LTC1624 Datasheet, PDF (17/28 Pages) Linear Technology – High Efficiency SO-8 N-Channel Switching Regulator Controller

LTC1624
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.
Specify the maximum inductor current to safely handle
I_L(PEAK). Make sure the inductor’s saturation current rat-
ing (current when inductance begins to fall) exceeds the
maximum current rating set by R_SENSE.
SEPIC Converter: R_SENSE Selection for Maximum
Output Current
R_SENSE is chosen based on the required output current.
Remember the LTC1624 current comparator has a maxi-
mum threshold of 160mV/R_SENSE. The current compara-
tor threshold sets the peak of the inductor current, yielding
a maximum average output current I_OUT(MAX) equal to
I_L1(PEAK) less half the peak-to-peak ripple current, ∆I_L,
divided by the output-input voltage ratio (see equation for
I_L1(PEAK))_.
Allowing a margin for variations in the LTC1624 (without
considering variation in R_SENSE), assuming 30% ripple
current in the inductor, yields:
( ) ^{( ) RSENSE}
=
100mV
I_{OUT MAX}
 _{VIN MIN} 


 ^{VOUT + VD}
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_OUT + V_IN. Average forward current in normal
operation is equal to output current. Peak current is:
( ) ( ) _{( )} ^{ID1 PEAK}
= I_OUT
MAX


V_OUT
+
V_D
 ^{VIN MIN}

+ 1

+ ∆I_L
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
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_IN(MAX). The input
capacitor ripple current is:
( ) IRIPPLE RMS = ∆IL
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The output capacitor ripple current is:
( ) IRIPPLE RMS = IOUT VOUT
V_IN
The output capacitor ripple voltage (RMS) is:
V_OUT(RIPPLE) = 2(∆I_L)(ESR)
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_OUT(V_OUT/ V_IN) while approximately
– I_OUT flows though C1 during the on-time. This current
waveform creates a triangular ripple voltage on C1:
( )( ) ^
∆V_C1 = _{}
I_OUT
200kHz
C1




V_IN
+
V_OUT
V_OUT
+

V_D
The maximum voltage on C1 is then:
V_C1(MAX) = V_IN + ∆V_C1/2 (typically close to V_IN(MAX)).
The ripple current though C1 is:
( ) ^{IRIPPLE C1 = IOUT}
V_OUT
V_IN
The maximum ripple current occurs at I_OUT(MAX) and
V_IN(MIN). The capacitance of C1 should be large enough so
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