ltc1624 Linear Technology Corporation, ltc1624 Datasheet - Page 17

• Manufacturer Part Number: ltc1624
• Description: High Efficiency So-8 N-channel Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS
10 H inductance is obtained with a current rating of 4A.
Splitting the two windings creates two 10 H inductors
with a current rating of 2A each. Therefore substitute
(2)(10 H) = 20 H for L in the equations.
Specify the maximum inductor current to safely handle
I
ing (current when inductance begins to fall) exceeds the
maximum current rating set by R
SEPIC Converter: R
Output Current
R
Remember the LTC1624 current comparator has a maxi-
mum threshold of 160mV/R
tor threshold sets the peak of the inductor current, yielding
a maximum average output current I
I
divided by the output-input voltage ratio (see equation for
I
Allowing a margin for variations in the LTC1624 (without
considering variation in R
current in the inductor, yields:
SEPIC Converter: Output Diode
The output diode conducts current only during the switch
off-time. Peak reverse voltage for SEPIC converters is
equal to V
operation is equal to output current. Peak current is:
Schottky diodes such as MBR130LT3 are recommended.
SEPIC Converter: Input and Output Capacitors
The output capacitor is normally chosen by its effective
series resistance (ESR), because this is what determines
L(PEAK)
L1(PEAK)
L1(PEAK)
SENSE
R
I
D PEAK
SENSE
1
. Make sure the inductor’s saturation current rat-
is chosen based on the required output current.
)
.
less half the peak-to-peak ripple current, I
OUT
I
OUT MAX
I
+ V
100
OUT MAX
IN
mV
U
. Average forward current in normal
SENSE
INFORMATION
V
U
Selection for Maximum
V
SENSE
V
OUT
V
OUT
SENSE
IN MIN
IN MIN
), assuming 30% ripple
SENSE
. The current compara-
V
V
D
W
D
OUT(MAX)
.
1
I
L
U
equal to
L
,
output ripple voltage. The input capacitor needs to be sized
to handle the ripple current safely.
Since the output capacitor’s ESR affects efficiency, use
low ESR capacitors for best performance. SEPIC regula-
tors, like step-down regulators, have a triangular current
waveform but have maximum ripple at V
capacitor ripple current is:
The output capacitor ripple current is:
The output capacitor ripple voltage (RMS) is:
The input capacitor can see a very high surge current when
a battery is suddenly connected, and solid tantalum
capacitors can fail under this condition. Be sure to specify
surge tested capacitors.
SEPIC Converter: Coupling Capacitor (C1)
The coupling capacitor C1 in Figure 7 sees a nearly
rectangular current waveform. During the off-time the
current through C1 is I
– I
waveform creates a triangular ripple voltage on C1:
The maximum voltage on C1 is then:
The ripple current though C1 is:
The maximum ripple current occurs at I
V
IN(MIN)
V
OUT
V
I
I
I
C1(MAX)
RIPPLE RMS
RIPPLE RMS
RIPPLE C
OUT(RIPPLE)
V
C
flows though C1 during the on-time. This current
1
. The capacitance of C1 should be large enough so
= V
1
200
IN
= 2( I
I
I
OUT
kHz C
OUT
+ V
I
OUT
12
I
L
L
OUT
C1
V
)(ESR)
1
V
OUT
/2 (typically close to V
V
IN
V
(V
OUT
IN
OUT
V
IN
/V
IN
V
V
OUT
OUT
) while approximately
IN(MAX)
LTC1624
V
OUT(MAX)
D
. The input
IN(MAX)
17
and
).