Datasheet

0 0.5 1 1.5 2 2.5 3 3.5 4
I - Output Current - A
O
50
55
60
65
70
75
80
85
90
95
100
Efficiency - %
V = 2.5 V
O
V = 1.8 V
O
V = 1.2 V
O
V = 1 V
O
Vin = 3.3 V,
DCR = 18 m ,
Fs = 500 kHz,
Tj = 25°C
W
V = 2.5 V
O
V = 1.8 V
O
V = 1.2 V
O
V = 1 V
O
Vin = 5 V,
DCR = 18 m ,
Fs = 500 kHz,
Tj = 25°C
W
0 0.5 1 1.5 2 2.5 3 3.5 4
I - Output Current - A
O
50
55
60
65
70
75
80
85
90
95
100
Efficiency - %
V = 3.3 V
O
TPS54478
SLVSAS2 – JUNE 2011 www.ti.com
TYPICAL CHARACTERISTICS CURVES (continued)
EFFICIENCY vs LOAD CURRENT EFFICIENCY vs LOAD CURRENT
Figure 19. Figure 20.
OVERVIEW
The TPS54478 is a 6-V, 4-A, synchronous step-down (buck) converter with two integrated n-channel MOSFETs.
To improve performance during line and load transients the device implements a constant frequency, peak
current mode control which reduces output capacitance and simplifies external frequency compensation design.
The wide switching frequency of 200 kHz to 2000 kHz allows for efficiency and size optimization when selecting
the output filter components. The switching frequency is adjusted using a resistor to ground on the RT/CLK pin.
The device has an internal phase lock loop (PLL) on the RT/CLK pin that is used to synchronize the power
switch turn on to a falling edge of an external system clock.
The TPS54478 has a typical default start up voltage of 2.6 V. 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 total
operating current for the TPS54478 is typically 525 μA when not switching and under no load. When the device
is disabled, the supply current is less than 2.5 μA.
The integrated 30 mΩ MOSFETs allow for high efficiency power supply designs with continuous output currents
up to 4 amperes.
The TPS54478 reduces the external component count by integrating the boot recharge diode. The bias voltage
for the integrated high side MOSFET is supplied by a capacitor between the BOOT and PH pins. The boot
capacitor voltage is monitored by an UVLO circuit and turns off the high side MOSFET when the voltage falls
below a preset threshold. This BOOT circuit allows the TPS54478 to operate approaching 100%. The output
voltage can be stepped down to as low as the 0.600 V reference.
TPS54478 features monotonic starup under pre-bias conditions. The low side Fet turns on for a very short time
period every cycle before the output voltage reaches the pre-biased voltage. This ensures the boot cap has
enough charge to turn on the top Fet when the output voltage reaches the pre-biased voltage.
The TPS54478 has a power good comparator (PWRGD) with 2% hysteresis.
The TPS54478 minimizes excessive output overvoltage transients by taking advantage of the overvoltage power
good comparator. When the regulated output voltage is greater than 107% of the nominal voltage, the
overvoltage comparator is activated, and the high side MOSFET is turned off and masked from turning on until
the output voltage is lower than 105%.
The SS/TR (slow start/tracking) pin is used to minimize inrush currents or provide power supply sequencing
during power up. A small value capacitor should be coupled to the pin for slow start. The SS/TR pin is
discharged before the output power up to ensure a repeatable restart after an overtemperature fault, UVLO fault
or disabled condition. To optimize the output startup waveform, two levels of SS current are implemented. The
first slope has more current so that the converter can get out of the region requiring small minimum ON time.
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