LM2653

SNVS050E – NOVEMBER 1999 – REVISED APRIL 2013

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INDUCTOR

The most critical parameters for the inductor are the inductance, peak current and the DC resistance. The

inductance is related to the peak-to-peak inductor ripple current, the input and the output voltages:

(2)

A higher value of ripple current reduces inductance, but increases the conductance loss, core loss, current stress

for the inductor and switch devices. It also requires a bigger output capacitor for the same output voltage ripple

requirement. A reasonable value is setting the ripple current to be 30% of the DC output current. Since the ripple

current increases with the input voltage, the maximum input voltage is always used to determine the inductance.

The DC resistance of the inductor is a key parameter for the efficiency. Lower DC resistance is available with a

bigger winding area. A good tradeoff between the efficiency and the core size is letting the inductor copper loss

equal 2% of the output power.

OUTPUT CAPACITOR

frequency, PWM mode is approximated by:

(3)

The ESR term usually plays the dominant role in determining the voltage ripple. A low ESR aluminum electrolytic

or tantalum capacitor (such as Nichicon PL series, Sanyo OS-CON, Sprague 593D, 594D, AVX TPS, and CDE

polymer aluminum) is recommended. An electrolytic capacitor is not recommended for temperatures below

−25°C since its ESR rises dramatically at cold temperature. A tantalum capacitor has a much better ESR

specification at cold temperature and is preferred for low temperature applications.

The output voltage ripple in constant frequency mode has to be less than the sleep mode voltage hysteresis to

avoid entering the sleep mode at full load:

(4)

BOOST CAPACITOR

A 0.1

μF ceramic capacitor is recommended for the boost capacitor. The typical voltage across the boost

capacitor is 6.7V.

SOFT-START CAPACITOR

A soft-start capacitor is used to provide the soft-start feature. When the input voltage is first applied, or when the

SD(SS) pin is allowed to go high, the soft-start capacitor is charged by a current source (approximately 2

μA).

When the SD(SS) pin voltage reaches 0.6V (shutdown threshold), the internal regulator circuitry starts to

operate. The current charging the soft-start capacitor increases from 2

μA to approximately 10 μA. With the

SD(SS) pin voltage between 0.6V and 1.3V, the level of the current limit is zero, which means the output voltage

is still zero. When the SD(SS) pin voltage increases beyond 1.3V, the current limit starts to increase. The switch

duty cycle, which is controlled by the level of the current limit, starts with narrow pulses and gradually gets wider.

At the same time, the output voltage of the converter increases towards the nominal value, which brings down

the output voltage of the error amplifier. When the output of the error amplifier is less than the current limit

voltage, it takes over the control of the duty cycle. The converter enters the normal current-mode PWM

operation. The SD(SS) pin voltage is eventually charged up to about 2V.

The soft-start time can be estimated as:

(5)

During start-up, the internal circuit is monitoring the soft-start voltage. When the softstart voltage reaches 2V, the

undervoltage and overvoltage protections are enabled.

If the output voltage doesn't rise above 80% of the normal value before the soft-start reaches 2V. The

undervoltage protection will kick in and shut the device down. You can avoid this by either increasing the value of

the soft-start capacitor, or using a LDELAY capacitor.

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