LT1308 データシートの表示（PDF） - Linear Technology
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LT1308 Datasheet PDF : 8 Pages
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LT1308
PIN FUNCTIONS
SW (Pin 5): Switch Pin. Connect inductor/diode here.
Minimize trace area at this pin to keep EMI down.
VIN (Pin 6): Supply Pin. Must have local bypass capacitor
right at the pin, connected directly to ground.
LBI (Pin 7): Low-Battery Detector Input. 200mV refer-
ence. Voltage on LBI must stay between ground and
700mV. Low-battery detector does not function with
SHDN pin grounded. If not used, float LBI pin.
LBO (Pin 8): Low-Battery Detector Output. Open collec-
tor, can sink 10µA. A 1MΩ pullup is recommended. LBO
is high impedance when SHDN is grounded.
BLOCK DIAGRAM
VIN
6
R5
40k
VOUT
R1
(EXTERNAL)
FB
Q1
FB
2
R2
(EXTERNAL)
VIN
R6
40k
+
gm
–
ERROR
AMPLIFIER
Q2
×10
R3
30k
R4
140k
RAMP
GENERATOR
600kHz
OSCILLATOR
VC
1
BIAS
+
Σ
+
SHUTDOWN
LBI
+
7+
–
A1
ENABLE
–
200mV
COMPARATOR
–
FF
+
A2
R
Q
S
A4
DRIVER
A=3
SHDN
3
LBO
8
SW
5
Q3
0.03Ω
4
GND
1308 BD
APPLICATIONS INFORMATION
LAYOUT HINTS
The LT1308 switches current at high speed, mandating
careful attention to layout for proper performance. You will
not get advertised performance with careless layouts.
Figure 2 shows recommended component placement.
Follow this closely in your PC layout. Note the direct path
of the switching loops. Input capacitor CIN must be placed
close (< 5mm) to the IC package. As little as 10mm of wire
or PC trace from CIN to VIN will cause problems such as
inability to regulate or oscillation. A 10µF ceramic bypass
capacitor is the only input capacitance required provided
the battery has a low inductance path to the circuit. The
battery itself provides the bulk capacitance the device
requires for proper operation. If the battery is located some
GROUND PLANE
1
8
2 LT1308
7
3
6
VIN
4
5
L
MULTIPLE
CIN
VIAs
GND
COUT
D
VOUT
1308 F02
Figure 2. Recommended Component Placement. Traces
Carrying High Current Are Direct. Trace Area at FB Pin and VC
Pin is Kept Low. Lead Length to Battery Should Be Kept Short.
Ground Plane Should Be Placed Under All Components
5
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