LTC3703 datasheet(22/32 Pages) LINER

22 / 32 page

LTC3703

3703f

Boost Converter: Output Capacitor Selection

In boost mode, the output capacitor requirements are

more demanding due to the fact that the current waveform

is pulsed instead of continuous as in a buck converter. The

choice of component(s) is driven by the acceptable ripple

voltage which is affected by the ESR, ESL and bulk

capacitance as shown in Figure 15. The total output ripple

voltage is:

∆=









ESR

OUT

O MAX

OUT

MAX

()

•–

where the first term is due to the bulk capacitance and

second term due to the ESR.

APPLICATIO S I FOR ATIO

discussion in Input Capacitor section for the buck con-

verter). With this combination, the ripple voltage can be

improved significantly. The low ESR ceremic capacitor

will minimize the ESR step, while the electrolytic will

supply the required bulk capacitance.

Boost Converter: Input Capacitor Selection

The input capacitor of a boost converter is less critical than

the output capacitor, due to the fact that the inductor is in

series with the input and the input current waveform is

continuous. The input voltage source impedance deter-

mines the size of the input capacitor, which is typically in

the range of 10

µF to 100µF. A low ESR capacitor is

recommended though not as critical as for the output

capacitor.

The RMS input capacitor ripple current for a boost con-

verter is:

RMS CIN

IN MIN

MAX

()

.•

•

= 03

Please note that the input capacitor can see a very high

surge current when a battery is suddenly connected to the

input of the converter and solid tantalum capacitors can

fail catastrophically under these conditions. Be sure to

specify surge-tested capacitors!

Boost Converter: Current Limit Programming

The LTC3703 provides current limiting in boost mode by

monitoring the VDS of the main switch during its on-time

and comparing it to the voltage at IMAX. To set the current

limit, calculate the expected voltage drop across the

MOSFET at the maximum desired inductor current and

maximum junction temperature. The maximum inductor

current is a function of both duty cycle and maximum load

current, so the limit must be set for the maximum expected

duty cycle (minimum VIN) in order to ensure that the

current limit does not kick in at loads < IO(MAX):

PROG

OMAX

MAX

DS ON

OUT

IN MIN

O MAX

DS ON









()

(

)

–

()

•(

)

Figure 15. Output Voltage Ripple

Waveform for a Boost Converter

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

∆VESR

∆VCOUT

VOUT

(AC)

The choice of output capacitor is driven also by the RMS

ripple current requirement. The RMS ripple current is:

RMS COUT

O MAX

IN MIN

()

(

)

()

•

–

≈

At lower output voltages (less than 30V), it may be

possible to satisfy both the output ripple voltage and RMS

ripple current requirements with one or more capacitors of

a single capacitor type. However, at output voltages above

30V where capacitors with both low ESR and high bulk

capacitance are hard to find, the best approach is to use a

combination of aluminum and ceramic capacitors (see

MAIN

MAX

DS ON

OUT

MAX

MILLER

TH IL

SYNC

MAX

DS ON









()







()(

)













()







()

()

–

•

–

()

LTC3703 Datasheet(PDF) 22 Page - Linear Technology

LTC3703 Datasheet(HTML) 22 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL