Datasheet
SLVS485 − AUGUST 2003
www.ti.com
11
Soft Start
The TPS6420x has an internal soft start circuit that limits the inrush current during start up. This prevents
possible voltage drops of the input voltage in case a battery or a high impedance power source is connected
to the input of the TPS6420x. During soft start the current limit is increased from 25% of its maximum to the
maximum within about 250 µs.
100% Duty Cycle Low Dropout Operation
The TPS6420x offers the lowest possible input to output voltage difference while still maintaining regulation with
the use of the 100% duty cycle mode. In this mode the P-channel switch is constantly turned on. This is
particularly useful in battery powered applications to achieve longest operation time by taking full advantage
of the whole battery voltage range.
Enable
A voltage higher than the EN trip point of 1.3 V up to the input voltage forces the TPS64200 into shutdown. In
shutdown, the power switch, drivers, voltage reference, oscillator, and all other functions are turned off. The
supply current is reduced to less than 1 µA in shutdown. Pulling enable low starts up the TPS64200 with the
softstart as described under the chapter softstart.
Undervoltage Lockout
The undervoltage lockout circuit prevents the device from misoperation at low input voltages. Basically, it
prevents the converter from turning on the external PMOS under undefined conditions.
Current Limit
The ISENSE input is used to set the current limit for the external PMOS. The sense resistor must be connected
between V
I
and source of the external PMOS. The ISENSE pin is connected to the source of the external
PMOS. The maximum current is calculated by:
I
(cur lim)
+
V
(ISENSE)
R
S
(1)
For low cost solutions the r
DS(on)
of the external PMOS can also be used to set the current limit. In this case
the ISENSE pin is connected to the drain of the PMOS. The current in the PMOS is automatically sampled by
the TPS6420x some 10 ns after the PMOS is turned on. The ISENSE pin should always be connected to either
the source of the PMOS or the drain if an additional sense resistor is used. Otherwise there is no working
overcurrent protection and no soft start in the system. The maximum drain current if the r
DS(on)
is used as a
sense resistor is calculated by:
I
(cur lim)
+
V
(ISENSE)
r
DS(on)
(2)
Short-Circuit Protection
With a controller only limited short circuit protection is possible because the temperature of the external
components is not supervised. In an overload condition, the current in the external diode may exceed the
maximum rating. To protect the diode against overcurrent, the off-time of the TPS6420x is increased when the
voltage at the feedback pin is lower than its nominal value. The off-time when the output is shorted (feedback
voltage is zero) is about 4 µs. This allows the current in the external diode to drop until the PMOS is turned on
again and the overcurrent protection switches off the PMOS again. The off-time is directly proportional to the
voltage at feedback.