LTC1624CS datasheet(15/28 Pages) LINER

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LTC1624

Allowing a margin for variations in the LTC1624 (without

considering variation in RSENSE), assuming 30% ripple

current in the inductor, yields:

SENSE

OUT MAX

IN MIN

OUT













()

100

Boost Converter: Output Diode

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. Check to be sure the diode’s current rating exceeds

the maximum current set by RSENSE. Schottky diodes such

as Motorola MBR130LT3 are recommended.

Boost Converter: Output Capacitors

The output capacitor is normally chosen by its effective

series resistance (ESR), because this is what determines

output ripple 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:

OUT

IRIPPLE RMS

() ≈

−

Output ripple is then simply: VOUT = RESR (∆IL(RMS)).

Boost Converter: Input Capacitors

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

µF to 100µF. A low ESR is recommended although not

as critical as the output capacitor and can be on the order

of 0.3

Ω. Input capacitor ripple current for the LTC1624

used as a boost converter is:

APPLICATIONS INFORMATION

kHz L V

OUT

IRIPPLE ≈

() −

()

()( )( )

200

The input capacitor can see a very high surge current when

a battery is suddenly connected and solid tantalum capaci-

tors can fail under this condition. Be sure to specify surge

tested capacitors.

Boost Converter: Duty Cycle Limitations

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%

SEPIC Converter Applications

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.

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-boost converter. The two inductors L1 and L2 can be

independent but also can be wound on the same core since

Figure 7. SEPIC Converter

RSENSE

CIN

VIN

1624 F07

VIN

VFB

LTC1624

SENSE–

BOOST

GND

COUT

VOUT

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