Datasheet

LM2750
SNVS180L –APRIL 2002–REVISED MAY 2013 www.ti.com
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 operation 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 2× the input voltage. The LM2750 regulates the output voltage by controlling
the resistance 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 essential 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 degradation. 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 management 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.
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