VCL0

VSC

LM3477A

VSC

LM3477

VHYS

LM3477

(32mV)

VHYS

LM3477A

(11mV)

Se

D(MIN)

D(MAX)

Hysteretic

Hysteretic

PWM

PWM

LM3477

SNVS141K – OCTOBER 2000 – REVISED MARCH 2013

www.ti.com

FUNCTIONAL DESCRIPTION

GENERAL DESCRIPTION

The LM3477/A is a switching regulator controller for topologies incorporating a high side switch. The most

common of these topologies is the step-down, or buck, converter. Other topologies such as the inverting (buck-

boost) and inverse SEPIC (zeta) converters can be realized. This datasheet will focus on buck converter

applications.

The LM3477/A employs current mode control architecture. Among the many benefits of this architecture are

superior line and load regulation, cycle-by-cycle current limiting, and simple loop compensation. The LM3477/A

features a patented adjustable slope compensation scheme to enable flexible inductor selection. The LM3477/A

has a combination of features that allow its use in a wide variety of applications. The input voltage can range

from 2.97V to 35V, with the output voltage being positive or negative depending on the topology. The current

limit can be scaled to safely drive a wide range of loads. An internal soft-start is provided to limit initial in-rush

current. Output over voltage and input under voltage protection ensure safe operation of the LM3477/A.

REGIONS OF OPERATION

Pulse width modulation (PWM) is the normal mode of operation. In PWM, the output voltage is well regulated

and has a ripple frequency equal to the switching frequency (500kHz). In low load conditions, the part operates in

hysteretic mode. In this mode, the output voltage is regulated between a high and low value that results in a

higher ripple magnitude and lower ripple frequency than in PWM mode (see OVER VOLTAGE PROTECTION).

Figure 19. Operating Regions of the LM3477/A

The important differences between the LM3477 and the LM3477A are summarized in Figure 19. The voltages in

Figure 19 can be referred to the switch current by dividing through by RSN. The LM3477A has a lower hysteretic

threshold voltage VHYS, and thus will operate in PWM mode for a larger load range than the LM3477. Typically,

VHYS = 32mV for the LM3477, while VHYS = 11mV for the LM3477A. The difference in area between the shaded

regions give a graphical representation of this. The lightly shaded region is the extra PWM operating area gained

by using the LM3477A. Thus the benefits of operating in PWM mode such as a well regulated output voltage with

low noise ripple are extended to a larger load range when the LM3477A is used. While less significant, the other

noteworthy difference between the two parts is in the short circuit current limit VSC.

VSC is a ceiling limit for the peak sense voltage VSNpk (see SHORT CIRCUIT PROTECTION). VSC is lower in the

LM3477A than in the LM3477 (see the Electrical Characteristics for limits).

OVER VOLTAGE PROTECTION

The LM3477/A has over voltage protection (OVP) for the output voltage. OVP is sensed at and is in respect to

the feedback pin (pin 3). If at anytime the voltage at the feedback pin rises to VFB + VOVP, OVP is triggered. See

Electrical Characteristics for limits on VFB and VOVP.

10

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