6
LT1372/LT1377
The LT1372/LT1377 are current mode switchers. This
means that switch duty cycle is directly controlled by
switch current rather than by output voltage. Referring to
the block diagram, the switch is turned “On” at the start of
each oscillator cycle. It is turned “Off” when switch current
reaches a predetermined level. Control of output voltage is
obtained by using the output of a voltage sensing error
amplifier to set current trip level. This technique has
several advantages. First, it has immediate response to
input voltage variations, unlike voltage mode switchers
which have notoriously poor line transient response.
Second, it reduces the 90° phase shift at mid-frequencies
in the energy storage inductor. This greatly simplifies
closed-loop frequency compensation under widely vary-
ing input voltage or output load conditions. Finally, it
allows simple pulse-by-pulse current limiting to provide
maximum switch protection under output overload or
short conditions. A low dropout internal regulator pro-
vides a 2.3V supply for all internal circuitry. This low
dropout design allows input voltage to vary from 2.7V to
25V with virtually no change in device performance. A
500kHz (LT1372) or 1MHz (LT1377) oscillator is the basic
clock for all internal timing. It turns “On” the output switch
via the logic and driver circuitry. Special adaptive anti-sat
circuitry detects onset of saturation in the power switch
and adjusts driver current instantaneously to limit switch
saturation. This minimizes driver dissipation and provides
very rapid turn-off of the switch.
A 1.245V bandgap reference biases the positive input of
the error amplifier. The negative input of the amplifier is
brought out for positive output voltage sensing. The error
amplifier has nonlinear transconductance to reduce out-
put overshoot on start-up or overload recovery. When
the feedback voltage exceeds the reference by 40mV,
error amplifier transconductance increases ten times,
which reduces output overshoot. The feedback input also
invokes oscillator frequency shifting, which helps pro-
tect components during overload conditions. When the
feedback voltage drops below 0.6V, the oscillator fre-
quency is reduced 5:1. Lower switching frequency allows
full control of switch current limit by reducing minimum
switch duty cycle.
Unique error amplifier circuitry allows the LT1372/LT1377
to directly regulate negative output voltages. The negative
feedback amplifier’s 100k source resistor is brought out
for negative output voltage sensing. The NFB pin regulates
at –2.49V while the amplifier output internally drives the
FB pin to 1.245V. This architecture, which uses the same
main error amplifier, prevents duplicating functions and
maintains ease of use. Consult Linear Technology Market-
ing for units that can regulate down to –1.25V.
The error signal developed at the amplifier output is
brought out externally. This pin (V
C
) has three different
functions. It is used for frequency compensation, current
limit adjustment and soft starting. During normal regula-
tor operation this pin sits at a voltage between 1V (low
output current) and 1.9V (high output current). The error
amplifier is a current output (g
m
) type, so this voltage can
be externally clamped for lowering current limit. Like-
wise, a capacitor coupled external clamp will provide soft
start. Switch duty cycle goes to zero if the V
C
pin is pulled
below the control pin threshold, placing the LT1372/
LT1377 in an idle mode.
Positive Output Voltage Setting
The LT1372/LT1377 develops a 1.245V reference (V
REF
)
from the FB pin to ground. Output voltage is set by
connecting the FB pin to an output resistor divider
(Figure 1). The FB pin bias current represents a small
error and can usually be ignored for values of R2 up to 7k.
The suggested value for R2 is 6.19k. The NFB pin is
normally left open for positive output applications.
Figure 1. Positive Output Resistor Divider
R1
V
OUT
= V
REF
1 +
R2
FB
PIN
V
REF
V
OUT
()
R1
R2
R1 = R2
– 1
()
V
OUT
1.245
LT1372 • F01
OPERATIO
U
APPLICATIO S I FOR ATIO
WUUU
