Datasheet
SLVS519A − MAY 2004 − REVISED OCTOBER 2004
www.ti.com
9
320 kΩ
125 kΩ
(1)
R1
R2
Input
1 kΩ
Voltage
Supply
(1)
75 kΩ for the TPS54357
Figure 1. Circuit Using External UVLO Function
For applications which require an undervoltage lock out
(UVLO) threshold greater than 4.49 V (6.6 V for
TPS54357), external resistors may be implemented, see
Figure 1, to adjust the start voltage threshold. For example,
an application needing an UVLO start voltage of
approximately 7.8 V using the equation (1), R1 is
calculated to the nearest standard resistor value of
5.36 kΩ. Using equation (2), the input voltage stop
threshold is calculated as 6.48 V.
Enable (ENA) and Internal Slow Start
The TPS5435x has an internal digital slow start that ramps
the reference voltage to its final value in 1150 switching
cycles. The internal slow start time (10% − 90%) is
approximated by the following expression:
T
SS_INTERNAL
(ms)
1.15k
ƒ
s
(kHz) n
Use n in Table 2
Table 2. Slow Start Characteristics
DEVICE n
TPS54352 1.485
TPS54353 1.2
TPS54354 1
TPS54355 1.084
TPS54356 0.818
TPS54357 0.900
Once the TPS5435x device is in normal regulation, the
ENA pin is high. If the ENA pin is pulled below the stop
threshold of 0.5 V, switching stops and the internal slow
start resets. If an application requires the TPS5435x to be
disabled, use open drain or open collector output logic to
interface to the ENA pin (see Figure 2). The ENA pin has
an internal pullup current source. Do not use external
pullup resistors.
5 µA
Disabled
Enabled
R
SS
C
SS
Figure 2. Interfacing to the ENA Pin
Extending Slow Start Time
In applications that use large values of output capacitance
there may be a need to extend the slow start time to
prevent the startup current from tripping the current limit.
The current limit circuit is designed to disable the high side
MOSFET and reset the internal voltage reference for a
short amount of time when the high side MOSFET current
exceeds the current limit threshold. If the output
capacitance and load current cause the startup current to
exceed the current limit threshold, the power supply output
will not reach the desired output voltage. To extend the
slow start time and to reduce the startup current, an
external resistor and capacitor can be added to the ENA
pin. The slow start capacitance is calculated using the
following equation:
Use n in Table 2
C
SS
(µF) = 5.55 10
−3
n T
ss
(ms)
The R
SS
resistor must be 2 kΩ and the slow start capacitor
must be less than 0.47 µF.
Switching Frequency (RT)
The TPS5435x has an internal oscillator that operates at
twice the PWM switching frequency. The internal oscillator
frequency is controlled by the RT pin. Grounding the RT
pin sets the PWM switching frequency to a default
frequency of 250 kHz. Floating the RT pin sets the PWM
switching frequency to 500 kHz.
Connecting a resistor from RT to AGND sets the frequency
according to the following equation (also see Figure 30).
RT(kW)
46000
ƒ
s
(kHz) 35.9
The RT pin controls the SYNC pin functions. If the RT pin
is floating or grounded, SYNC is an output. If the switching
frequency has been programmed using a resistor from RT
to AGND, then SYNC functions as an input.
The internal voltage ramp charging current increases
linearly with the set frequency and keeps the feed forward
modulator constant (Km = 8) regardless of the frequency
set point.
(3)
(4)
(5)