LTC1149 查看數據表（PDF） - Linear Technology
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LTC1149 Datasheet PDF : 20 Pages
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LTC1149
LTC1149-3.3/LTC1149-5
UU W U
APPLICATIO S I FOR ATIO
BOLD LINES INDICATE HIGH CURRENT PATHS
+
1N4148
1N4148
0.068µF
P-CHANNEL
+ CIN
VIN
D1
1µF
+
1
PGATE
16
CAP
0.047µF
2
VIN
3
VCC
4
PDRIVE
15
SD2
14
RGND
13
NGATE
SHUTDOWN
5
VCC
12
PGND
6
CT
11
SGND
7
ITH
VFB/ 10
SHDN1
CT
3300pF 8 SENSE–
SENSE+ 9
1k
100pF
R1
R2
1000pF
N-CHANNEL
–
L
–
+ COUT
VOUT
RSENSE
+
1149 F08
OUTPUT DIVIDER REQUIRED WITH
ADJUSTABLE VERSION ONLY
Figure 8. LTC1149 Series Layout Diagram (see Layout Checklist)
in series with each sense lead to help decouple Pins 8
and 9. However, when these resistors are used, the
capacitor should be no larger than 1000pF.
4. Does the (+) plate of CIN connect to the source of the
P-channel MOSFET as closely as possible? An addi-
tional 0.1µF ceramic capacitor between VIN and power
ground may be required in some applications.
5. Is the VCC decoupling capacitor connected closely
between Pin 5 of the LTC1149 and power ground? This
capacitor carries the MOSFET driver peak currents.
6. Is the SHDN1 Pin 10 (fixed output versions only)
actively pulled to ground during normal operation? The
SHDN1 pin is high impedance and must not be allowed
to float. In adjustable versions, Pin 10 is the feedback
pin and is very sensitive to pickup from the switch node.
Care must be taken to isolate VFB from possible capaci-
tive coupling of the inductor switch signal.
correctly in both continuous and Burst Mode operation.
The waveform to monitor is the voltage on the timing
capacitor Pin 6.
In continuous mode (ILOAD > IBURST) the voltage on Pin 6
should be a sawtooth with a 0.9VP-P swing. This voltage
should never dip below 2V as shown in Figure 9a.
When load currents are low (ILOAD < IBURST) Burst Mode
operation should occur with the CT pin waveform periodi-
cally falling to ground 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.
3.3V
0V
(a) CONTINUOUS MODE OPERATION
3.3V
Troubleshooting Hints
Since efficiency is critical to LTC1149 series applications,
it is very important to verify that the circuit is functioning
(b) Burst Mode OPERATION
0V
1149 F09
Figure 9. CT Pin 6 Waveforms
15
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