Datasheet

I =
C(chg)
C V
t
OUT OUT
SS
´
t >
SS
C V
(I I )
OUT OUT
OUT(oc) EXT
-
´
SS(ofst)
DCHG SS(dchg) SS
SS(rst)
V
t R C ln
V
æ ö
ç ÷
= ´ ´
ç ÷
è ø
( )
( )
BP SS(rst)
CHG SS(chg) SS
BP SS(ofst)
V V
t R C ln
V V
æ ö
-
ç ÷
= ´ ´
ç ÷
-
è ø
( )
CHG DCHG
RSTRT min
t t t= +
TPS40210, TPS40211
SLUS772E –MARCH 2008– REVISED OCTOBER 2011 www.ti.com
Also take note of V
BP
. Its value varies depending on input conditions. For example, a converter operating from a
slowly rising input initializes V
BP
at a fairly low value and increases during the entire startup sequence. If the
controller has a voltage above 8 V at the input and the DIS pin is used to stop and then restart the converter, V
BP
is approximately 8 V for the entire startup sequence. The higher the voltage on BP, the shorter the startup time is
and conversely, the lower the voltage on BP, the longer the startup time is.
The soft-start time (t
SS
) must be chosen long enough so that the converter can start up without going into an
overcurrent state. Since the over current state is triggered by sensing the peak voltage on the ISNS pin, that
voltage must be kept below the overcurrent threshold voltage V
ISNS(oc)
. The voltage on the ISNS pin is a function
of the load current of the converter, the rate of rise of the output voltage and the output capacitance, and the
current sensing resistor. The total output current that must be supported by the converter is the sum of the
charging current required by the output capacitor and any external load that must be supplied during startup. This
current must be less than the I
OUT(oc)
value used in Equation 6 or Equation 7 (depending on the operating mode
of the converter) to determine the current sense resistor value. In these equations, the actual input voltage at the
time that the controller reaches the final output voltage is the important input voltage to use in the calculations. If
the input voltage is slowly rising and is at less than the nominal input voltage when the startup time ends, the
output current limit is less than I
OUT(oc)
at the nominal input voltage. The output capacitor charging current must
be reduced (decrease C
OUT
or increase the t
SS
) or I
OUT(oc)
must be increased and a new value for R
ISNS
calculated.
(14)
(15)
Where:
• I
C(chg)
is the output capacitor charging current in A
• C
OUT
is the total output capacitance in F
• V
OUT
is the output voltage in V
• t
SS
is the soft start time from Equation 13
• I
OUT(oc)
is the desired over current trip point in A
• I
EXT
is any external load current in A
The capacitor on the SS pin (C
SS
) also plays a role in overcurrent functionality. It is used as the timer between
restart attempts. The SS pin is connected to GND through a resistor, R
SS(dchg)
, whenever the controller senses an
overcurrent condition. Switching stops and nothing else happens until the SS pin discharges to the soft-start
reset threshold, V
SS(rst)
. At this point, the SS pin capacitor is allowed to charge again through the charging
resistor R
SS(chg)
, and the controller restarts from that point. The shortest time between restart attempts occurs
when the SS pin discharges from V
SS(ofst)
(approximately 700 mV) to V
SS(rst)
(150 mV) and then back to V
SS(ofst)
and switching resumes. In actuality, this is a conservative estimate since switching does not resume until the
V
SSE
ramp rises to a point where it is commanding more output voltage than exists at the output of the controller.
This occurs at some SS pin voltage greater than V
SS(ofst)
and depends on the voltage that remains on the output
overvoltage the converter while switching has been halted. The fastest restart time can be calculated by using
Equation 16, Equation 17 and Equation 18.
(16)
(17)
(18)
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