Datasheet
( )
( ) ( ) ( )
( )
ILIM max ILIM max ILIMOFST max
SCP max
DS(on) min
I R V
I
R
´ +
=
( )
( ) ( ) ( )
( )( )
ILIM min ILIM min ILIMOFST min
SCP min
DS on max
I R V
I
R
´ +
=
7
ILIM
I
ILIM
9 mA
BP
R
R
+
R
ILIM
15
SW
+
t
BLNK
+
+
550 mV
VDD
LDRV On
t
BLNK
HDRV On
SQ
Q R
CLK
UP/DN
3-Bit Counter
Q0
Q1
Q2
Fault
VDD
HDRV
LDRV
UDG-07001
TPS40195
SLUS720E –FEBRUARY 2007–REVISED JULY 2012
www.ti.com
where
• N
DAC
is the number of 1-V DAC ramp cycles from Table 2
• f
SW
is the switching frequency in Hz (4)
Selecting the Short Circuit Threshold
An over current is detected by sensing a voltage drop across the low-side FET when it is on, and across the
high-side FET when it is on. If the voltage drop across either FET exceeds the short circuit threshold in any given
switching cycle, a counter increments one count. If the voltage across the high-side FET was higher that the
short circuit threshold, that FET is turned off early. If the voltage drop across either FET does not exceed the
short circuit threshold during a cycle, the counter is decremented for that cycle. If the counter fills up (a count of
7) a fault condition is declared and the drivers turn off both MOSFETs. After a timeout of approximately 40 ms,
the controller attempts to restart. If a short circuit remains present at the output, the current quickly ramps up to
the short circuit threshold and another fault condition is declared and the process of waiting for the 40 ms and
attempting to restart repeats.
The current limit threshold for the low-side FET is programmable by the user. To set the threshold a resistor is
connected from the ILIM pin to GND. A current source inside the IC connected to the ILIM pin and this resistor
set a voltage that is the threshold used for the overcurrent detection threshold. The low side threshold will
increase as the low side on time decreases due to blanking time and comparator response time. See Figure 5 for
changes in the threshold as the low-side FET conduction time decreases. Refer to Figure 21 for details on the
functional equivalent schematic.
Figure 21. Overcurrent
(5)
where
• I
S.P.
is the short circuit current
• I
ILIM
is ILIM pin bias current, 9.0μA (typ)
• R
ILIM
is the resistance connected from ILIM to GND
16 Copyright © 2007–2012, Texas Instruments Incorporated