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SLVS839B – JULY 2008 – REVISED OCTOBER 2008
TYPICAL CHARACTERISTICS (continued)
OVERVIEW
The TPS54331 is a 28-V, 3-A, step-down (buck) converter with an integrated high-side n-channel MOSFET. To
improve performance during line and load transients, the device implements a constant frequency, current mode
control which reduces output capacitance and simplifies external frequency compensation design. The
TPS54331 has a pre-set switching frequency of 570kHz.
The TPS54331 needs a minimum input voltage of 3.5V to operate normally. The EN pin has an internal pull-up
current source that can be used to adjust the input voltage under-voltage lockout (UVLO) with two external
resistors. In addition, the pull-up current provides a default condition when the EN pin is floating for the device to
operate. The operating current is 110
µA
typically when not switching and under no load. When the device is
disabled, the supply current is 1µA typically.
The integrated 80 mΩ high-side MOSFET allows for high efficiency power supply designs with continuous output
currents up to 3A.
The TPS54331 reduces the external component count by integrating the boot recharge diode. The bias voltage
for the integrated high-side MOSFET is supplied by an external capacitor on the BOOT to PH pin. The boot
capacitor voltage is monitored by an UVLO circuit and will turn the high-side MOSFET off when the voltage falls
below a preset threshold of 2.1V typically. The output voltage can be stepped down to as low as the reference
voltage.
By adding an external capacitor, the slow start time of the TPS54331 can be adjustable which enables flexible
output filter selection.
To improve the efficiency at light load conditions, the TPS54331 enters a special pulse skipping Eco-mode
TM
when the peak inductor current drops below 160mA typically.
The frequency foldback reduces the switching frequency during startup and over current conditions to help
control the inductor current. The thermal shut down gives the additional protection under fault conditions.
DETAILED DESCRIPTION
FIXED FREQUENCY PWM CONTROL
The TPS54331 uses a fixed frequency, peak current mode control. The internal switching frequency of the
TPS54331 is fixed at 570kHz.
ECO-MODE
TM
The TPS54331 is designed to operate in pulse skipping Eco-mode
TM
at light load currents to boost light load
efficiency. When the peak inductor current is lower than 160 mA typically, the COMP pin voltage falls to 0.5V
typically and the device enters Eco-mode
TM
. When the device is in Eco-mode
TM
, the COMP pin voltage is
clamped at 0.5V internally which prevents the high side integrated MOSFET from switching. The peak inductor
current must rise above 160mA for the COMP pin voltage to rise above 0.5V and exit Eco-mode
TM
. Since the
integrated current comparator catches the peak inductor current only, the average load current entering
Eco-mode
TM
varies with the applications and external output filters.
VOLTAGE REFERENCE (Vref)
The voltage reference system produces a ±2% initial accuracy voltage reference (±3.5% over temperature) by
scaling the output of a temperature stable bandgap circuit. The typical voltage reference is designed at 0.8V.
BOOTSTRAP VOLTAGE (BOOT)
The TPS54331 has an integrated boot regulator and requires a 0.1µF ceramic capacitor between the BOOT and
PH pin to provide the gate drive voltage for the high-side MOSFET. A ceramic capacitor with an X7R or X5R
grade dielectric is recommended because of the stable characteristics over temperature and voltage. To improve
drop out, the TPS54331 is designed to operate at 100% duty cycle as long as the BOOT to PH pin voltage is
greater than 2.1V typically.
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