16
LTC1266
LTC1266 -3.3/LTC1266 -5
Troubleshooting Hints
Since efficiency is critical to LTC1266 series applications,
it is very important to verify that the circuit is functioning
correctly in both continuous and Burst Mode
operation.
The waveform to monitor is the voltage on the timing
capacitor, Pin 6.
In continuous mode (I
LOAD
> I
BURST
) the voltage on the C
T
pin should be a sawtooth with a 0.9V
P-P
swing. This
voltage should never dip below 2V as shown in Figure 9a.
When load currents are low (I
LOAD
< I
BURST
) Burst Mode
operation should occur with the C
T
pin waveform periodi-
cally falling to ground for periods of time as shown in
Figure 9b.
If Pin 6 is observed falling to ground at high output
currents, it indicates poor decoupling or improper ground-
ing. Refer to the Board Layout Checklist.
Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1266 series. These items are also illustrated graphi-
cally in the layout diagram of Figure 10. Check the follow-
ing in your layout:
1. Are the signal and power grounds segregated? The
LTC1266 signal ground (Pin 12) must return to the (–)
plate
of C
OUT
. The power ground returns to the source
of the bottom-side MOSFET, anode of the Schottky
diode and (–) plate of C
IN
, which should have as short
lead lengths as possible.
2. Does the LTC1266 SENSE
–
(Pin 8) connect to a point
close to R
SENSE
and the (+) plate of C
OUT
? In adjustable
applications, the resistive divider R1 and R2 must be
connected between the (+) plate of C
OUT
and signal
ground.
Figure 9. C
T
Waveforms
Figure 10. LTC1266 Layout Diagram (See Layout Checklist)
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
LTC1266
L
SHUTDOWN
BOLD LINES INDICATE
HIGH CURRENT PATHS
OUTPUT DIVIDER REQUIRED WITH
ADJUSTABLE VERSION ONLY
C
IN
C
B
C
OUT
PGND
TDRIVE
PINV
BINH
V
IN
C
T
I
TH
SENSE
–
BDRIVE
LB
OUT
LB
IN
SGND
SHDN
V
FB
SENSE
+
470Ω
R1
R2
R
SENSE
V
OUT
1266 F10
V
IN
1000pF
3300pFC
T
PWR V
IN
+
+
+
+
–
–
3.3V
0V
3.3V
0V
1266 F09
(a) Continuous Mode Operation
(b) Burst Mode Operation
APPLICATIO S I FOR ATIO
WUUU
