COUT

CFLY

FLY

SW

SW

OUT

ESR

ESR

4

C

F

1

R

2

R

˜

u

˜

Reg

2×

V '

2×V '

LM2750

Output Resistance Model

LM2750

www.ti.com

SNVS180L – APRIL 2002 – REVISED MAY 2013

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 provides 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

issues is recommended and a thorough analysis of the application circuit should be performed when using the

part outside 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 the POWER EFFICIENCY

AND POWER DISSIPATION section, to come).

The schematic of Figure 13 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 (2×), and an output resistance

output resistance of the LM2750 is 5

Ω (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

increases. To prevent droop in output voltage, the voltage drop across the regulator is reduced, V' increases, and

regulator, V' equals the input voltage, and the output voltage is "on the edge" of regulation. Additional output

current causes the output voltage to fall out of regulation, and the LM2750 operation is similar to a basic open-

loop doubler. As in a voltage doubler, increase in output current results in output voltage drop proportional to the

output resistance of the doubler. The out-of-regulation LM2750 output voltage can be approximated by:

Again, this equation only applies at low input voltage and high output current where the LM2750 is not regulating.

See Output Current vs. Output Voltage curves in the Typical Performance Characteristics section for more

details.

Figure 13. LM2750 Output Resistance Model

A more complete calculation of output resistance takes into account the effects of switching frequency, flying

capacitance, and capacitor equivalent series resistance (ESR). This equation is shown below:

Switch resistance (5

Ω typ.) dominates the output resistance equation of the LM2750. With a 1.7MHz typical

switching frequency, the 1/(F×C) component of the output resistance contributes only 0.6

Ω to the total output

resistance. Increasing the flying capacitance will only provide minimal improvement to the total output current

capability of the LM2750. In some applications it may be desirable to reduce the value of the flying capacitor

below 1µF to reduce solution size and/or cost, but this should be done with care so that output resistance does

not increase to the point that undesired output voltage droop results. If ceramic capacitors are used, ESR will be

a negligible factor in the total output resistance, as the ESR of quality ceramic capacitors is typically much less

than 100m

Ω.

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