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LT3689/LT3689-5

3689fa

Another way to lower the start-up voltage is by using a

resistor divider on the EN/UVLO pin (see the Shutdown

and Undervoltage Lockout section). A resistor divider on

EN/UVLO pin programs the turn-on threshold to slightly

higher than the minimum VIN voltage required to run at full

load. Below the EN/UVLO high voltage, the part will stay

shut off and the output cap will remain discharged during

the worse case slow VIN ramp. When the EN/UVLO pin

crosses the EN/UVLO high threshold, the part will turn on

and the empty output capacitor will provide enough load

to bring the output voltage in regulation. This technique

signiﬁcantly lowers the start-up voltage of the circuit. The

plot in Figure 7 depicts the minimum load required to start

and run (as a function of input voltage). It also depicts the

beneﬁt of programming the EN/UVLO threshold to lower

the start-up voltage at low load currents. At light loads, the

inductor current becomes discontinuous and the effective

duty cycle can be very high. This reduces the minimum input

voltage to approximately 300mV above VOUT. At higher load

currents, the inductor current is continuous and the duty

cycle is limited by the maximum duty cycle of the LT3689,

requiring a higher input voltage to maintain regulation.

Soft-Start

The LT3689 has an internal soft-start that gradually ramps

up the switch current limit from about 100mA to the

switch’s maximum current limit in typical value of 150μs,

as shown in Figure 8. This feature limits the inrush current

during start-up and prevents the switch current from spik-

ing when the EN/UVLO pin crosses the UVLO threshold.

A soft-start sequence is also initiated right after a VIN

overvoltage/undervoltage lockout, or thermal shutdown

fault in order to prevent the switch current from suddenly

jumping to its maximum current limit.

APPLICATIONS INFORMATION

Figure 7. LT3689 Minimum VIN to Start and Run vs Load

Figure 8. Internal Soft-Start

20μs/DIV

VOUT

2V/DIV

VIN = 12V

VOUT = 5V

f = 2MHz

CIN = 1μF CERAMIC + 100μF ELECTROLYTIC

IVIN

0.2A/DIV

0.5A/DIV

3689 F08

LOAD (mA)

7.0

7.5

100

1000

6.5

6.0

5.5

5.0

8.0

3589 F07a

TO START:

EN/UVLO TIED TO VIN

TO RUN

VOUT = 5V

L = 8.2μH

fSW = 800KHz

TA = 25°C

TO START: EN/UVLO HIGH

THRESHOLD = 6.15V

LOAD (mA)

5.0

5.5

100

1000

4.5

4.0

3.5

3.0

6.0

3589 F07b

TO START:

EN/UVLO TIED TO VIN

TO RUN

VOUT = 3.3V

L = 8.2μH

fSW = 800KHz

TA = 25°C

TO START: EN/UVLO HIGH

THRESHOLD = 4.85V

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