LT1305 LINER [Linear Technology], LT1305 Datasheet - Page 5

• Manufacturer Part Number: LT1305
• Description: Micropower High Power DC/DC Converter with Low-Battery Detector
• Manufacturer: LINER [Linear Technology]

BLOCK
OPERATION
Operation of the LT1305 is best understood by referring to
the Block Diagram in Figure 1. When C1’s negative input,
related to the output voltage by the appropriate resistor-
divider ratio, is higher than the 1.24V reference voltage,
C1’s output is low. C2, A3 and the oscillator are turned off,
drawing no current. Only the reference and C1 consume
current, typically 120 A. When C1’s negative input drops
below 1.24V and overcomes C1’s 6mV hysteresis, C1’s
output goes high, enabling the oscillator, current compara-
tor C2 and driver A3. Quiescent current increases to 2mA
as the device goes into active switching mode. Q1 then
turns on in controlled saturation for nominally 6 s or until
current comparator C2 trips, whichever comes first. The
switch then turns off for approximately 1.5 s, then turns on
again. The LT1305’s switching causes current to alter-
nately build up in L1 and dump into output capacitor C4 via
D1, increasing the output voltage. When the output is high
enough to cause C1’s output to go high, switching action
ceases. Capacitor C4 is left to supply current to the load
DIAGRAM
U
R1
R2
V
IN
+
W
C5
4
FB
REFERENCE
1
1.24V
GND
6
V
IN
5
LBI
Figure 1. LT1305 Block Diagram
–
+
COMPARATOR
HYSTERETIC
C1
–
+
C3
OSCILLATOR
until V
the entire cycle repeats. Figure 2 details relevant wave-
forms. C1’s cycling causes low-to-mid-frequency ripple
voltage on the output. Ripple can be reduced by making the
output capacitor large. The 220 F unit specified results in
ripple of 50mV to 100mV on the 5V output. Paralleling two
capacitors will decrease ripple by approximately 50%.
AC COUPLED
100mV/DIV
COMPARATOR
OFF
2
5V/DIV
1A/DIV
LBO
V
CURRENT
OUT
V
OUT
SW
I
L
decreases enough to force C1’s output high, and
DRIVER
A3
Figure 2. Burst Mode Operation
C2
SHUTDOWN
+
–
3
36mV
L1
Q2
1
R1
3
50 s/DIV
7
8
Q1
160
SW
PGND
D1
+
LT1305 • F01
LT1305
C4
LT1305 • F02
5