Datasheet

6
LT1303/LT1303-5
W
IDAGRA
B
L
O
C
K
S
If switch current reaches 1A, causing C2 to trip, switch on-
time is reduced and off-time increases slightly. This allows
continuous operation during bursts. C2 monitors the
voltage across 3Ω resistor R1 which is directly related to
the switch current. Q2’s collector current is set by the
emitter-area ratio to 0.6% of Q1’s collector current. When
R1’s voltage drop exceeds 18mV, corresponding to 1A
switch current, C2’s output goes high, truncating the on-
time portion of the oscillator cycle and increasing off-time
OPERATION
U
Operation of the LT1303 is best understood by referring to
the Block Diagram in Figure 2. 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 140µA. 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 LT1303’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 4 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 100µF unit specified results in
ripple of 50mV to 100mV on the 5V output. A 220µF
capacitor will decrease ripple by approximately 50%.
–
+
–
+
–
+
6 7
8
3
25
1
4
V
IN
R1
3Ω
Q2
1×
Q1
160×
SHUTDOWN
LBOLBIGND
PGND
R1
474k
R2
156k
C3
C2
C1
FB SW
OFF
OSCILLATOR
A3
DRIVER
CURRENT 
COMPARATOR
HYSTERETIC
COMPARATOR
REFERENCE
1.24V
18mV
LT1303 BD02
Figure 3. LT1303-5 Block Diagram
20µs/DIV
V
OUT
100mV/DIV
AC COUPLED
LT1303 F04
Figure 4. Burst Mode Operation in Action
I
L
1A/DIV
V
SW
5V/DIV