Datasheet

LM2650
SNVS133C –JUNE 1999–REVISED APRIL 2013 www.ti.com
– The PWM circuit drives the pulse stream into the low-pass filter made up of L1 and C
OUT
. The filter
passed the DC component of the stream and attenuates the AC components. The output of the filter is
the DC voltage V
OUT
superimposed with a small ripple voltage. Since the DC component of any periodic
waveforms the average value of the waveform, V
OUT
can be found using:
(1)
– Here T is the switching period in seconds V(t) is the pulse stream. Under DC steady-state conditions, (1)
yields
(2)
– Here V
IN
is the input voltage, and therefore the height of the pulses, in volts, is the width of the pulses in
seconds, and D is the ratio of t
ON
to T, the duty or the duty cycle.
– The output voltage is programmed using the resistive divider made up for R1 and R2,
(3)
– As Q1 turns on, its source voltage swings up to just below the input voltage. The LM2650 uses a simple
technique called "bootstrapping" to pull the positive supply rail of DH (at BOOT) up along with the source
voltage of Q1, but to a voltage above the input voltage. Because the source of Q1 and the positive supply
rail of DH make the same voltage swing together, DH maintains the positive gate-to-source voltage
required to turn Q1 on. Q12 plays an active role in pulling the supply rail of DH up and is therefore said to
pull itself up by its "bootstraps", thus the name of the technique and of the BOOT pin.
– In the typical application, a capacitor CB is connected outside the IC between the BOOT and SW pins.
When Q2 is on, the input supply charges CB through VRegH and the internal diode D.
THE HYSTERETIC CIRCUIT AND LOOP (Figure 16)
Except for C2, the hysteretic circuit borrows all its circuit blocks from the PWM circuit.
The hysteretic comparator C2 is a voltage comparator with built-in hysteresis V
HYST
of typically 30mV centered at
1.25V.
The diode D2 is the body diode of Q2. The hysteretic circuit uses D2 as a rectifier instead of switching Q2 as a
synchronous rectifier.
When the load current drops below the prescribed sleep-in threshold, the LM2650 shuts down the PWM loop and
starts up the hysteretic loop. The hysteretic loop supports light loads more efficiently because it uses less power
to support its own operation; it uses less bias power because it's a simpler loop having less circuit blocks to bias,
and it switches slower, so it incurs lower switching losses.
The hysteretic control loop does not switch at a constant frequency. Instead, it monitors V
OUT
and switches only
when V
OUT
reaches either side of a narrow window centered on the desired output voltage. C2 directs the
switching based on its reading of the feedback voltage. Switching in this manner yields a regulated voltage
consisting of the desired output voltage and an AC ripple voltage. The magnitude of the AC component can be
approximated using
(4)
For example, with V
OUT
set to 5V, V
OUT_PP
is approximately 120mV,
(5)
When it starts up, the hysteretic loop turns Q1 on. While Q1 is on, the input power supply charges C
OUT
and
supplies current to the load. Current from the supply reaches C and the load via the series path provided by Q1
and L1. As the feedback voltage just surpasses the upper hysteretic threshold of C2, the output of C2 changes
from high to low, and HD responds by pulling the gate of Q1 down turning Q1 off. As Q1 turns off, L1 generates
a negative-going voltage transient that D2 clamps at just below ground. D2 remains on only briefly as the current
in L1 runs out. While both Q1 and D2 are off, C
OUT
alone supplies current to the load. As the feedback voltage
just surpasses the lower hysteretic threshold of C2, the output of C2 changes states from low to high, and DH
responds by pulling the gate of Q1 up turning Q1 on and starting the hysteretic cycle over.
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