LT1513/LT1513-2
BLOCK DIAGRAM
S/S
4k
I
FB
50k*
V
FB
1.245V
REF
*REMOVE ON LT1513-2
OPERATION
The LT1513 is a current mode switcher. This means that
switch duty cycle is directly controlled by switch current
rather than by output voltage or current. 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
and current is obtained by using the output of a dual
feedback voltage sensing error amplifier to set switch
current trip level. This technique has the advantage of
simplified loop frequency compensation. A low dropout
internal regulator provides a 2.3V supply for all internal
circuitry on the LT1513. This low dropout design allows
input voltage to vary from 2.7V to 25V. A 500kHz oscillator
is the basic clock for all internal timing. It turns “on” the
output switch via the logic and driver circuitry. Special
adaptive antisat 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 unique error amplifier design has two inverting inputs
which allow for sensing both output voltage and current. A
1.245V bandgap reference biases the noninverting input.
The first inverting input of the error amplifier is brought out
for positive output voltage sensing. The second inverting
input is driven by a “current” amplifier which is sensing
output current via an external current sense resistor. The
current amplifier is set to a fixed gain of – 12.5 which
provides a – 100mV current limit sense voltage.
The LT1513-2 option removes the feedback resistors
around the I
FB
amplifier and connects its output to the FB
signal. This provides a ground referenced current sense
voltage suitable for external current programming and
makes amplifier input and output available for external
loop compensation.
The error signal developed at the amplifier output is
brought out externally and is used for frequency compen-
sation. During normal regulator operation this pin sits at a
voltage between 1V (low output current) and 1.9V (high
output current). Switch duty cycle goes to zero if the V
C
pin
is pulled below the V
C
pin threshold, placing the LT1513 in
an idle mode.
6
+
–
–
W
V
IN
SHUTDOWN
DELAY AND RESET
500kHz
OSC
LOW DROPOUT
2.3V REG
SW
ANTISAT
SYNC
LOGIC
DRIVER
SWITCH
+
I
FBA
–
COMP
+
EA
V
C
A
V
≈
6
IA
0.04Ω
–
LT1513 • BD
Figure 2
U
LINER [ LINEAR TECHNOLOGY ]
