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 to-

wards the nominal value, which brings down the output volt-

age of the error amplifier. When the output of the error ampli-

fier 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:

T

During start-up, the internal circuit is monitoring the soft-start

voltage. When the softstart voltage reaches 2V, the under-

voltage and overvoltage protections are enabled.

If the output voltage doesn’t rise above 80% of the normal

value before the soft-start reaches 2V, undervoltage protec-

tion shut down the device. You can avoid this by either in-

creasing the value of the soft-start capacitor, or using a LDE-

LAY capacitor.

LDELAY CAPACITOR

The LDELAY capacitor (CDELAY) provides a means to con-

trol undervoltage latch protection. By changing CDELAY, the

user can adjust the time delay between the output voltage

dropping below 80% of its nominal value and the part shut-

ting off due to undervoltage latch protection. The LDELAY

circuit consists ofa5µA current source in series with a user

defined capacitor, CDELAY. The 5 µA current source is

turned on whenever the output voltage is below 80% of its

nominal value, otherwise this current source is off. With the

output voltage below 80% of its nominal value, the 5 µA cur-

rent source begins to charge CDELAY, as shown in

Figure 2.

If the potential across CDELAY reaches 2V, undervoltage

latch protection will be enabled and the part will shutdown. If

the output voltage recovers to above 80% of its nominal

value before the potential across CDELAY reaches 2V, und-

ervoltage latch protection will remain disabled. Hence, CDE-

LAY sets a time delay by the following equation:

T

Undervoltage latch protection can be disabled by tying the

LDELAY pin to the ground.

COMPENSATION COMPONENTS

In the control to output transfer function, the first pole F

be estimated as 1/(2

πR

output capacitor is 1/(2

πESRC

quency pole F

F

where D = V

and V

The total loop gain G is approximately 1000/I

is in amperes.

A Gm amplifier is used inside the LM2655. The output resis-

tor R

gether with R

F

In some applications, the ESR zero F

by F

ESR zero, F

The rule of thumb is to have more than 45˚ phase margin at

the crossover frequency (G=1).

If C

good choices for most applications. If the ESR zero is too

low to be cancelled by F

If the transient response to a step load is important, choose

R

APPLICATION CIRCUITS

PROGRAMMABLE OUTPUT VOLTAGE

Using the adjustable output version of the LM2655 as shown

in

Figure 3, output voltages between 1.24V and 13V can be

achieved. Use the following formula to select the appropriate

resistor values:

R

where V

Select resistors between 10k

Ω and 100kΩ.(1% or higher ac-

curacy metal film resistors for R

DS101284-22

FIGURE 2. Undervoltage latch protection.

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