LTC1435CS datasheet(8/20 Pages) LINER

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LTC1435

APPLICATIONS INFORMATION

The basic LTC1435 application circuit is shown in Figure

1, High Efficiency Step-Down Converter. External compo-

nent selection is driven by the load requirement and

begins with the selection of RSENSE. Once RSENSE is

known, COSC and L can be chosen. Next, the power

MOSFETs and D1 are selected. Finally, CIN and COUT are

selected. The circuit shown in Figure 1 can be configured

for operation up to an input voltage of 28V (limited by the

external MOSFETs).

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current.

The LTC1435 current comparator has a maximum thresh-

old of 150mV/RSENSE and an input common mode range

of SGND to INTVCC. The current comparator threshold

sets the peak of the inductor current, yielding a maximum

average output current IMAX equal to the peak value less

half the peak-to-peak ripple current

∆IL.

Allowing a margin for variations in the LTC1435 and

external component values yields:

SENSE

MAX

100

The LTC1435 works well with values of RSENSE from

0.005

Ω to 0.2Ω.

COSC Selection for Operating Frequency

The LTC1435 uses a constant frequency architecture with

the frequency determined by an external oscillator capaci-

tor COSC. Each time the topside MOSFET turns on, the

voltage COSC is reset to ground. During the on-time, COSC

is charged by a fixed current. When the voltage on the

capacitor reaches 1.19V, COSC is reset to ground. The

process then repeats.

The value of COSC is calculated from the desired operating

frequency:

CpF

OSC ()

–













1.37(10 )

Frequency (kHz)

A graph for selecting COSC vs frequency is given in Figure

2. As the operating frequency is increased the gate charge

OPERATING FREQUENCY (kHz)

300

250

200

150

100

200

300

400

LTC1435 • F02

500

Figure 2. Timing Capacitor Value

losses will be higher, reducing efficiency (see Efficiency

Considerations). The maximum recommended switching

frequency is 400kHz.

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. So why would

anyone ever choose to operate at lower frequencies with

larger components? The answer is efficiency. A higher

frequency generally results in lower efficiency because of

MOSFET gate charge losses. In addition to this basic

trade-off, the effect of inductor value on ripple current and

low current operation must also be considered.

The inductor value has a direct effect on ripple current. The

inductor ripple current

∆IL decreases with higher induc-

tance or frequency and increases with higher VIN or VOUT:

∆I

OUT

= ()( )









1–

Accepting larger values of

∆IL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is

∆IL = 0.4(IMAX). Remember, the

maximum

∆IL occurs at the maximum input voltage.

The inductor value also has an effect on low current

operation. The transition to low current operation begins

when the inductor current reaches zero while the bottom

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