section, the soft start curent has a temperature coefficient

and would lead one to believe there would be significant

variation with temperature. Though the current has a tem-

perature coefficient the actual programmed external soft

start time does not show this extreme of a temperature

variation. As you can see in the following transient plots:

V

Ω,C

Trace:

1) V

2) V

3) I

4) V

20051169

T

20051170

T

20051171

T

When programming the softstart time externally, simply use

the equation given in the Soft-Start Capacitor section above.

This equation uses the typical room temperature value of the

soft start current, 12µA, to set the soft start time.

DELAY CAPACITOR

The LM2702 has internal circuitry that can be used to set a

delay time preventing control of the PMOS switch via SWC

until a desired amount of time after the switcher starts up.

The PMOS control circuitry remains inactive until V

OUT

reaches 85% of the nominal output voltage. When this oc-

curs, C

pin reaches 1.265V the PMOS switch will become active and

can be controlled using the SWC pin. If no C

PMOS switch can be controlled immediately after V

OUT

reaches 85% of the nominal output voltage. The delay time

can be calculated using the equation:

T

INTRODUCTION TO COMPENSATION

The LM2702 contains a current mode PWM boost converter.

The signal flow of this control scheme has two feedback

loops, one that senses switch current and one that senses

output voltage.

To keep a current programmed control converter stable

above duty cycles of 50%, the inductor must meet certain

criteria. The inductor, along with input and output voltage,

will determine the slope of the current through the inductor

(see Figure 2 (a)). If the slope of the inductor current is too

great, the circuit will be unstable above duty cycles of 50%.

A 4.7µH inductor is recommended for most applications. If

the duty cycle is approaching the maximum of 85%, it may

be necessary to increase the inductance by as much as 2X.

See Inductor and Diode Selection for more detailed inductor

sizing.

The LM2702 provides a compensation pin (V

the voltage loop feedback. It is recommended that a series

combination of R

network, as shown in the typical application circuit. For any

given application, there exists a unique combination of R

C

and C

circuit in terms of its transient response. The series combi-

nation of R

the following equations:

20051105

FIGURE 2. (a) Inductor current. (b) Diode current.

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