Datasheet

Hysteretic
V
CL0
V
SC
LM3477A
V
SC
LM3477
V
HYS
LM3477
(32mV)
V
HYS
LM3477A
(11mV)
S
e
D
(MIN)
D
(MAX)
Hysteretic
Hysteretic
PWM
PWM
LM3477A
LM3477
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 R
SN
. The LM3477A has a lower hysteretic
threshold voltage V
HYS
, and thus will operate in PWM mode for a larger load range than the LM3477. Typically,
V
HYS
= 32mV for the LM3477, while V
HYS
= 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 V
SC
.
V
SC
is a ceiling limit for the peak sense voltage V
SNpk
(see SHORT CIRCUIT PROTECTION). V
SC
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 V
FB
+ V
OVP
, OVP is triggered. See
Electrical Characteristics for limits on V
FB
and V
OVP
.
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