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SLVS456C − OCTOBER 2003 − REVISED OCTOBER 2004
Input Voltage Supply
320 kΩ
125 kΩ
R1
R2
1 kΩ
Figure 1. Circuit Using External UVLO Function
The equations for selecting the UVLO resistors are:
Extending Slow Start Time
VIN(start) 1 kW
R1 +
* 1kW
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 desied output voltage. To extend the slow
start time and to reduce the startup current, an external
resistor and capcitor can be added to the ENA pin. The
slow start capacitance is calculated using the following
equation:
1.24 V
(1)
(R1 ) 1 kW) 1.02 V
1 kW
VIN(stop) +
(2)
For applications which require an undervoltage lock out
(UVLO) threshold greater than 4.49 V, 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
Once the ENA pin voltage exceeds 0.5 V, the TPS54350
starts operation. The TPS54350 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:
e−3
C
(µF) = 5.55
T (ms)
ss
SS
The R resistor must be 2 kΩ and the slow start capacitor
must be less than 0.47 µF.
SS
1.15k
+
Switching Frequency (RT)
T
SS_INTERNAL(ms)
ƒ
s(kHz)
(3)
The TPS54350 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.
Once the TPS54350 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 TPS54350 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.
Connecting a resistor from RT to AGND sets the frequency
according to the following equation (also see Figure 30).
46000
RT(kW) +
ƒ
s(kHz)–35.9
(4)
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.
5 µA
R
Disabled
Enabled
SS
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.
C
SS
Figure 2. Interfacing to the ENA Pin
7
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