Datasheet

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SOFTSTART
SHORT-CIRCUIT PROTECTION
THERMAL SHUTDOWN
ENABLE
UNDERVOLTAGE LOCKOUT
TPS62020
TPS62021
TPS62026
SLVS076C – JUNE 2003 – REVISED DECEMBER 2004
DETAILED DESCRIPTION (continued)
• R
L
= DC resistance of the inductor
• V
O
max = nominal output voltage plus maximum output voltage tolerance
The TPS6202x series has an internal softstart 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 TPS6202x.
The softstart is implemented with a digital circuit increasing the switch current in steps of typically I
LIM
/8, I
LIM
/4,
I
LIM
/2 and then the typical switch current limit of 1.1 A as specified in the electrical parameter table. The start-up
time mainly depends on the output capacitor and load current, see Figure 13 .
As soon as the output voltage falls below 50% of the nominal output voltage, the converter switching frequency
as well as the current limit is reduced to 50% of the nominal value. Since the short-circuit protection is enabled
during start up the device does not deliver more than half of its nominal current limit until the output voltage
exceeds 50% of the nominal output voltage. This needs to be considered in case a load acting as a current sink
is connected to the output of the converter.
As soon as the junction temperature of typically 150 ° C is exceeded the device goes into thermal shutdown. In
this mode, the P-Channel switch and N-Channel rectifier are turned off. The device continues its operation when
the junction temperature falls below typically 150 ° C again.
Pulling the EN low forces the part into shutdown mode, with a shutdown current of typically 0.1 µA. In this mode,
the P-Channel switch and N-Channel rectifier are turned off and the whole device is in shut down. If an output
voltage is present during shut down, which could be an external voltage source or super cap, the reverse
leakage current is specified under electrical parameter table. For proper operation the enable (EN) pin must be
terminated and should not be left floating.
Pulling EN high starts up the device with the softstart as described under the section Softstart.
The undervoltage lockout circuit prevents device misoperation at low input voltages. It prevents the converter
from turning on the switch or rectifier MOSFET with undefined conditions.
12