3

LT1641

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

ON Low to GATE Low

Figures 1, 2

6

µs

ON High to GATE High

Figures 1, 2

1.7

µs

FB Low to PWRGD Low

Figures 1, 3

3.2

µs

FB High to PWRGD High

Figures 1, 3

1.5

µs

Figures 1, 4

0.5

1

2

µs

PI FU CTIO S

ON (Pin 1): The ON pin is used to implement undervoltage

lockout. When the ON pin is pulled below the 1.233V High-

to-Low threshold voltage, an undervoltage condition is

detected and the GATE pin is pulled low to turn the

MOSFET off. When the ON pin rises above the 1.313V

Low-to-High threshold voltage, the MOSFET is turned on

again.

Pulsing the ON pin low after a current limit fault will reset

the fault latch and allow the part to turn back on.

FB (Pin 2): “Power Good” Comparator Input. It monitors

the output voltage with an external resistive divider. When

the voltage on the FB pin is lower than the High-to-Low

threshold of 1.233V, the PWRGD pin is pulled low and

released when the FB pin is pulled above the 1.313V Low-

to-High threshold.

The FB pin also effects foldback current limit (see Figure 7

and related discussion).

PWRGD (Pin 3): Open Collector Output to GND. The

PWRGD pin is pulled low whenever the voltage at the FB

pin falls below the High-to-Low threshold voltage. It goes

into a high impedance state when the voltage on the FB pin

exceeds the Low-to-High threshold voltage. An external

pull-up resistor can pull the pin to a voltage higher or lower

GND (Pin 4): Chip Ground.

TIMER (Pin 5): Timing Input. An external timing capacitor

at this pin programs the maximum time the part is allowed

to remain in current limit.

When the part goes into current limit, an 80

µA pull-up

current source starts to charge the timing capacitor. When

the voltage on the TIMER pin reaches 1.233V, the GATE

pin is pulled low; the pull-up current will be turned off and

the capacitor is discharged by a 3

µA pull-down current.

When the TIMER pin falls below 0.5V, the GATE pin turns

on once the ON pin is pulsed low to reset the internal fault

latch. If the ON pin is not cycled low, the GATE pin remains

latched off.

By connecting a 0.01

µF capacitor from the GATE pin to the

center tap of a resistive divider at the ON pin, the part

automatically restarts after a current limit fault. With a

short at the output, the part cycles on and off with a 3.75%

on-time duty cycle.

GATE (Pin 6): The High Side Gate Drive for the External

N-Channel. An internal charge pump guarantees at least

10V of gate drive for supply voltages above 20V and 4.5V

gate drive for supply voltages between 9V and 20V. The

rising slope of the voltage at the GATE is set by an external

capacitor connected from the GATE pin to GND and an

internal 10

µA pull-up current source from the charge

pump output.

When the current limit is reached, the GATE pin voltage will

be adjusted to maintain a constant voltage across the

sense resistor while the timer capacitor starts to charge.

If the TIMER pin voltage exceeds 1.233V, the GATE pin will

be pulled low.

The GATE pin is pulled to GND whenever the ON pin is

undervoltage lockout threshold or the TIMER pin rises

above 1.233V.

AC ELECTRICAL CHARACTERISTICS

Note 1: Absolute Maximum Ratings are those values beyond which the life

of a device may be impaired.

Note 2: All currents into device pins are positive; all currents out of device

pins are negative. All voltages are referenced to ground unless otherwise

specified.