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SLVS413C – OCTOBER 2002 – REVISED AUGUST 2007
Table 1. Terminal Functions
TERMINAL
NAME
EN
FB
GND
NC
SW
V
IN
DBV NO.
4
3
2
–
1
5
DRV NO.
3
4
1
5
6
2
I/O
DESCRIPTION
This is the enable pin of the device. Pulling this pin to ground forces the device into shutdown
mode reducing the supply current to less than 1
μA.
This pin should not be left floating and needs
to be terminated.
This is the feedback pin of the device. Connect this pin to the external voltage divider to program
the desired output voltage.
Ground
No connection
Connect the inductor and the Schottky diode to this pin. This is the switch pin and is connected to
the drain of the internal power MOSFET.
Supply voltage pin
I
I
–
–
I
I
DETAILED DESCRIPTION
OPERATION
The TPS61040/41 operates with an input voltage range of 1.8 V to 6 V and can generate output voltages up to
28 V. The device operates in a pulse-frequency-modulation (PFM) scheme with constant peak current control.
This control scheme maintains high efficiency over the entire load current range, and with a switching frequency
up to 1 MHz, the device enables the use of very small external components.
The converter monitors the output voltage, and as soon as the feedback voltage falls below the reference voltage
of typically 1.233 V, the internal switch turns on and the current ramps up. The switch turns off as soon as the
inductor current reaches the internally set peak current of typically 400 mA (TPS61040) or 250 mA (TPS61041).
See the
section for more information. The second criteria that turns off the switch is the
maximum on-time of 6
μs
(typical). This is just to limit the maximum on-time of the converter to cover for extreme
conditions. As the switch is turned off the external Schottky diode is forward biased delivering the current to the
output. The switch remains off for a minimum of 400 ns (typical), or until the feedback voltage drops below the
reference voltage again. Using this PFM peak current control scheme the converter operates in discontinuous
conduction mode (DCM) where the switching frequency depends on the output current, which results in very high
efficiency over the entire load current range. This regulation scheme is inherently stable, allowing a wider
selection range for the inductor and output capacitor.
PEAK CURRENT CONTROL
The internal switch turns on until the inductor current reaches the typical dc current limit (I
LIM
) of 400 mA
(TPS61040) or 250 mA (TPS61042). Due to the internal propagation delay of typical 100 ns, theactualcurrent
exceeds the dc current limit threshold by a small amount. The typical peak current limit can be calculated:
V
I
+
I
)
IN
100 ns
peak(typ)
LIM
L
V
I
+
400 mA
)
IN
100 ns for the TPS61040
peak(typ)
L
V
I
+
250 mA
)
IN
100 ns for the TPS61041
peak(typ)
L
(1)
The higher the input voltage and the lower the inductor value, the greater the peak.
By selecting the TPS61040 or TPS61041, it is possible to tailor the design to the specific application current limit
requirements. A lower current limit supports applications requiring lower output power and allows the use of an
inductor with a lower current rating and a smaller form factor. A lower current limit usually has a lower output
voltage ripple as well.
Copyright © 2002–2007, Texas Instruments Incorporated
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