Datasheet

I
PEAK
+
I
RIPPLE
2
) I
OC
I
RIPPLE
+
V
IN
* V
OUT
L
V
OUT
V
IN
1
f
SW
TPS40140
SLUS660H –SEPTEMBER 2005–REVISED JUNE 2013 www.ti.com
In Figure 5-9, normal operation is occurring between t0 and t1 as shown by V
OUT
being at the regulated
voltage, ©) and normal switching on the SW NODE (B) and COMP at it's nominal level, (D). At t1, an
overcurrent load is experienced. The increased current forces COMP to increase to the ILIM level as
shown in (D). If the COMP voltage is above the ILIM voltage for 7 switching cycles, the controller enters a
hiccup mode. During this time the controller is not switching and the switching MOSFETs are turned off.
The TRKx voltage goes through 7 cycles of charging and discharging the soft-start capacitor. At the end of
the 7 cycles the controller attempts another normal re-start. If the fault has been cleared, the output
voltage comes up to the regulation level as shown at time t3. If the fault has not cleared, the COMP
voltage again rises above the ILIM voltage and the hiccup mode repeats.
If the overcurrent condition exists for seven (7) PWM clock cycles the converter turns off the upper and
lower MOSFETs and initiates a hiccup mode restart. In hiccup mode, the TRKx pin is periodically charged
and discharged. After seven hiccup cycles, the controller attempts another soft-start cycle to restore
normal operation. If the overload condition persists, the controller returns to the hiccup mode. This
condition may continue indefinitely.
5.26 CALCULATING OVERCURRENT PROTECTION LEVEL
In order to set the desired overcurrent (I
OC
), a few variables must be known. The input and output voltage,
the output inductor value and it's DC resistance (DCR), as well as the switching frequency. Also known
are the ramp voltage which is 0.5 V and the V
SHARE
voltage, V
SH
which is 1.8 V. See the list of variables
and their values at the end of this section.
The overcurrent set point is in terms of the DC output current, but the current sense circuit monitors the
peak of the current. Therefore, the current ripple is needed and is calculated from the values of:
• input voltage (V
IN
)
• output voltage (V
OUT
)
• switching frequency (f
SW
)
• output inductance (L)
The ripple current is given by Equation 11.
(11)
The peak current detected is given by Equation 12, and used in Equation 14.
(12)
It is this I
PEAK
current that is detected by the current sense circuit. The two resistors needed to set the
peak overcurrent protection threshold and their connection for each channel is shown in Figure 5-10.
DESIGN HINT
Resistor R2 may be connected to the output voltage, Vout, or to the output of the differential
amplifier, DIFFO, if used.
26 APPLICATION INFORMATION Copyright © 2005–2013, Texas Instruments Incorporated
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