Datasheet
24
LT1956/LT1956-5
1956f
couples energy to L2 and ensures equal voltages across
L2 and L1 during steady state. Instead of using a trans-
former for L1 and L2, uncoupled inductors were used
because they require less height than a single transformer,
can be placed separately in the circuit layout for optimized
space savings and reduce overall cost. This is true even
when the uncoupled inductors are sized (twice the value of
inductance of the transformer) in order to keep ripple
current comparable to the transformer solution. If a single
transformer becomes available to provide a better height/
cost solution, refer to the dual output SEPIC circuit de-
scription in Design Note 100 for correct transformer
connection.
During switch on-time, in steady state, the voltage across
both L1 and L2 is positive and equal; with energy (and
current) ramping up in each inductor. The current in L2 is
provided by the coupling capacitor C4. During switch off-
time, current ramps downward in each inductor. The
APPLICATIO S I FOR ATIO
WUUU
V
OUT1
**
5V
V
OUT2
**
†
–5V
*SUMIDA CDRH4D28-150
**SEE FIGURE 14c FOR V
OUT1
, V
OUT2
LOAD CURRENT RELATIONSHIP
†
IF LOAD CAN GO TO ZERO, AN OPTIONAL
PRELOAD OF 500Ω CAN BE
USED TO IMPROVE REGULATION
V
IN
9V TO 12V
(TRANSIENTS
TO 36V)
GND
1956 F14a
C2
0.1µF
C
F
220pF
R
C
2.2k
D1
B0540W
C5
10µF
6.3V
CER
C6
10µF
6.3V CER
C3
2.2µF
50V
CERAMIC
C4
10µF
6.3V
CER
D2
MMSD914TI
D3
B0540W
L1*
15µH
L2*
C
C
3300pF
R1
15.4k
R2
4.99k
+
+
+
BOOST
V
IN
LT1956
SHDN
SYNC
SW
FB
V
C
GND
Figure 14a. Dual Polarity Output Converter
V
OUT1
LOAD CURRENT (mA)
0
0
V
OUT2
MAXIMUM LOAD CURRENT (mA)
50
150
200
250
500
350
200
400
1956 F15b
100
400
450
300
600
800
Figure 14b. V
OUT2
(–5V) Maximum
Allowable Load Current vs V
OUT1
(5V) Load Current
V
OUT2
LOAD CURRENT (mA)
0
4.75
|V
OUT2
| (V)
4.85
4.95
5.05
5.15
100 200 300 400
1956 F14c
500 600
5.25
4.80
4.90
5.00
5.10
5.20
5.30
V
OUT1
LOAD CURRENT
750mA
V
OUT1
LOAD CURRENT
250mA
V
OUT1
LOAD CURRENT
500mA
V
OUT2
LOAD CURRENT (mA)
0
EFFICIENCY (%)
60
80
100
400
1956 F14d
40
20
50
70
90
30
10
0
100
200
300
500
V
OUT1
LOAD CURRENT
750mA
V
OUT1
LOAD CURRENT
250mA
Figure 14c. V
OUT2
(–5V) Output
Voltage vs Load Current
Figure 14d. Dual Polarity Output
Converter Efficiency