LM1577 datasheet(13/27 Pages) NSC

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Application Hints (Continued)

STEP-UP (BOOST) REGULATOR

Figure 4 shows the LM1577-ADJ/LM2577-ADJ used as a

Step-Up Regulator. This is a switching regulator used for

producing an output voltage greater than the input supply

voltage.

The

LM1577-12/LM2577-12

and

LM1577-15/

LM2577-15 can also be used for step-up regulators with 12V

or 15V outputs (respectively), by tying the feedback pin di-

rectly to the regulator output.

A basic explanation of how it works is as follows. The

LM1577/LM2577 turns its output switch on and off at a fre-

quency of 52 kHz, and this creates energy in the inductor (L).

When the NPN switch turns on, the inductor current charges

up at a rate of V

IN/L, storing current in the inductor. When the

switch turns off, the lower end of the inductor flies above V

IN,

discharging its current through diode (D) into the output ca-

pacitor (C

OUT) at a rate of (VOUT −VIN)/L. Thus, energy

stored in the inductor during the switch on time is transferred

to the output during the switch off time. The output voltage is

controlled by the amount of energy transferred which, in turn,

is controlled by modulating the peak inductor current. This is

done by feeding back a portion of the output voltage to the

error amp, which amplifies the difference between the feed-

back voltage and a 1.230V reference. The error amp output

voltage is compared to a voltage proportional to the switch

current (i.e., inductor current during the switch on time).

The comparator terminates the switch on time when the two

voltages are equal, thereby controlling the peak switch cur-

rent to maintain a constant output voltage.

Voltage and current waveforms for this circuit are shown in

Figure 5, and formulas for calculating them are given in Fig-

ure 6.

STEP-UP REGULATOR DESIGN PROCEDURE

The following design procedure can be used to select the ap-

propriate external components for the circuit in

Figure 4,

based on these system requirements.

Given:

IN (min) = Minimum input supply voltage

OUT = Regulated output voltage

LOAD(max) = Maximum output load current

Before proceeding any further, determine if the LM1577/

LM2577 can provide these values of V

OUT and ILOAD(max)

when operating with the minimum value of V

IN. The upper

limits for V

OUT and ILOAD(max) are given by the following

equations.

OUT ≤ 60V

and

OUT ≤ 10xVIN(min)

These limits must be greater than or equal to the values

specified in this application.

1. Inductor Selection (L)

Voltage Options:

1. For 12V or 15V output

DS011468-11

FIGURE 5. Step-Up Regulator Waveforms

Duty Cycle

Average

Inductor

Current

IND(AVE)

Inductor

Current

Ripple

∆I

IND

Peak

Inductor

Current

IND(PK)

Peak Switch

Current

SW(PK)

Switch

Voltage

When Off

SW(OFF)

OUT +VF

Diode

Reverse

Voltage

OUT −VSAT

Average

Diode

Current

D(AVE)

LOAD

Peak Diode

Current

D(PK)

Power

Dissipation

LM1577/2577

VF = Forward Biased Diode Voltage

ILOAD = Output Load Current

FIGURE 6. Step-Up Regulator Formulas

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