increases to the point that there is zero voltage drop across

the 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 out-

put voltage can be approximated by:

V

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.

A more complete calculation of output resistance takes into

account the effects of switching frequency, flying capaci-

tance, 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/(FxC) component of the output resistance

contributes only 0.6

Ω to the total output resistance. Increas-

ing the flying capacitance will only provide minimal improve-

ment 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 resis-

tance 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.

THERMAL SHUTDOWN

The LM2750 implements a thermal shutdown mechanism to

protect the device from damage due to overheating. When

switches into shutdown mode. The LM2750 releases thermal

shutdown when the junction temperature of the part is re-

Thermal shutdown is most-often triggered by self-heating,

which occurs when there is excessive power dissipation in

the device and/or insufficient thermal dissipation. LM2750

power dissipation increases with increased output current

and input voltage (see Power Efficiency and Power Dissi-

pation section). When self-heating brings on thermal shut-

down, thermal cycling is the typical result. Thermal cycling is

the repeating process where the part self-heats, enters ther-

mal shutdown (where internal power dissipation is practically

zero), cools, turns-on, and then heats up again to the ther-

mal shutdown threshold. Thermal cycling is recognized by a

pulsing output voltage and can be stopped be reducing the

internal power dissipation (reduce input voltage and/or out-

put current) or the ambient temperature. If thermal cycling

occurs under desired operating conditions, thermal dissipa-

tion performance must be improved to accommodate the

power dissipation of the LM2750. Fortunately, the LLP pack-

age has excellent thermal properties that, when soldered to

a PCB designed to aid thermal dissipation, allows the

LM2750 to operate under very demanding power dissipation

conditions.

OUTPUT CURRENT LIMITING

The LM2750 contains current limit circuitry that protects the

device in the event of excessive output current and/or output

shorts to ground. Current is limited to 300mA (typ.) when the

output is shorted directly to ground. When the LM2750 is

current limiting, power dissipation in the device is likely to be

quite high. In this event, thermal cycling should be expected

(see Thermal Shutdown section).

PROGRAMMING THE OUTPUT VOLTAGE OF THE

LM2750-ADJ

As shown in the application circuit of Figure 2, the output

voltage of the LM2750-ADJ can be programmed with a

simple resistor divider (see resistors R1 and R2). The values

of the feedback resistors set the output voltage, as deter-

mined by the following equation:

V

In the equation above, the "1.23V" term is the nominal

voltage of the feedback pin when the feedback loop is cor-

rectly established and the part is operating normally. The

sum of the resistance of the two feedback resistors should

be between 15k

Ω and 20kΩ:

15k

If larger feedback resistors are desired, a 10pF capacitor

should be placed in parallel with resistor R1.

20035109

FIGURE 1. LM2750 Output Resistance Model

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