LTC4367 datasheet(15/30 Pages) AD

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LTC7000A

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

sense and current monitoring functions. RFLT should be

at least 2000x larger than RSNS (minimum 100Ω) to pro-

vide robustness during short circuit events. The current

injected into the SNS+ and SNS– pins during a short cir-

cuit event depends on the voltage on POWER, RSNS,

external MOSFET RDS(ON), timer capacitor value and the

value of RFLT.

ISET Pin (LTC7000A Only)

The current comparator has an adjustable threshold

voltage, ΔVTH, of 20mV to 75mV and is set by placing a

resistor to ground on the ISET pin. The ISET pin is biased

with an internal 10µA current source. Floating ISET

enables the current comparator to have an accurate 30mV

threshold voltage which allows for lower value sense

resistors and reduces the external power dissipation.

By placing a 40kΩ to 150kΩ resistor between ISET and

ground, the sense threshold voltage can be programmed

to values between 20mV and 75mV. The value of resistor

for a particular sense threshold voltage can be selected

using Figure 3 or the following equation:

RISET =

Δ VTH

0.5µA

Where 20mV< ΔVTH < 75mV.

ISET RESISTOR TO GROUND (k)

90 120 150 180 210 240

7000A F03

Figure 3. RISET Selection

Fault Timer and Fault Flag

The LTC7000A/LTC7000A-1 includes an adjustable fault

timer. Connecting a capacitor from the TIMER pin to

ground sets the delay period before the external MOSFET

is turned off during an overcurrent fault condition. The

same capacitor also sets the cooldown period before the

external MOSFET is allowed to turn back on. Once a fault

condition is detected, a 100µA current charges the TIMER

pin. When the voltage on the TIMER pin reaches 1.3V, the

FAULT pin pulls low to indicate the detection of a fault

condition and provide warning of an impending power

loss. After the TIMER voltage crosses the 1.4V thresh-

old, TGDN is immediately pulled to TS turning off the

external MOSFET. The on-time of the external MOSFET,

TOVER_CURRENT, during an overcurrent event is given by

the following equation:

TOVER_CURRENT =

1.4V • CTIMER

100µA

+ 1.5µs

The warning time, TWARNING, generated by an overcurrent

event is given by the following equation:

TWARNING =

0.1V • CTIMER

100µA

+ 1.5µs

If the overcurrent fault condition disappears before

TIMER has reached 1.4V, TIMER is discharged by a

2.5µA current. If TIMER had reached 1.3V (FAULT has

gone low) and the overcurrent fault condition disappears,

TIMER is discharged with a 2.5µA current and FAULT will

be reset when TIMER reaches 0.4V. The on-time and

warning times are shown graphically in Figure 4.

TIME

VTMR (V)

7000A F04

TOVER_CURRENT

14ms/µF

TWARNING

1ms/µF

TFAULT

13ms/µF

1.4

1.3

Figure 4. Fault Timer Trip Points

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