LTC1266-5_15 Datasheet, PDF (16/20 Pages) Linear Technology – Synchronous Regulator Controller for N- or P-Channel MOSFETs

LTC1266
LTC1266-3.3/LTC1266-5
APPLICATIO S I FOR ATIO
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.
3.3V
0V
(a) Continuous Mode Operation
3.3V
0V
(b) Burst Mode Operation
1266 F09
Figure 9. C_T Waveforms
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.
C_B ^CIN
+
BOLD LINES INDICATE
HIGH CURRENT PATHS
V_IN
1
16
L
–
TDRIVE BDRIVE
2
PWR V_IN
PGND 15
3
PINV
14
LB_OUT
4
LTC1266
13
BINH
LB_IN
5
V_IN
12
SGND
–
6
C_T
11
SHDN
SHUTDOWN
7
I_TH
10
V_FB
CT
3300pF _{8 SENSE–}
SENSE^{+ 9}
470Ω
R1
+ ^COUT
V_OUT
R2
R_SENSE
+
1000pF
OUTPUT DIVIDER REQUIRED WITH
ADJUSTABLE VERSION ONLY
1266 F10
Figure 10. LTC1266 Layout Diagram (See Layout Checklist)
16