Texas Instruments SLVU013, User Manual

Page: 29 / 64

TPS56xx Functions
2-9 Design Procedure
resistor-divider network is designed so that the voltage applied to OCP is
100 mV for the desired output current limit point. If the voltage on OCP
exceeds 100 mV, a fault latch is set and the output drivers are turned off. The
latch remains set until VCC (pin 15) goes below the undervoltage lockout
value.
The following equations summarize the relationships discussed above.
The on state drain-to-source voltage of the high-side MOSFET, Q1, is:
ǒ
V
HISENSE
–V
IOUTLO
Ǔ
+
I
O
R
DS(on)
where R
DS(on)
is the value obtained from the high-side MOSFET and includes
correction for elevated temperature if necessary.
The voltage difference, V
HISENSE
– V
IOUTLO
, is internally amplified by a fixed
gain of two to produce the IOUT (pin 1) signal.
V
IOUT
+
ǒ
V
HISENSE
–V
IOUTLO
Ǔ
2
The V
IOUT
signal is scaled for the desired current limit level and applied to the
OCP pin:
V
OCP
+
V
IOUT
R13
R13
)
R7
Therefore, to set the power supply output current, first calculate the quantity
I
O
×
R
DS(on)
for the value of
I
O
desired for current limit. Variations in
R
DS(on)
,
including its temperature dependence, should be considered, since this
parameter can vary a significant amount for typical MOSFETs. Next, multiply
this voltage by two. Finally, set the R7 and R13 voltage divider to produce 100
mV at the desired current limit point.
For this design, the maximum output current is 6 A. In most power supply
designs, the exact current limit set point rarely needs to be very accurate.
Limiting the current to a level that is not destructive is the main consideration.
In this case, the nominal current limit is set for approximately 25% above the
maximum, including an elevated temperature correction factor of 1.4 applied
to a nominal R
DS(on)
of 11 m
Ω
. The current limit set point, I
OCP
, is given by:
I
OCP
+
I
O(Max)
1.25
+
6 (1.25)
+
7.5 Amps
For the above current level, V
IOUT(Trip)
is given by:
V
IOUT(Trip)
+
ǒ
I
O
R
DS(on)
Ǔ
2
+
(7.5 A
0.011 1.4 ) 2
+
0.23
V
Choose R13 = 750
Ω
.
Now, calculate R7 using:
R7
+
ǒ
V
IOUT(Trip)
0.1
V
*
1
Ǔ
R13
+
983
+u
1 k
In the above calculations, the nominal value for R
DS(on)
of 11 m
Ω
was used.
Worst case analysis should always be performed to insure that current limit
does not interfere with delivering maximum load.