24
LT1576/LT1576-5
APPLICATIONS INFORMATION
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with large inductors. Output ripple voltage for the positive-
to-negative converter will be much higher than a buck
converter. Ripple current in the output capacitor will also
be much higher. The following equations can be used to
calculate operating conditions for the positive-to-negative
converter.
Maximum load current:
I
I
VV
VVfL
VV
VV VV
MAX
P
IN OUT
OUT IN
OUT IN
OUT IN OUT F
=
−
()( )
+
()()()
()
−
()
+−
()
+
()
2
035
035
.
.
I
P
= Maximum rated switch current
V
IN
= Minimum input voltage
V
OUT
= Output voltage
V
F
= Catch diode forward voltage
0.35 = Switch voltage drop at 1.5A
Example: with V
IN(MIN)
= 5.5V, V
OUT
= 5V, L = 30µH,
V
F
= 0.5V, I
P
= 1.5A: I
MAX
= 0.6A. Note that this equation
does not take into account that maximum rated switch
current (I
P
) on the LT1576 is reduced slightly for duty
cycles above 50%. If duty cycle is expected to exceed 50%
(input voltage less than output voltage), use the actual I
P
value from the Electrical Characteristics table.
Operating duty cycle:
DC
VV
VVV
OUT F
IN OUT F
=
+
−+ +03.
(This formula uses an average value for switch loss, so it
may be several percent in error.)
With the conditions above:
DC =
+
−++
=
505
55 03 5 05
51
.
.. .
%
This duty cycle is close enough to 50% that I
P
can be
assumed to be 1.5A.
OUTPUT DIVIDER
If the adjustable part is used, the resistor connected to
V
OUT
(R2) should be set to approximately 5k. R1 is
calculated from:
R
RV
OUT
1
2121
121
=
−
()
.
.
INDUCTOR VALUE
Unlike buck converters, positive-to-negative converters
cannot use large inductor values to reduce output ripple
voltage. At 200kHz, values larger than 75µH make almost
no change in output ripple. The graph in Figure 16 shows
peak-to-peak output ripple voltage for a 5V to –5V con-
verter versus inductor value. The criteria for choosing the
INDUCTOR SIZE (µH)
0
OUTPUT RIPPLE VOLTAGE (mV
P-P
)
150
120
90
60
30
0
60
1576 F16
15
30
45
75
DISCONTINUOUS
I
LOAD
= 0.25A
DISCONTINUOUS
I
LOAD
= 0.1A
5V TO –5V CONVERTER
OUTPUT CAPACITOR’S
ESR = 0.1Ω
CONTINUOUS
I
LOAD
> 0.38A
Figure 16. Ripple Voltage on Positive-to-Negative Converter
