LT3990 Просмотр технического описания (PDF) - Linear Technology

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LT3990

62V, 350mA Step-Down Regulator with 2.5µA Quiescent Current and Integrated Diodes

Linear Technology 

LT3990

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

1.21

–

1

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 LT3990 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

787

0.3

511

0.4

374

0.5

287

0.6

232

0.8

169

1.0

127

1.2

102

1.4

84.5

1.6

69.8

1.8

59

2.0

51.1

2.2

44.2

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 fre-

quency because the LT3990 switch has finite minimum

on and off times. The switch can turn off for a minimum

of ~160ns but the minimum on-time is a strong function

of temperature. Use the minimum switch on-time curve

(see Typical Performance Characteristics) to design for

an application’s maximum temperature, while adding

about 30% for part-to-part variation. The minimum and

maximum duty cycles that can be achieved taking these

on and off times into account 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, 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

LT3990’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 switch-

ing frequency will increase the minimum input voltage.

3990f



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