Datasheet

FT
S
100 mV
I
R
=
SENSE SUM
12SUM SENSE
SET
R R
V I
R
æ ö
´
= ´
ç ÷
è ø
SET
LIMIT
SUM SENSE
R
I 0.675 V
R R
æ ö
= ´
ç ÷
´
è ø
LIMIT SET
0.0198 A
I R
æ ö
= ´
ç ÷
W
è ø
TPS2458
www.ti.com SLUS916B –FEBRUARY 2009–REVISED MAY 2010
12-V Fast Trip and Current Limiting
Figure 19 shows a simplified block diagram of the circuitry associated with the fast trip and current limit circuitry
in the 12-V channel, which requires an external N-channel pass FET and three external resistors. These resistors
allow the user to independently set the fast trip threshold and the current limit threshold, as described below.
12-V Fast Trip Function
The 12-V fast trip function is designed to protect the channel against short-circuit events. If the voltage across
R
SENSE
exceeds a nominal threshold of 100 mV, the device will immediately disable the pass transistor and
declare a fault condition. The nominal fast trip threshold is shown in Equation 2.
(2)
12-V Current Limit Function
The 12-V current limit function regulates the PASS pin voltage to prevent the current through the channel from
exceeding I
LIMIT
. The current limit circuitry includes two amplifiers, A
1
and A
2
, as shown in Figure 19. Amplifier A
1
forces the voltage across external resistor R
SET
to equal the voltage across external resistor R
SENSE
. The current
that flows through R
SET
also flows through external resistor R
SUM
, generating a voltage on the 12SUM pin is
shown in Equation 3.
(3)
Amplifier A
2
senses the voltage on the 12SUM pin. As long as this voltage is less than the reference voltage on
its positive input (nominally 0.675 V), the amplifier sources current to PASS. When the voltage on the 12SUM pin
exceeds the reference voltage, amplifier A
2
begins to sink current from the PASS pin. The gate-to-source voltage
of pass FET MPASS drops until the voltages on the two inputs of amplifier A
2
balance. The current flowing
through the channel then nominally is shown in Equation 4.
(4)
The recommended value of R
SUM
is 6810 Ω. This resistor should never equal less than 675 Ω to prevent
excessive currents from flowing through the internal circuitry. Using the recommended values of R
SENSE
= 5 m
and R
SUM
= 6810 Ω gives Equation 5.
(5)
Copyright © 2009–2010, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Link(s): TPS2458