Datasheet

5
LT1305
BLOCK DIAGRAM
W
–
+
–
+
–
+
6 7
8
3
25
1
4
++
C5 C4
L1
D1
V
IN
V
IN
R1
3Ω
Q2
1×
Q1
160×
SHUTDOWN
LBOLBIGND
PGND
SW
R1
R2
C3
C2
C1
FB
OFF
OSCILLATOR
A3
DRIVER
CURRENT 
COMPARATOR
HYSTERETIC
COMPARATOR
REFERENCE
1.24V
36mV
LT1305 • F01
Figure 1. LT1305 Block Diagram
OPERATION
U
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 120µA. 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 6µs or until
current comparator C2 trips, whichever comes first. The
switch then turns off for approximately 1.5µs, 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
until V
OUT
decreases enough to force C1’s output high, and
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 220µF unit specified results in
ripple of 50mV to 100mV on the 5V output. Paralleling two
capacitors will decrease ripple by approximately 50%.
V
OUT
100mV/DIV
AC COUPLED
V
SW
5V/DIV
I
L
1A/DIV
50µs/DIV
LT1305 • F02
Figure 2. Burst Mode Operation