LTC1624
U
W U U
APPLICATIONS INFORMATION
Allowing a margin for variations in the LTC1624 (without
considering variation in RSENSE), assuming 30% ripple
current in the inductor, yields:
0.3 V
V
− V
(
)(
)
IN OUT
IN
C I
≈
IN RIPPLE
200kHz L V
OUT
V
Theinputcapacitorcanseeaveryhighsurgecurrentwhen
abatteryissuddenlyconnectedandsolidtantalumcapaci-
tors can fail under this condition. Be sure to specify surge
tested capacitors.
IN MIN
(
)
100mV
R
=
SENSE
V
+ V
I
OUT
D
OUT MAX
(
)
Boost Converter: Output Diode
Boost Converter: Duty Cycle Limitations
The output diode conducts current only during the switch
off-time. Peak reverse voltage for boost converters is
equal to the regulator output voltage. Average forward
current in normal operation is equal to output current.
Remember boost converters are not short-circuit pro-
tected. Checktobesurethediode’scurrentratingexceeds
themaximumcurrentsetbyRSENSE.Schottkydiodessuch
as Motorola MBR130LT3 are recommended.
The minimum on-time of 450ns sets a limit on how close
VIN can approach VOUT without the output voltage over-
shooting and tripping the overvoltage comparator. Unless
very low values of inductances are used, this should never
be a problem. The maximum input voltage in continuous
mode is:
VIN(MAX) = 0.91VOUT + 0.5V
For DC = 9%
Boost Converter: Output Capacitors
SEPIC Converter Applications
The output capacitor is normally chosen by its effective
series resistance (ESR), because this is what determines
output ripple voltage.
The LTC1624 is also well-suited to SEPIC (Single Ended
Primary Inductance Converter) converter applications.
The SEPIC converter shown in Figure 7 uses two induc-
tors. The advantage of the SEPIC converter is the input
voltage may be higher or lower than the output voltage.
Since the output capacitor’s ESR affects efficiency, use
low ESR capacitors for best performance. Boost regula-
tors have large RMS ripple current in the output capacitor
that must be rated to handle the current. The output
capacitor ripple current (RMS) is:
The first inductor L1 together with the main N-channel
MOSFET switch resemble a boost converter. The second
inductor L2 and output diode D1 resemble a flyback or
buck-boostconverter. ThetwoinductorsL1andL2canbe
independentbutalsocanbewoundonthesamecoresince
V
− V
IN
OUT
C
I
≈I
OUT RIPPLE RMS
OUT
(
)
V
IN
V
IN
Output ripple is then simply: VOUT = RESR (∆IL(RMS)).
+
R
SENSE
C
IN
Boost Converter: Input Capacitors
V
IN
SENSE
–
The input capacitor of a boost converter is less critical due
to the fact that the input current waveform is triangular,
and does not contain large square wave currents as found
in the output capacitor. The input voltage source imped-
ance determines the size of the capacitor that is typically
10µF to 100µF. A low ESR is recommended although not
as critical as the output capacitor and can be on the order
of 0.3Ω. Input capacitor ripple current for the LTC1624
used as a boost converter is:
L1
C1
D1
BOOST
V
OUT
+
LTC1624
GND
L2
V
M1
TG
R2
R1
C
+
B
C
SW
OUT
FB
1624 F07
Figure 7. SEPIC Converter
15
Linear [ Linear ]
