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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

FB Resistor Network

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resis-

tors according to:

R1=R2 VOUT – 1

1.21

Reference designators refer to the Block Diagram. Note

that choosing larger resistors will decrease the quiescent

current of the application circuit.

Setting the Switching Frequency

The LT3970 uses a constant frequency PWM architecture

that can be programmed to switch from 200kHz to 2.2MHz

by using a resistor tied from the RT pin to ground. A table

showing the necessary RT value for a desired switching

frequency is in Table 1.

Table 1. Switching Frequency vs RT Value

SWITCHING FREQUENCY (MHz)

RT VALUE (kΩ)

0.2

768

0.3

499

0.4

357

0.5

280

0.6

226

0.8

158

1.0

124

1.2

100

1.4

80.6

1.6

68.1

1.8

57.6

2.0

49.9

2.2

42.2

Operating Frequency Trade-Offs

Selection of the operating frequency is a trade-off between

efficiency, component size, minimum dropout voltage and

maximum input voltage. The advantage of high frequency

operation is that smaller inductor and capacitor values may

be used. The disadvantages are lower efficiency, lower

maximum input voltage, and higher dropout voltage. The

highest acceptable switching frequency (fSW(MAX)) for a

given application can be calculated as follows:

( ) fSW(MAX)

=

VOUT

tON(MIN) VIN

+

–

VD

VSW

+

VD

where VIN is the typical input voltage, VOUT is the output

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

and VSW is the internal switch drop (~0.5V at max load).

This equation shows that slower switching frequency is

necessary to accommodate high VIN/VOUT ratio.

Lower frequency also allows a lower dropout voltage. The

input voltage range depends on the switching frequency

because the LT3970 switch has finite minimum on and off

times. The switch can turn on for a minimum of ~150ns and

turn off for a minimum of ~160ns (note that the minimum

on-time is a strong function of temperature). This means

that the minimum and maximum duty cycles are:

DCMIN = fSW • tON(MIN)

DCMAX = 1 – fSW • tOFF(MIN)

where fSW is the switching frequency, the tON(MIN) is the

minimum switch on-time (~150ns), and the tOFF(MIN) is

the minimum switch off-time (~160ns). These equations

show that duty cycle range increases when switching

frequency is decreased.

A good choice of switching frequency should allow ad-

equate input voltage range (see next section) and keep

the inductor and capacitor values small.

Input Voltage Range

The minimum input voltage is determined by either the

LT3970’s minimum operating voltage of 4.2V or by its

maximum duty cycle (as explained in previous section).

The minimum input voltage due to duty cycle is:

VIN(MIN)

=

1–

VOUT + VD

fSW • tOFF(MIN)

–

VD

+

VSW

where VIN(MIN) is the minimum input voltage, VOUT is

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

VSW is the internal switch drop (~0.5V at max load), fSW

is the switching frequency (set by RT), and tOFF(MIN) is

the minimum switch off-time (160ns). Note that higher

switching frequency will increase the minimum input

voltage. If a lower dropout voltage is desired, a lower

switching frequency should be used.

3970fa

10

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