 4): 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 be-

tween 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 consider-

ation 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.

BIAS (Pin 5): The BIAS pin is used to improve efficiency

when operating at higher input voltages and light load

current. Connecting this pin to the regulated output volt-

age forces most of the internal circuitry to draw its

operating current from the output voltage rather than the

input supply. This is a much more efficient way of doing

business if the input voltage is much higher than the

output.

Minimum output voltage setting for this mode of

operation is 3.3V. Efficiency improvement at VIN = 20V,

VOUT = 5V, and IOUT = 25mA is over 10%.

VC (Pin 6): The VC pin is the output of the error amplifier

and the input of the peak switch current comparator. It is

normally used for frequency compensation, but can do

double duty as a current clamp or control loop override.

BOOST Pin Current

SWITCH CURRENT (A)

0.25

0.50

0.75

1.00

1576 G19

1.25

1.50

VC Pin Shutdown Threshold

JUNCTION TEMPERATURE (

°C)

–50

1.0

0.8

0.6

0.4

0.2

100

1576 G20

–25

125

TYPICAL PERFORMANCE CHARACTERISTICS

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