Datasheet

LM2757
SNVS536E –OCTOBER 2007–REVISED MAY 2013 www.ti.com
OPERATION DESCRIPTION
Overview
The LM2757 is a switched capacitor converter that produces a regulated output voltage of either 5V, 4.5V or
4.1V depending on the mode selected. The core of the part is a highly efficient charge pump that utilizes fixed
frequency pre-regulation to minimize ripple and power losses over wide input voltage and output current ranges.
A description of the principal operational characteristics of the LM2757 is detailed in the Circuit Description and
Efficiency Performance sections. These sections refer to details in the Block Diagram.
Circuit Description
The core of the LM2757 is a two-phase charge pump controlled by an internally generated non-overlapping
clock. The charge pump operates by using external flying capacitors C
1
, C
2
to transfer charge from the input to
the output. At input voltages below 3.9V (typ.) for the 5V mode, the LM2757 operates in a 2x gain, with the input
current being equal to 2x the load current. At input voltages above 3.9V (typ.) for the 5V mode, the part utilizes a
gain of 3/2x, resulting in an input current equal to 3/2 times the load current. For the 4.5V mode, the LM2757
operates in a 2x gain when the input voltage is below 3.35V (typ.) and transitions to a 3/2x gain when the input
voltage is above 3.35V (typ.). For the 4.1V mode, the device utilizes the 3/2x gain for the entire input voltage
range.
The two phases of the switched capacitor switching cycle will be referred to as the "phase one" and the "phase
two". During phase one, one flying capacitor is charged by the input supply while the other flying capacitor is
connected to the output and delivers charge to the load . After half of the switching cycle [ t = 1/(2×F
SW
) ], the
LM2757 switches to phase two. In this configuration, the capacitor that supplied charge to the load in phase one
is connected to the input to be recharged while the capacitor that had been charged in the previous phase is
connected to the output to deliver charge. With this topology, output ripple is reduced by delivering charge to the
output in every phase.
The LM2757 uses fixed frequency pre-regulation to regulate the output voltage. The input and output
connections of the flying capacitors are made with internal MOS switches. Pre-regulation limits the gate drive of
the MOS switch connected between the voltage input and the flying capacitors. Controlling the on resistance of
this switch limits the amount of charge transferred into and out of each flying capacitor during the charge and
discharge phases, and in turn helps to keep the output ripple very low.
Efficiency Performance
Charge-pump efficiency is derived in the following two ideal equations (supply current and other losses are
neglected for simplicity):
I
IN
= G × I
OUT
E = (V
OUT
× I
OUT
) ÷ (V
IN
× I
IN
) = V
OUT
÷ (G × V
IN
)
In the equations, G represents the charge pump gain. Efficiency is at its highest as G×V
IN
approaches V
OUT
.
Refer to the efficiency graph in the Typical Performance Characteristics section for detailed efficiency data. The
transition between gains of 3/2, and 2 are clearly distinguished by the sharp discontinuity in the efficiency curve.
Enable and Voltage Mode Selection
The LM2757 is enabled when either one of the mode select pins (M0, M1) has a logic High voltage applied to it.
There are 450kΩ pulldown resistors connected internally to each of the mode select pins. The voltage mode is
selected according to the following table.
M0 M1 Output Voltage Mode
0 0 Device Shutdown, Output High Impedance
0 1 5V
1 0 4.5V
1 1 4.1V
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