LTC1624
U
W U U
APPLICATIONS INFORMATION
10µH inductance is obtained with a current rating of 4A.
Splitting the two windings creates two 10µH inductors
with a current rating of 2A each. Therefore substitute
(2)(10µH) = 20µH for L in the equations.
outputripplevoltage. Theinputcapacitorneedstobesized
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 VIN(MAX). The input
capacitor ripple current is:
Specify the maximum inductor current to safely handle
IL(PEAK). Make sure the inductor’s saturation current rat-
ing (current when inductance begins to fall) exceeds the
maximum current rating set by RSENSE
.
∆I
L
I
=
RIPPLE RMS
(
)
SEPIC Converter: RSENSE Selection for Maximum
Output Current
12
The output capacitor ripple current is:
RSENSE is chosen based on the required output current.
Remember the LTC1624 current comparator has a maxi-
mum threshold of 160mV/RSENSE. The current compara-
torthresholdsetsthepeakoftheinductorcurrent,yielding
a maximum average output current IOUT(MAX) equal to
IL1(PEAK) less half the peak-to-peak ripple current, ∆IL,
divided by the output-input voltage ratio (see equation for
V
V
OUT
I
=I
OUT
RIPPLE RMS
(
)
IN
The output capacitor ripple voltage (RMS) is:
VOUT(RIPPLE) = 2(∆IL)(ESR)
IL1(PEAK) .
)
Theinputcapacitorcanseeaveryhighsurgecurrentwhen
a battery is suddenly connected, and solid tantalum
capacitors can fail under this condition. Be sure to specify
surge tested capacitors.
Allowing a margin for variations in the LTC1624 (without
considering variation in RSENSE), assuming 30% ripple
current in the inductor, yields:
SEPIC Converter: Coupling Capacitor (C1)
V
IN MIN
(
)
D
100mV
R
=
SENSE
The coupling capacitor C1 in Figure 7 sees a nearly
rectangular current waveform. During the off-time the
current through C1 is IOUT(VOUT/VIN) while approximately
–IOUT flows though C1 during the on-time. This current
waveform creates a triangular ripple voltage on C1:
I
V
+ V
OUT MAX
OUT
(
)
SEPIC Converter: Output Diode
The output diode conducts current only during the switch
off-time. Peak reverse voltage for SEPIC converters is
equal to VOUT + VIN. Average forward current in normal
operation is equal to output current. Peak current is:
V
I
OUT
OUT
∆V =
C1
V + V
+ V
D
200kHz C1
IN
OUT
V
+ V
D
The maximum voltage on C1 is then:
OUT
I
=I
+1 + ∆I
L
D1 PEAK
OUT MAX
(
)
(
)
V
IN MIN
(
)
VC1(MAX) = VIN + ∆VC1/2 (typically close to VIN(MAX)).
The ripple current though C1 is:
Schottky diodes such as MBR130LT3 are recommended.
V
V
OUT
SEPIC Converter: Input and Output Capacitors
I
=I
OUT
RIPPLE C1
( )
IN
The output capacitor is normally chosen by its effective
series resistance (ESR), because this is what determines
The maximum ripple current occurs at IOUT(MAX) and
VIN(MIN). The capacitance of C1 should be large enough so
17
Linear [ Linear ]
