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LTC3835

3835fc

APPLICATIONS INFORMATION

RSENSE Selection For Output Current

RSENSE is chosen based on the required output current.

The current comparator has a maximum threshold of

100mV/RSENSE and an input common mode range of

SGND to 10V. 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,

ΔIL.

Allowing a margin for variations in the IC and external

component values yields:

SENSE

MAX

When using the controller in very low dropout conditions,

the maximum output current level will be reduced due to the

internal compensation required to meet stability criterion for

buck regulators operating at greater than 50% duty factor. A

curve is provided to estimate this reduction in peak output

current level depending upon the operating duty factor.

Operating Frequency and Synchronization

The choice of operating frequency, is a trade-off between

efﬁciency and component size. Low frequency operation

improves efﬁciency by reducing MOSFET switching losses,

both gate charge loss and transition loss. However, lower

frequency operation requires more inductance for a given

amount of ripple current.

The internal oscillator of the LTC3835 runs at a nominal

400kHz frequency when the PLLLPF pin is left ﬂoating

and the PLLIN/MODE pin is a DC low or high. Pulling the

PLLLPF to INTVCC selects 530kHz operation; pulling the

PLLLPF to SGND selects 250kHz operation.

Alternatively, the LTC3835 will phase-lock to a clock

signal applied to the PLLIN/MODE pin with a frequency

between 140kHz and 650kHz (see Phase-Locked Loop

and Frequency Synchronization).

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

ΔIL decreases with higher

inductance or frequency and increases with higher VIN:

ΔI

OUT

⎛

⎝⎜

⎞

⎠⎟

()( )

–

Accepting larger values of

ΔIL allows the use of low in-

ductances, but results in higher output voltage ripple and

greater core losses. A reasonable starting point for setting

ripple current is

ΔIL=0.3(IMAX). The maximum ΔIL occurs

at the maximum input voltage.

The inductor value also has secondary effects. The tran-

sition to Burst Mode operation begins when the average

inductor current required results in a peak current below

10% of the current limit determined by RSENSE. Lower

inductor values (higher

ΔIL) will cause this to occur at

lower load currents, which can cause a dip in efﬁciency in

the upper range of low current operation. In Burst Mode

operation, lower inductance values will cause the burst

frequency to decrease.

Inductor Core Selection

Once the value for L is known, the type of inductor must

be selected. High efﬁciency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite or molypermalloy

cores. Actual core loss is independent of core size for a

ﬁxed inductor value, but it is very dependent on inductance

selected. As inductance increases, core losses go down.

Unfortunately, increased inductance requires more turns

of wire and therefore copper losses will increase.

Ferrite designs have very low core loss and are preferred

at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard,” which means that

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