Datasheet
I
SCP(min)
u
ǒ
C
OUT
V
OUT
t
START
Ǔ
) I
LOAD
)
ǒ
I
RIPPLE
2
Ǔ
(7)
R
ILIM
+
I
SCP
R
DS(onMAX)
) V
ILIM (offset)
I
ILIM
W
(8)
I
SCP(max)
+
1.09 I
ILIM(max)
R
ILIM
* 0.09 R
VDD
I
R
VDD
* 0.045 V ) 75 mV
R
DS(ON)min
(A)
(9)
I
SCP(min)
+
1.09 I
ILIM(min)
R
ILIM
* 0.09 R
VDD
I
R
VDD
* 0.045 V ) 30 mV
R
DS(ON)max
(A)
(10)
C
ILIM(max)
+
V
OUT
0.2
V
IN
R
ILIM
f
SW
(Farads)
(11)
TPS40077
SLUS714D – JANUARY 2007 – REVISED APRIL 2009 .....................................................................................................................................................
www.ti.com
The second-tier protection incorporates a fault counter. The fault counter is incremented on each cycle with an
overcurrent pulse and decremented on a clock cycle without an overcurrent pulse. When the counter reaches
seven (7), a fault condition is declared by the controller. When this happens, the outputs are placed in a state
defined in Table 2 . Seven soft-start cycles are initiated (without activity on the HDRV and LDRV outputs) and the
PWM is disabled during this period. The counter is decremented on each soft-start cycle. When the counter is
decremented to zero, the PWM is re-enabled and the controller attempts to restart. If the fault has been
removed, the output starts up normally. If the output is still present, the counter counts seven overcurrent pulses
and re-enters the second-tier fault mode. Refer to Figure 25 for typical fault-protection waveforms.
The minimum short-circuit limit setpoint (I
SCP(min)
) depends on t
START
, C
OUT
, V
OUT
, ripple current in the inductor
(I
RIPPLE
), and the load current at turnon (I
LOAD
).
The short-circuit limit programming resistor (R
ILIM
) is calculated from:
where
• I
ILIM
is the current into the ILIM pin (110 µ A, typical)
• V
ILIM(offset)
is the offset voltage of the ILIM comparator ( – 50 mV, typical)
• I
SCP
is the short-circuit protection current
To find the range of the overcurrent values, use the following equations:
The TPS40077 provides short-circuit protection only. Therefore, it is recommended that the minimum short-circuit
protection level be placed at least 20% above the maximum output current required from the converter. The
maximum output of the converter should be the steady state maximum output plus any transient specification
that may exist.
The ILIM capacitor maximum value can be found from:
Note that this is a recommended maximum value. If a smaller value can be used, it should be. For most
applications, consider using half the maximum value above.
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