LT3689 datasheet(13/32 Pages) LINER

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LT3689/LT3689-5

3689fa

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose 1% resistors

according to:

R1= R2

VOUT

0.8V

For reference designators, refer to the Block Diagram.

Setting the Switching Frequency

The LT3689 uses a constant-frequency PWM architecture

that can be programmed to switch from 350kHz to 2.2MHz

by using a resistor tied from the RT pin to ground. Table 1

shows the RT values for various switching frequencies.

Table 1. Switching Frequency vs RT

SWITCHING FREQUENCY (MHz)

RT (kΩ)

0.35

48.7

0.5

31.6

0.6

24.9

0.7

20.5

0.8

16.9

0.9

14.7

12.7

1.2

9.53

1.4

7.5

1.6

6.04

1.8

4.87

4.02

2.2

3.16

APPLICATIONS INFORMATION

Operating Frequency Tradeoffs

Selection of the operating frequency is a tradeoff between

efﬁciency, component size and maximum input voltage.

The advantage of high frequency operation is that smaller

inductor and capacitor values may be used. The disad-

vantages are lower efﬁciency, and narrower input voltage

range at constant-frequency. The highest constant-switch-

ing frequency (fSW(MAX)) for a given application can be

calculated as follows:

SW MAX

OUT

ON MIN

()

–

()

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

voltage, VD is the catch diode drop (~0.5V) and VSW is

the internal switch drop (~0.5V at maximum load). If the

LT3689 is programmed to operate at a frequency higher

than fSW(MAX) for a given VIN input voltage, the LT3689

enters pulse-skipping mode, where it skips switching

cycles to maintain regulation. At frequencies higher than

fSW(MAX), the LT3689 no longer operates with constant-

frequency. The LT3689 enters pulse-skipping mode at

frequencies higher than fSW(MAX) because of the limita-

tion on the LT3689’s minimum on-time of 130ns. As the

switching frequency is increased above fSW(MAX), the part

is required to switch for shorter periods of time to maintain

the same duty cycle. Delays associated with turning off

the power switch dictate the minimum on-time of the part.

When the required on-time decreases below the minimum

on-time of 130ns, the switch pulse width remains ﬁxed at

130ns (instead of becoming narrower to accommodate

the duty cycle requirement). The inductor current ramps

up to a value exceeding the load current and the output

ripple increases. The part then remains off until the output

voltage dips below the programmed value before it begins

switching again.

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