LTC1439CG datasheet(12/32 Pages) LINER

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LTC1438/LTC1439

APPLICATIONS INFORMATION

A graph for selecting COSC vs frequency is given in Figure

2. As the operating frequency is increased the gate charge

losses will be higher, reducing efficiency (see Efficiency

Considerations). The maximum recommended switching

frequency is 400kHz. When using Figure 2 for

synchronizable applications, choose COSC corresponding

to a frequency approximately 30% below your center

frequency. (See Phase-Locked Loop and Frequency

Sychronization).

OPERATING FREQUENCY (kHz)

300

250

200

150

100

200

300

400

LTC1435 • F02

500

VPLLLPF = 0V

Figure 2. Timing Capacitor Value

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









()( )

–

The basic LTC1439 application circuit is shown in Fig-

ure 1. External component 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 config-

ured 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 LTC1438/LTC1439 current comparator has a maxi-

mum threshold 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 some margin for variations in the LTC1438/

LTC1439 and external component values yield:

SENSE

MAX

100

The LTC1438/LTC1439 work well with values of RSENSE

from 0.005

Ω to 0.2Ω.

COSC Selection for Operating Frequency

The LTC1438/LTC1439 use a constant frequency archi-

tecture with the frequency determined by an external

oscillator capacitor on COSC.EachtimethetopsideMOSFET

turns on, the voltage on COSC is reset to ground. During the

on-time, COSC is charged by a fixed current plus an

additional current which is proportional to the output

voltage of the phase detector (VPLLLPF)(LTC1439 only).

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. Assuming the phase-locked loop has no exter-

nal oscillator input (VPLLLPF = 0V):

CpF

OSC()

)













−

137 10

Frequency (kHz)

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