LTC1435AI datasheet(13/20 Pages) LINER

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LTC1435A

APPLICATIONS INFORMATION

on and off again. It is determined by internal timing delays

and the gate charge required to turn on the top MOSFET.

Low duty cycle applications may approach this minimum

on-time limit. If the duty cycle falls below what can be

accommodated by the minimum on-time, the LTC1435A

will begin to skip cycles. The output voltage will continue

to be regulated, but the ripple current and ripple voltage will

increase. Therefore this limit should be avoided.

The minimum on-time for the LTC1435A in a properly

configured application is less than 300ns but increases at

low ripple current amplitudes (see Figure 7). If an appli-

cation is expected to operate close to the minimum on-time

limit, an inductor value must be chosen that is low enough

to provide sufficient ripple amplitude to meet the minimum

on-time requirement. To determine the proper value, use

the following procedure:

1. Calculate on-time at maximum supply, tON(MIN) =

(1/f)(VOUT/VIN(MAX)).

2. Use Figure 7 to obtain the peak-to-peak inductor ripple

current as a percentage of IMAX necessary to achieve the

calculated tON(MIN).

3. Ripple amplitude

∆IL(MIN) = (% from Figure 7)(IMAX)

where IMAX = 0.1/RSENSE.

4. LMAX = t

ON MIN

IN MAX

OUT

L MIN

()

–

∆













Choose an inductor less than or equal to the calculated LMAX

to ensure proper operation.

Because of the sensitivity of the LTC1435A current com-

parator when operating close to the minimum on-time limit,

it is important to prevent stray magnetic flux generated by

the inductor from inducing noise on the current sense re-

sistor, which may occur when axial type cores are used. By

orienting the sense resistor on the radial axis of the induc-

tor (see Figure 8), this noise will be minimized.

Figure 7. Minimum On-Time vs Inductor Ripple Current

INDUCTOR RIPPLE CURRENT (% OF IMAX)

200

250

300

350

400

RECOMMENDED

REGION FOR MIN

ON-TIME AND

MAX EFFICIENCY

1435A F07

INDUCTOR

1435A F08

Figure 8. Allowable Inductor/RSENSE Layout Orientations

Efficiency Considerations

The efficiency of a switching regulator is equal to the out-

put power divided by the input power times 100%. It is often

useful to analyze individual losses to determine what is

limiting the efficiency and which change would produce the

most improvement. Efficiency can be expressed as:

Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percentage

of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1435A circuits. LTC1435A VINcurrent,INTVCC

current, I2R losses, and topside MOSFET transition losses.

1. The VIN current is the DC supply current given in the

electrical characteristics which excludes MOSFET driver

and control currents. VIN current results in a small

(< 1%) loss which increases with VIN.

2. INTVCC current is the sum of the MOSFET driver and

control currents. The MOSFET driver current results from

switching the gate capacitance of the power MOSFETs.

Each time a MOSFET gate is switched from low to high

to low again, a packet of charge dQ moves from INTVCC

to ground. The resulting dQ/dt is a current out of INTVCC

that is typically much larger than the control circuit cur-

rent. In continuous mode, IGATECHG = f(QT + QB), where

QT and QB are the gate charges of the topside and bot-

tom side MOSFETs.

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