LT1513/LT1513-2
W
BLOCK DIAGRAM
V
IN
SW
SHUTDOWN
DELAY AND RESET
LOW DROPOUT
2.3V REG
S/S
ANTISAT
500kHz
LOGIC
COMP
DRIVER
SWITCH
SYNC
OSC
+
I
FBA
4k
50k*
I
–
FB
–
–
+
+
–
V
EA
IA
0.04Ω
FB
A
V
≈ 6
V
C
1.245V
REF
LT1513 • BD
*REMOVE ON LT1513-2
Figure 2
U
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
BlockDiagram,theswitchisturned“on”atthestartofeach
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
powerswitchandadjustsdrivercurrentinstantaneouslyto
limit switch saturation. This minimizes driver dissipation
and provides very rapid turn-off of the switch.
1.245V bandgap reference biases the noninverting input.
Thefirstinvertinginputoftheerroramplifierisbroughtout
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 IFB 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
outputcurrent). SwitchdutycyclegoestozeroiftheVC pin
is pulled below the VC pin threshold, placing the LT1513 in
an idle mode.
A unique error amplifier design has two inverting inputs
which allow for sensing both output voltage and current. A
6
Linear [ Linear ]
