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LM3691

SNVS506H – MAY 2008 – REVISED SEPTEMBER 2011

www.ti.com

THERMAL SHUTDOWN PROTECTION

The LM3691 has a thermal overload protection function that operates to protect itself from short-term misuse and

overload conditions. When the junction temperature exceeds around 150°C, the device inhibits operation. Both

the PFET and the NFET are turned off. When the temperature drops below 130°C, normal operation resumes.

Prolonged operation in thermal overload conditions may damage the device and is considered bad practice.

OVER-TEMPERATURE MAXIMUM LOAD RECOMMENDATIONS

VIN

Maximum Load

2.5V to 5.5V

1000 mA

2.3V to 2.5V

650 mA

Application Information

INDUCTOR SELECTION

DC bias current characteristics of inductors must be considered. Different manufacturers follow different

saturation current rating specifications, so attention must be given to details. DC bias curves should be requested

from them as part of the inductor selection process.

over the ambient temp range. The inductor’s DC resistance should be less than 0.1

Ω for good efficiency at

high current condition. The inductor AC loss (resistance) also affects conversion efficiency. Higher Q factor at

switching frequency usually gives better efficiency at light load to middle load.

Table 1 lists suggested inductors and suppliers.

INPUT CAPACITOR SELECTION

A ceramic input capacitor of 4.7 µF, 6.3V/10V is sufficient for most applications. Place the input capacitor as

to improve input voltage filtering. Use X7R, X5R or B types; do not use Y5V or F. DC bias characteristics of

ceramic capacitors must be considered when selecting case sizes like 0402. Minimum input capacitance to

guarantee good performance is 2.2 µF at maximum input voltage DC bias including tolerances and over

ambient temp range.

The input filter capacitor supplies current to the PFET (high-side) switch in the first half of each cycle and

reduces voltage ripple imposed on the input power source. A ceramic capacitor's low ESR provides the best

noise filtering of the input voltage spikes due to this rapidly changing current. Select an input filter capacitor with

sufficient ripple current rating. The input current ripple can be calculated as:

(1)

OUTPUT CAPACITOR SELECTION

Use a 4.7

μF, 6.3V ceramic capacitor, X7R, X5R or B types; do not use Y5V or F. DC bias voltage characteristics

of ceramic capacitors must be considered. DC bias characteristics vary from manufacturer to manufacturer, and

DC bias curves should be requested from them as part of the capacitor selection process. The output filter

capacitor smooths out current flow from the inductor to the load, helps maintain a steady output voltage during

transient load changes and reduces output voltage ripple. These capacitors must be selected with sufficient

capacitance and sufficiently low ESR to perform these functions. Minimum output capacitance to guarantee

good performance is 2.2 µF at the output voltage DC bias including tolerances and over ambient temp

range.

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