LP3942 datasheet(18/20 Pages) NSC

18 / 20 page

Charge Pump Operational

Description (Continued)

regulation LP3942 output voltage can be approximated by:

OUT=1.5xVIN –IOUT xROUT. Again, this equation only

applies at low input voltage and high output current where

the LP3942 is not regulating. See Output Current vs. Output

Voltage curves in the Typical Performance Characteristics

section for more details.

On 2x mode the functionality is similar, only the output

voltage is set to 5.0V and out-of-regulation output voltage

can be estimated by: V

OUT =2.0xVIN –IOUT xROUT. Output

resistance is approximately same as in 1.5x mode.

THERMAL SHUTDOWN

The LP3942 implements a thermal shutdown mechanism to

protect the device from damage due to overheating. When

the junction temperature rises to 160˚C (typ), the part

switches into Startup mode. The LP3942 releases thermal

shutdown when the junction temperature of the part is re-

duced to 140˚C (typ). Thermal shutdown is most-often trig-

gered by self-heating, which occurs when there is excessive

power dissipation in the device and/or insufficient thermal

dissipation. LP3942 power dissipation increases with in-

creased output current and input voltage (see Power Effi-

ciency and Power Dissipation section). Because of auto-

matic recovery from thermal shutdown function, thermal

cycling is the typical result. Thermal cycling is the repeating

process where the part self-heats, enters thermal shutdown,

cools, turns-on, and then heats up again to the thermal

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 dissipation performance must be improved to ac-

commodate the power dissipation of the LP3942. Fortu-

nately, the LLP package has excellent thermal properties

that, when soldered to a PCB designed to aid thermal dissi-

pation, allows the LP3942 to operate under very demanding

power dissipation conditions.

OUTPUT CURRENT LIMITING

The LP3942 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 300 mA (typ) when the

output is shorted directly to ground. When the LP3942 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).

Charge Pump Application

Information

OUTPUT VOLTAGE RIPPLE

The amount of voltage ripple on the output of the LP3942 is

highly dependent on the application conditions: output cur-

rent and the output capacitor, specifically. A simple approxi-

mation of output ripple is determined by calculating the

amount of voltage droop that occurs when the output of the

LP3942 is not being driven. This occurs during the charge

phase (

Φ1 ). During this time, the load is driven solely by the

charge on the output capacitor. The magnitude of the ripple

thus follows the basic discharge equation for a capacitor (I =

C x dV/dt), where discharge time is one-half the switching

period, or 0.5/FSW. Put simply,

A more thorough and accurate examination of factors that

affect ripple requires including effects of phase non-overlap

times and output capacitor equivalent series resistance

(ESR). In order for the LP3942 to operate properly, the two

phases of operation must never coincide. (If this were to

happen all switches would be closed simultaneously, short-

ing input, output, and ground). Thus, non-overlap time is built

into the clocks that control the phases. Since the output is

not being driven during the non-overlap time, this time

should be accounted for in calculating ripple. Actual output

capacitor discharge time is approximately 60% of a switch-

ing period, or 0.6/FSW.

The ESR of the output capacitor also contributes to the

output voltage ripple, as there is effectively an AC voltage

drop across the ESR due to current switching in and out of

the capacitor. The following equation is a more complete

calculation of output ripple than presented previously, taking

into account phase non-overlap time and capacitor ESR.

A low-ESR ceramic capacitor is recommended on the output

to keep output voltage ripple low. Placing multiple capacitors

in parallel can reduce ripple significantly, both by increasing

capacitance and reducing ESR. When capacitors are in

parallel, ESR is in parallel as well. The effective net ESR is

determined according to the properties of parallel resistance.

Two identical capacitors in parallel have twice the capaci-

tance and half the ESR as compared to a single capacitor of

the same make. On a similar note, if a large-value, high-ESR

capacitor (tantalum, for example) is to be used as the pri-

mary output capacitor, the net output ESR can be signifi-

cantly reduced by placing a low-ESR ceramic capacitor in

parallel with this primary output capacitor.

CAPACITORS

The LP3942 requires 4 external capacitors for proper opera-

tion. Surface-mount multi-layer ceramic capacitors are highly

recommended. These capacitors are small, inexpensive and

have very low equivalent series resistance (

≤ 10 mΩ. typ.).

Tantalum capacitors, OS-CON capacitors, and aluminum

electrolytic capacitors generally are not recommended for

use with the LP3942 due to their high ESR, as compared to

ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R

temperature characteristic are preferred for use with the

LP3942. These capacitors have tight capacitance tolerance

(as good as ±10%), hold their value over temperature (X7R:

±15% over −55˚C to +125˚C; X5R: ±15% over −55˚C to

+85˚C), and typically have little voltage coefficient. Capaci-

tors with Y5V and/or Z5U temperature characteristic are

generally not recommended. These types of capacitors typi-

cally have wide capacitance tolerance (+80%, −20%), varies

significantly over temperature (Y5V: +22%, −82% over

−30˚C to +85˚C range; Z5U: +22%, −56% over +10˚C to

+85˚C range), and has poor voltage coefficients. Under

some conditions, a nominal 1 µF Y5V or Z5U capacitor could

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