LM2757

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SNVS536F – OCTOBER 2007 – REVISED JULY 2015

Feature Description (continued)

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.

8.3.1 Efficiency Performance

Charge-pump efficiency is derived in Equation 1 and Equation 2 (supply current and other losses are neglected

for simplicity):

(1)

where

•

G = the charge pump gain

•

E = efficiency

(2)

data. The transition between gains of 3/2 and 2 are clearly distinguished by the sharp discontinuity in the

efficiency curve.

8.3.2 Soft Start

The LM2757 employs soft-start circuitry to prevent excessive input inrush currents during start-up. At start-up,

the output voltage gradually rises from 0 V to the nominal output voltage. This occurs in 300 µs (typical). Soft-

start is engaged when the device is enabled.

8.3.3 Thermal Shutdown

Protection from damage related to overheating is achieved with a thermal shutdown feature. When the junction

temperature rises to 145°C (typical), the device switches into shutdown mode. The LM2757 disengages thermal

shutdown when the junction temperature of the part is reduced to 135°C (typical). Due to the high efficiency of

the LM2757, thermal shutdown and/or thermal cycling are not encountered when the part is operated within

specified input voltage, output current, and ambient temperature operating ratings. If thermal cycling is seen

under these conditions, the most likely cause is an inadequate PCB layout that does not allow heat to be

sufficiently dissipated out of the device.

8.3.4 Current-Limit Protection

conditions where excessive current is drawn. Output current is limited to 250 mA (typical).

8.4 Device Functional Modes

8.4.1 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 450-k

Ω pulldown resistors connected internally to each of the mode select pins. The voltage mode is

selected according to Table 1.

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