TPS61045
SLVS440A – JANUARY 2003 – REVISED SEPTEMBER 2003
www.ti.com
Typical Characteristics (continued)
V
O
= 5 V/Div
CTRL 2 V/Div
I
I
= 50 mA/Div
500
µs/Div
Figure 15 . Soft Start
DETAILED DESCRIPTION
OPERATION
The TPS61045 operates with an input voltage range of 1.8 V to 6.0 V and generates 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 of up
to 1 MHz, the device enables the use of small external components.
The converter monitors the output voltage and when the feedback voltage falls below the reference voltage of
1.233 V (typ) the main switch turns on and the current ramps up. The main switch turns off when the inductor
current reaches the internally set peak current of 375 mA (typ). Refer to the
peak current controlsection
for more
information. The second criteria that turns off the main switch is the maximum on-time of 6 µs (typ). This limits
the maximum on-time of the converter in extreme conditions. As the switch is turned off, the external Schottky
diode is forward biased delivering the current to the output. The main switch remains off until the minimum off
time of 400 ns (typ) has passed and the feedback voltage is 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 input voltage, output voltage and output current. This gives a high efficiency
over the entire load current range. This regulation scheme is inherently stable which allows a wider range for the
selection of the inductor and output capacitor.
PEAK CURRENT CONTROL
The internal switch is turned on until the inductor current reaches the typical dc current limit (I
LIM
) of 375 mA. Due
to the internal current limit delay of 100 ns (typ) the actual current exceeds the dc current limit threshold by a
small amount. The typical peak current limit can be calculated:
V
I
+
I
)
I 100 ns
P(typ)
(LIM)
L
(1)
I
+
400 mA
)
I
P(typ)
L
V
100 ns
The higher the input voltage and the lower the inductor value, the greater the current limit overshoot.
8
TI [ TEXAS INSTRUMENTS ]
