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LT3970

40V, 350mA Step-Down Regulator with 2.5µA Quiescent Current and Integrated Diodes

Linear Technology 

LT3970/LT3970-3.3/LT3970-5

APPLICATIONS INFORMATION

The highest allowed VIN during normal operation

(VIN(OP-MAX)) is limited by minimum duty cycle and can

be calculated by the following equation:

VIN(OP-MAX)

=

VOUT + VD

fSW • tON(MIN)

–

VD

+

VSW

where tON(MIN) is the minimum switch on-time (~150ns).

However, the circuit will tolerate inputs up to the absolute

maximum ratings of the VIN and BOOST pins, regardless of

chosen switching frequency. During such transients where

VIN is higher than VIN(OP-MAX), the switching frequency will

be reduced below the programmed frequency to prevent

damage to the part. The output voltage ripple and inductor

current ripple may also be higher than in typical operation,

however the output will still be in regulation.

Inductor Selection

For a given input and output voltage, the inductor value

and switching frequency will determine the ripple current.

The ripple current increases with higher VIN or VOUT and

decreases with higher inductance and faster switching

frequency. A good starting point for selecting the induc-

tor value is:

L

=

3

VOUT +

fSW

VD

Table 2. Inductor Vendors

VENDOR

Coilcraft

Sumida

Toko

Würth Elektronik

Coiltronics

Murata

URL

www.coilcraft.com

www.sumida.com

www.tokoam.com

www.we-online.com

www.cooperet.com

www.murata.com

where VD is the voltage drop of the catch diode (~0.7V),

L is in μH and fSW is in MHz. The inductor’s RMS current

rating must be greater than the maximum load current

and its saturation current should be about 30% higher.

For robust operation in fault conditions (start-up or short

circuit) and high input voltage (>30V), the saturation

current should be above 500mA. To keep the efficiency

high, the series resistance (DCR) should be less than

0.1Ω, and the core material should be intended for high

frequency applications. Table 2 lists several vendors and

suitable types.

This simple design guide will not always result in the

optimum inductor selection for a given application. As a

general rule, lower output voltages and higher switching

frequency will require smaller inductor values. If the ap-

plication requires less than 350mA load current, then a

lesser inductor value may be acceptable. This allows use

of a physically smaller inductor, or one with a lower DCR

resulting in higher efficiency. There are several graphs in

the Typical Performance Characteristics section of this data

sheet that show the maximum load current as a function

of input voltage for several popular output voltages. Low

inductance may result in discontinuous mode operation,

which is acceptable but reduces maximum load current.

For details of maximum output current and discontinu-

ous mode operation, see Linear Technology Application

Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN

> 0.5), there is a minimum inductance required to avoid

subharmonic oscillations. See Application Note 19.

Input Capacitor

Bypass the input of the LT3970 circuit with a ceramic

capacitor of X7R or X5R type. Y5V types have poor

performance over temperature and applied voltage, and

should not be used. A 1μF to 4.7μF ceramic capacitor

is adequate to bypass the LT3970 and will easily handle

3970fa

11

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