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SLVS456C − OCTOBER 2003 − REVISED OCTOBER 2004
100 ns, the ENA pin is pulled low, the high-side MOSFET
is disabled, and the internal digital slow-start is reset to 0 V.
ENA is held low for approximately the time that is
calculated by the following equation:
Deadtime Control
Adaptive dead time control prevents shoot through current
from flowing in the integrated high-side MOSFET and the
external low-side MOSFET during the switching
transitions by actively controlling the turn on times of the
drivers. The high-side driver does not turn on until the
voltage at the gate of the low-side MOSFET is below 1 V.
The low-side driver does not turn on until the voltage at the
gate of the high-side MOSFET is below 1 V.
2250
+
T
HICCUP(ms)
ƒ
s(kHz)
(7)
Once the hiccup time is complete, the ENA pin is released
and the converter initiates the internal slow-start.
Setting the Output Voltage
Low Side Gate Driver (LSG)
The output voltage of the TPS54350 can be set by feeding
back a portion of the output to the VSENSE pin using a
resistor divider network. In the application circuit of Figure
24, this divider network is comprised of resistors R1 and
R2. To calculate the resistor values to generate the
required output voltage use the following equation:
LSG is the output of the low-side gate driver. The 100-mA
MOSFET driver is capable of providing gate drive for most
popular MOSFETs suitable for this application. Use the
SWIFT Designer Software Tool to find the most
appropriate MOSFET for the application. Connect the LSG
pin directly to the gate of the low-side MOSFET. Do not use
a gate resistor as the resulting turn-on time may be too
slow.
R1 0.891
R2 +
VO * 0.891
(8)
Start with a fixed value of R1 and calculate the required R2
value. Assuming a fixed value of 10 kΩ for R1, the
following table gives the appropriate R2 value for several
common output voltages:
Integrated Pulldown MOSFET
The TPS54350 has a diode-MOSFET pair from PH to
PGND. The integrated MOSFET is designed for light−load
continuous−conduction mode operation when only an
external Schottky diode is used. The combination of
devices keeps the inductor current continuous under
conditions where the load current drops below the
inductor’s critical current. Care should be taken in the
selection of inductor in applications using only a low-side
Schottky diode. Since the inductor ripple current flows
through the integrated low-side MOSFET at light loads, the
inductance value should be selected to limit the peak
current to less than 0.3 A during the high-side FET turn off
time. The minimum value of inductance is calculated using
the following equation:
OUTPUT VOLTAGE (V)
R2 VALUE (KΩ)
1.2
1.5
1.8
2.5
3.3
28.7
14.7
9.76
5.49
3.74
Output Voltage Limitations
Due to the internal design of the TPS54350 there are both
upper and lower output voltage limits for any given input
voltage. Additionally, the lower boundary of the output
voltage set point range is also dependent on operating
frequency. The upper limit of the output voltage set point
is constrained by the maximum duty cycle of the device
and is shown in Figure 48. The lower limit is constrained
by the minimum controllable on time which may be as high
as 220 ns. The approximate minimum output voltage for a
given input voltage and range of operating frequencies is
shown in Figure 29 while the maximum operating
frequency versus input voltage for some common output
voltages is shown in Figure 30.
VO
VI
ǒ
Ǔ
VO 1 *
L(H) +
ƒ 0.6
s
(6)
Thermal Shutdown
The device uses the thermal shutdown to turn off the
MOSFET drivers and controller if the junction temperature
exceeds 165°C. The device is restarted automatically
when the junction temperature decreases to 7°C below the
thermal shutdown trip point and starts up under control of
the slow-start circuit.
The curves shown in these two figures are valid for output
currents greater than 0.5 A. As output currents decrease
towards no load (0 A), the minimum output voltage
decreases. For applications where the load current is less
than 100 mA, the curves shown in Figures 31 and 32 are
applicable. All of the data plotted in these curves are
approximate and take into account a possible 20 percent
deviation in actual operating frequency relative to the
intended set point.
Overcurrent Protection
Overcurrent protection is implemented by sensing the
drain-to-source voltage across the high-side MOSFET
and compared to a voltage level which represents the
overcurrent threshold limit. If the drain-to-source voltage
exceeds the overcurrent threshold limit for more than
10
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