Datasheet

19
17
16
14
13
12
R
LDRV
VIN
HDRV
SW
LDRV
PGND
ILIM
R
ILIM
I
ILIM
VDD
R
+
R
+
+
t
BLNK
LDRV On
+
t
BLNK
HDRV On
3-Bit
State
Machine
Q0
Q1
Q2 CLK
(A x V
ILIM
)
OC_FAULT
R
LDRV
(kW)
A
10
OPEN
20
3
7
15
UDG-09198
´
=
´
SC DS(on)HS
OC DS(on)LS
I R
A
I R
´ ´
= =
m
OC DS(on) OC DS(on)
ILIM
ILIM
I R I R
R
I 9.0 A
TPS40170
SLUS970A –MARCH 2011–REVISED NOVEMBER 2013 www.ti.com
Overcurrent Protection and Short-Circuit Protection (OCP and SCP)
The TPS40170 has the capability to set a two-level overcurrent protection. The first level of overcurrent
protection (OCP) is the normal overload setting based on low-side MOSFET voltage sensing. The second level
of protection is the heavy overload setting such as short-circuit based on the high-side MOSFET voltage sensing.
This protection takes effect immediately. The second level is termed short-circuit protection (SCP).
The OCP level is set by the ILIM pin voltage. A current (I
ILIM
) is sourced into the ILIM pin from which a resistor
R
ILIM
is connected to GND. Resistor R
ILIM
sets the first level of overcurrent limit. The OCP is based on the low-
side FET voltage at the switch-node (SW pin) when the LDRV is ON after a blanking time, which is the product of
inductor current and low-side FET turn-on resistance R
DS(on)
. The voltage is inverted and compared to ILIM pin
voltage. If it is greater than the ILIM pin voltage, then a 3-bit counter inside the device increments the fault-count
by 1 at the start of the next switching cycle. Alternatively, if it is less than the ILIM pin voltage, then the counter
inside the device decrements the fault-count by 1. When the fault-count reaches 7, an overcurrent fault
(OC_FAULT) is declared and both the HDRV and LDRV are turned OFF. The resistor R
ILIM
can be calculated by
the following Equation 6.
(6)
The SCP level is set by a multiple of the ILIM pin voltage. The multiplier has three discrete values, 3, 7 or 15
times, which can be selected by respectively choosing a 10-kΩ, open circuit, or 20-kΩ resistor from LDRV pin to
GND. This multiplier AOC information is translated during the t
CAL
time, which starts after the enable and UVLO
conditions are met.
The SCP is based on sensing the high-side FET voltage drop from V
VIN
to V
SW
when the HDRV is ON after a
blanking time, which is product of inductor current and high-side FET turn-on resistance R
DS(on)
. The voltage is
compared to the product of multiplier and the ILIM pin voltage. If it exceeds the product, then the fault-count is
immediately set to 7 and the OC_FAULT is declared. The HDRV is terminated immediately without waiting for
the duty cycle to end. When an OC_FAULT is declared, both the HDRV and LDRV are turned OFF. The
appropriate multiplier (A), can be selected using Equation 7.
(7)
Figure 19 shows the functional block of the two-level overcurrent protection.
Figure 19. OCP and SCP Protection Functional Block Diagram
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