10
LT1373
V
RIPPLE
= output ripple (V
P-P
)
g
m
= error amplifier transconductance (≈375µmho)
R
C
= series resistor on V
C
pin
V
OUT
= DC output voltage
To prevent irregular switching, V
C
pin ripple should be
kept below 50mV
P-P
. Worst-case V
C
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 V
C
pin reduces
switching frequency ripple to only a few millivolts. A low
value for R
C
will also reduce V
C
pin ripple, but loop phase
margin may be inadequate.
Switch Node Considerations
For maximum efficiency, switch rise and fall time are made
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, switch pin and output bypass capacitor leads as short
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.
Positive-to-Negative Converter with Direct Feedback
LT1373
V
IN
V
C
V
IN
2.7V TO 16V
1
3
†
MAX I
OUT
8
5
4
6, 7
*COILTRONICS CTX20-2 (407) 241-7876
GND
NFB
LT1373 • TA03
V
SW
S/S
D2
P6KE-15A
D3
1N4148
D1
MBRS130LT3
C1
22µF
C2
0.01µF
R1
5k
R3
2.49k
1%
R2
2.55k
1%
–V
OUT
†
–5V
C3
47µF
ON
OFF
V
IN
3V
5V
9V
I
OUT
0.3A
0.5A
0.75A
2
1
4
T1*
3
•
•
+
+
Dual Output Flyback Converter with Overvoltage Protection
LT1373
V
IN
FB
V
C
V
IN
4.75V TO 13V
1
3
8
52
4
6, 7
*DALE LPE-4841-100MB (605) 665-9301
GND
NFB
LT1373 • TA04
V
SW
S/S
P6KE-20A
1N4148
MBRS140T3
MBRS140T3
C1
100µF
R2
275k
1%
R1
302.6k
1%
C2
0.01µF
R3
5k
R5
2.49k
1%
R4
12.4k
1%
–V
OUT
–15V
V
OUT
15V
C3
47µF
C4
47µF
ON
OFF
2, 3
6, 7
5
T1*
4
8
1
•
•
•
+
+
+
TYPICAL APPLICATIONS N
U
The high speed switching current path is shown schemati-
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.
More Help
For more detailed information on switching regulator
circuits, please see AN19. Linear Technology also offers a
computer software program, SwitcherCAD
TM
, to assist in
designing switching converters. In addition, our applica-
tions department is always ready to lend a helping hand.
LOAD
V
OUT
L1
SWITCH
NODE
LT1373 • F03
V
IN
HIGH
FREQUENCY
CIRCULATING
PATH
Figure 3
APPLICATIO S I FOR ATIO
WUUU
SwitcherCAD is a trademark of Linear Technology Corporation.
