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TPS40074

SLUS617B – APRIL 2005 – REVISED OCTOBER 2006

Table 1. TERMINAL FUNCTIONS

TERMINAL

I/O

DESCRIPTION

NAME

NO.

The BOOST voltage is 8-V greater than the input voltage. The peak voltage on BOOST is equal to the SW node

voltage plus the voltage present at DBP less the bootstrap diode drop. This drop can be 1.4 V for the internal

BOOST

11

I

bootstrap diode or 300 mV for an external schottkey diode. The voltage differential between this pin and SW is

the available drive voltage for the high-side FET.

Output of the error amplifier, input to the PWM comparator. A feedback network is connected from this pin to the

COMP

6

O

FB pin to compensate the overall loop. This pin is internally clamped to a 3.4-V maximum output drive capability

for quicker recovery from a saturated feedback loop situation.

8-V regulator output used for the gate drive of the N-channel synchronous rectifier and as the supply for charging

DBP

9

O

the bootstrap capacitor. This pin should be bypassed to ground with a 1.0-µF ceramic capacitor.

Inverting input to the error amplifier. In normal operation the voltage on this pin is equal to the internal reference

FB

5

I

voltage, 0.7 V.

GND

3

-

Ground reference for the device.

Floating gate drive for the high-side N-channel MOSFET. This pin switches from BOOST (MOSFET on) to SW

HDRV

12

O

(MOSFET off).

Short circuit protection programming pin. This pin is used to set the overcurrent threshold. An internal current sink

from this pin to ground sets a voltage drop across an external resistor connected from this pin to VDD. The

ILIM

14

I

capacitor across the resistor from ILIM to VDD allows the ILIM threshold to decrease during the switch on time,

effectively programming the ILIM blanking time. See Applications Information section.

A resistor is connected from this pin to VDD programs the amount of input voltage feed-forward. The current fed

KFF

15

I

into this pin is used to control the slope of the PWM ramp and program undervoltage lockout. Nominal voltage at

this pin is maintained at 400 mV.

Gate drive for the N-channel synchronous rectifier. This pin switches from DBP (MOSFET on) to PGND (MOSFET

LDRV

8

O

off). For proper operation, the total gate charge of the MOSFET connected to LDRV should be less than 50nC.

4.2-V reference used for internal device logic and analog functions. This pin should be bypassed to GND with a

LVBP

17

O

0.1-µF ceramic capacitor. External loads less than 1 mA and electrically quiet may be applied.

This is an open drain output that pulls to ground when soft start is active, or when the FB pin is outside a ±10%

PGD

18

O

band around the 700 mV reference voltage.

Power ground reference for the device. There should be a low-impedance path from this pin to the source(s) of

PGND

7

the lower MOSFET(s).

RT

16

I

A resistor is connected from this pin to GND to set the switching frequency.

SA+

20

I

Noninverting input of the remote voltage sense amplifier.

SA-

1

I

Inverting input of the remote voltage sense amplifier.

SAO

2

O

Output of the remote voltage sense amplifier.

Soft-start programming pin. A capacitor connected from this pin to GND programs the soft-start time. The

capacitor is charged with an internal current source of 12 µA. The resulting voltage ramp on the SS pin is used as

a second non-inverting input to the error amplifier. The voltage at this error amplifier input is approximately 1 V

SS

4

I

less that that on the SS pin. Output voltage regulation is controlled by the SS voltage ramp until the voltage on

the SS pin reaches the internal offset voltage of 1 V plus the internal reference voltage of 700 mV. If SS is below

the internal offset voltage of 1 V (300 mV minimum ensured), the resulting output voltage is zero. Also provides

timing for fault recovery attempts. Maximum recommended capacitor is 22nF.

This pin is connected to the switched node of the converter and used for overcurrent sensing as well as gate drive

SW

10

I

timing. This pin is also the return path from the high-side FET for the floating high-side FET driver. A 1.5-

Ω

resistor is required in series with this pin to protect against substrate current issues.

SYNC

19

I

Logic input for pulse train to synchronize oscillator.

VDD

13

I

Supply voltage for the device.

6

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