LT1373
APPLICATIO S I FOR ATIO
W U U
U
VRIPPLE = output ripple (VP-P
)
Thehighspeedswitchingcurrentpathisshownschemati-
cally in Figure 3. Minimum lead length in this path is
essential to ensure clean switching and low EMI. The path
including the switch, output diode and output capacitor is
the only one containing nanosecond rise and fall times.
Keep this path as short as possible.
gm = error amplifier transconductance (≈375µmho)
RC = series resistor on VC pin
VOUT = DC output voltage
To prevent irregular switching, VC pin ripple should be
kept below 50mVP-P. Worst-case VC pin ripple occurs at
maximum output load current and will also be increased if
poor quality (high ESR) output capacitors are used. The
addition of a 0.001µF capacitor on the VC pin reduces
switching frequency ripple to only a few millivolts. A low
value for RC will also reduce VC pin ripple, but loop phase
margin may be inadequate.
SWITCH
NODE
L1
V
OUT
HIGH
FREQUENCY
CIRCULATING
PATH
V
IN
LOAD
Switch Node Considerations
LT1373 • F03
Formaximumefficiency,switchriseandfalltimearemade
as short as possible. To prevent radiation and high fre-
quency resonance problems, proper layout of the compo-
nents connected to the switch node is essential. B field
(magnetic) radiation is minimized by keeping output di-
ode, switchpinandoutputbypasscapacitorleadsasshort
as possible. E field radiation is kept low by minimizing the
length and area of all traces connected to the switch pin.
A ground plane should always be used under the switcher
circuitry to prevent interplane coupling.
Figure 3
More Help
For more detailed information on switching regulator
circuits, please see AN19. Linear Technology also offers a
computer software program, SwitcherCADTM, to assist in
designing switching converters. In addition, our applica-
tions department is always ready to lend a helping hand.
SwitcherCAD is a trademark of Linear Technology Corporation.
U
TYPICAL APPLICATIONS N
Positive-to-Negative Converter with Direct Feedback
Dual Output Flyback Converter with Overvoltage Protection
V
R2
275k
1%
R1
302.6k
1%
IN
2.7V TO 16V
T1*
+
2
4
3
C1
D2
+
•
V
IN
22µF
C3
P6KE-15A
D3
MBRS140T3
T1*
4.75V TO 13V
5
47µF
V
OUT
•
V
IN
1N4148
+
15V
†
2, 3
5
1
4
8
3
–V
C1
100µF
ON
+
OUT
V
S/S
SW
•
P6KE-20A
OFF
C3
–5V
R2
2.55k
1%
D1
MBRS130LT3
47µF
2
5
LT1373
4
8
1N4148
•
•
FB
S/S
V
IN
V
NFB
4
8
3
6, 7
ON
+
R3
2.49k
1%
SW
OFF
C4
47µF
†MAX I
V
GND
C
OUT
V
0.3A 3V
0.5A 5V
0.75A 9V
LT1373
1
6, 7
1
I
–V
OUT
IN
OUT
NFB
–15V
C2
0.01µF
R4
12.4k
1%
MBRS140T3
V
GND
C
R1
5k
1
6, 7
R5
2.49k
1%
C2
*COILTRONICS CTX20-2 (407) 241-7876
LT1373 • TA03
0.01µF
R3
5k
*DALE LPE-4841-100MB (605) 665-9301
LT1373 • TA04
10
Linear [ Linear ]
