LM2750
Operation Description
OVERVIEW
The LM2750 is a regulated switched capacitor doubler that,
by combining the principles of switched-capacitor voltage
boost and linear regulation, generates a regulated output
from an extended Li-Ion input voltage range. A two-phase
non-overlapping clock generated internally controls the op-
eration of the doubler. During the charge phase (φ1), the
flying capacitor (C
FLY
) is connected between the input and
ground through internal pass-transistor switches and is
charged to the input voltage. In the pump phase that follows
(φ2), the flying capacitor is connected between the input and
output through similar switches. Stacked atop the input, the
charge of the flying capacitor boosts the output voltage and
supplies the load current.
A traditional switched capacitor doubler operating in this
manner will use switches with very low on-resistance to
generate an output voltage that is 2x the input voltage. The
LM2750 regulates the output voltage by controlling the re-
sistance of the two input-connected pass-transistor switches
in the doubler.
PRE-REGULATION
The very low input current ripple of the LM2750, resulting
from internal pre-regulation, adds very little noise to the input
line. The core of the LM2750 is very similar to that of a basic
switched capacitor doubler: it is composed of four switches
and a flying capacitor (external). Regulation is achieved by
modulating the on-resistance of the two switches connected
to the input pin (one switch in each phase). The regulation is
done before the voltage doubling, giving rise to the term
"pre-regulation". It is pre-regulation that eliminates most of
the input current ripple that is a typical and undesirable
characteristic of a many switched capacitor converters.
INPUT, OUTPUT, AND GROUND CONNECTIONS
Making good input, output, and ground connections is es-
sential to achieve optimal LM2750 performance. The two
input pads, pads 8 and 9, must be connected externally. It is
strongly recommended that the input capacitor (C
IN
) be
placed as close as possible to the LM2750, so that the traces
from the input pads are as short and straight as possible. To
minimize the effect of input noise on LM2750 performance, it
is best to bring two traces out from the LM2750 all the way to
the input capacitor pad, so that they are connected at the
capacitor pad. Connecting the two input traces between the
input capacitor and the LM2750 input pads could make the
LM2750 more susceptible to noise-related performance deg-
radation. It is also recommended that the input capacitor be
on the same side of the PCB as the LM2750, and that traces
remain on this side of the board as well (vias to traces on
other PCB layers are not recommended between the input
capacitor and LM2750 input pads).
The two output pads, pads 1 and 2, must also be connected
externally. It is recommended that the output capacitor
(C
OUT
) be placed as close to the LM2750 output pads as
possible. It is best if routing of output pad traces follow
guidelines similar to those presented for the input pads and
capacitor. The flying capacitor (C
FLY
) should also be placed
as close to the LM2750 as possible to minimize PCB trace
length between the capacitor and the IC. Due to the pad-
layout of the part, it is likely that the trace from one of the
flying capacitor pads (C+ or C-) will need to be routed to an
internal or opposite-side layer using vias. This is acceptable,
and it is much more advantageous to route a flying capacitor
trace in this fashion than it is to place input traces on other
layers.
The GND pads of the LM2750 are ground connections and
must be connected externally. These include pads 3
(LM2750-5.0 only), 5, 6 and the die-attach pad (DAP). Large,
low impedance copper fills and via connections to an internal
ground plane are the preferred way of connecting together
the ground pads of the LM2750, the input capacitor, and the
output capacitor, as well as connecting this circuit ground to
the system ground of the PCB.
SHUTDOWN
When the voltage on the active-low-logic shutdown pin is
low, the LM2750 will be in shutdown mode. In shutdown, the
LM2750 draws virtually no supply current. There is a 200kΩ
pull-down resistor tied between the SD pin and GND that
pulls the SD pin voltage low if the pin is not driven by a
voltage source. When pulling the part out of shutdown, the
voltage source connected to the SD pin must be able to drive
the current required by the 200kΩ resistor. For voltage man-
agement purposes required upon startup, internal switches
connect the output of the LM2750 to an internal pull-down
resistor (1kΩ typ) when the part is shutdown. Driving the
output of the LM2750 by another supply when the LM2750 is
shutdown is not recommended, as the pull-down resistor
was not sized to sink continuous current.
SOFT START
The LM2750 employs soft start circuitry to prevent excessive
input inrush currents during startup. The output voltage is
programmed to rise from 0V to the nominal output voltage
(5.0V) in 500µs (typ.). Soft-start is engaged when a part, with
input voltage established, is taken out of shutdown mode by
pulling the SD pin voltage high. Soft-start will also engage
when voltage is established simultaneously to the input and
SD pins.
OUTPUT CURRENT CAPABILITY
The LM2750-5.0 is guaranteed to provide 120mA of output
current when the input voltage is within 2.9V-to-5.6V. Using
the LM2750 to drive loads in excess of 120mA is possible.
IMPORTANT NOTE: Understanding relevant application is-
sues is recommended and a thorough analysis of the appli-
cation circuit should be performed when using the part out-
side operating ratings and/or specifications to ensure
satisfactory circuit performance in the application. Special
care should be paid to power dissipation and thermal effects.
These parameters can have a dramatic impact on high-
current applications, especially when the input voltage is
high. (see "Power Efficiency and Power Dissipation" section,
to come).
The schematic of
is a simplified model of the
LM2750 that is useful for evaluating output current capability.
The model shows a linear pre-regulation block (Reg), a
voltage doubler (2x), and an output resistance (R
OUT
). Out-
put resistance models the output voltage droop that is inher-
ent to switched capacitor converters. The output resistance
of the LM2750 is 5Ω (typ.), and is approximately equal to
twice the resistance of the four LM2750 switches. When the
output voltage is in regulation, the regulator in the model
controls the voltage V’ to keep the output voltage equal to
5.0V
±
4%. With increased output current, the voltage drop
across R
OUT
increases. To prevent droop in output voltage,
the voltage drop across the regulator is reduced, V’ in-
creases, and V
OUT
remains at 5V. When the output current
8
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