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LT1576CS8 Datasheet(PDF) 24 Page - Linear Technology

Part # LT1576CS8
Description  1.5A, 200kHz Step-Down Switching Regulator
PDF  28 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT1576CS8 Datasheet(HTML) 24 Page - Linear Technology

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24
LT1576/LT1576-5
APPLICATIONS INFORMATION
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
VV
f L
VV
VV
V
V
MAX
P
IN
OUT
OUT
IN
OUT
IN
OUT
IN
OUT
F
=
−
()( )
+
()( )( )
()
−
()
+−
()
+
()
2
035
035
.
.
IP = Maximum rated switch current
VIN = Minimum input voltage
VOUT = Output voltage
VF = Catch diode forward voltage
0.35 = Switch voltage drop at 1.5A
Example: with VIN(MIN) = 5.5V, VOUT = 5V, L = 30µH,
VF = 0.5V, IP = 1.5A: IMAX = 0.6A. Note that this equation
does not take into account that maximum rated switch
current (IP) 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 IP
value from the Electrical Characteristics table.
Operating duty cycle:
DC
VV
VV
V
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
=
+
−+ +
=
50 5
55
03
5
0 5
51
.
..
.
%
This duty cycle is close enough to 50% that IP can be
assumed to be 1.5A.
OUTPUT DIVIDER
If the adjustable part is used, the resistor connected to
VOUT (R2) should be set to approximately 5k. R1 is
calculated from:
R
RVOUT
1
21 21
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 75
µH 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
150
120
90
60
30
0
60
1576 F16
15
30
45
75
DISCONTINUOUS
ILOAD = 0.25A
DISCONTINUOUS
ILOAD = 0.1A
5V TO –5V CONVERTER
OUTPUT CAPACITOR’S
ESR = 0.1
Ω
CONTINUOUS
ILOAD > 0.38A
Figure 16. Ripple Voltage on Positive-to-Negative Converter



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