LT1956IGN-5#PBF Linear Technology, LT1956IGN-5#PBF Datasheet - Page 26

• Manufacturer Part Number: LT1956IGN-5#PBF
• Description: IC SW REG STP-DWN HI VOLT 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1956IGN-5#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 1.5A
• Frequency - Switching: 500kHz
• Voltage - Input: 5.5 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LT1956/LT1956-5
PACKAGE DESCRIPTIO
Minimum inductor continuous mode:
For a 12V to –12V converter using the LT1956 with peak
switch current of 1.5A and a catch diode of 0.63V:
For a load current of 0.25A, this says that discontinuous
mode can be used and the minimum inductor needed is
found from:
In practice, the inductor should be increased by about
30% over the calculated minimum to handle losses and
variations in value. This suggests a minimum inductor of
7 H for this application.
Ripple Current in the Input and Output Capacitors
Positive-to-negative converters have high ripple current
in the input capacitor. For long capacitor lifetime, the
RMS value of this current must be less than the high
frequency ripple current rating of the capacitor. The
following formula will give an approximate value for RMS
ripple current. This formula assumes continuous mode
and large inductor value . Small inductors will give some-
what higher ripple current, especially in discontinuous
mode. The exact formulas are very complex and appear
in Application Note 44, pages 29 and 30. For our pur-
poses here I have simply added a fudge factor (ff). The
value for ff is about 1.2 for higher load currents and L
lower load currents.
ff = 1.2 to 2.0
26
15 H. It increases to about 2.0 for smaller inductors at
L
I
L
Capacitor I
CONT
MIN
MIN
2
(
500 10
( )(
f V
4 12 12 12 12 0 63
2 12 0 25
RMS
(
( )( . )
IN
•
3
( ) ( . )
V
( )(
)( . )
12 1 5
ff I
OUT
)(
1 5
2
OUT
)
2
(
I
V
P
IN
)
–
2
5 3
)(
I
V
OUT
.
V
V
OUT
OUT
IN
. )
H
U
1
)
(
V
0 370
OUT
.
V
IN
A
V
F
)
The output capacitor ripple current for the positive-to-
negative converter is similar to that for a typical buck
regulator—it is a triangular waveform with peak-to-peak
value equal to the peak-to-peak triangular waveform of the
inductor. The low output ripple design in Figure 14 places
the input capacitor between V
output. This placement of the input capacitor significantly
reduces the size required for the output capacitor (versus
placing the input capacitor between V
The peak-to-peak ripple current in both the inductor and
output capacitor (assuming continuous mode) is:
The output ripple voltage for this configuration is as low as
the typical buck regulator based predominantly on the
inductor’s triangular peak-to-peak ripple current and the
ESR of the chosen capacitor (see Output Ripple Voltage in
Applications Information).
Diode Current
Average diode current is equal to load current. Peak diode
current will be considerably higher.
Peak diode current:
Keep in mind that during start-up and output overloads,
average diode current may be much higher than with
normal loads. Care should be used if diodes rated less than
1A are used, especially if continuous overload conditions
must be tolerated.
I
I
I
DC Duty Cycle
Continuous Mode
Discontinuous Mode
P-P
COUT
OUT
(
V
(
DC V
RMS
IN
f L
V
•
•
IN
V
)
IN
OUT
I
P-P
)
12
V
2
OUT
( )( )(
L f V
V
OUT
(
V
IN
IN
2
V
(
and the regulated negative
)(
I
IN
OUT
IN
V
V
( )( )
OUT
L f
F
)(
V
V
F
V
IN
)
OUT
OUT
and ground).
)
)
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