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LT3688

3688f

age source. However, the full beneﬁt of the BIAS pin is not

realized unless it is at least 3V. Ensure that the maximum

voltage ratings of the BST and BIAS pins are not exceeded.

The minimum operating voltage of an LT3688 application

is limited by the minimum input voltage (3.6V) and by the

maximum duty cycle, as outlined in a previous section. For

proper start-up, the minimum input voltage is also limited

by the boost circuit. If the input voltage is ramped slowly,

or the LT3688 is turned on with its EN/UVLO pin when the

output is already in regulation, then the boost capacitor may

not be fully charged. Because the boost capacitor is charged

with the energy stored in the inductor, the circuit will rely

on some minimum load current to get the boost circuit

running properly. This minimum load will depend on input

and output voltages, and on the arrangement of the boost

circuit. The minimum load generally goes to zero once the

circuit has started. Figure 7 shows a plot of minimum load

to start and to run as a function of input voltage. In many

cases, the discharged output capacitor will present a load

to the switcher, which will allow it to start. The plots show

the worst-case situation where VIN is ramping very slowly.

For lower start-up voltage, the boost diode can be tied to

VIN; however, this restricts the input range to one-half of the

absolute maximum rating of the BST pin. 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 LT3688,

requiring a higher input voltage to maintain regulation.

There is one particular issue to note if sequencing is

used. If the BIAS pin is tied to VOUT2, it will be low during

the startup of VOUT1. This will prevent the boost circuit

from working on VOUT1 until it has risen to 90% of its

programmed value, increasing the required startup volt-

age. Using circuit in Figure 6b for VOUT1 will reduce the

startup voltage to its normal value. An alternative is to tie

BIAS to VOUT1, if it is greater than 2.8V.

Soft-Start and Individual Channel Shutdown

The RUN/SS (Run/Soft-Start) pins are used to place the

individual switching regulators in shutdown mode. They

also provide a soft-start function. To shut down either

Figure 7. The Minimum Input Voltage Depends on

Output Voltage, Load Current and Boost Circuit

regulator, pull the RUN/SS pin to ground with an open-

drain or collector. Note that if CONFIG is tied high or low

(not open), shutting down Channel 1 will also shut down

Channel 2 because of the sequencing function (See the

Conﬁguration and Sequencing section for more details).

2.5μA current sources pull up on each pin. If the RUN/SS

pin reaches ~0.2V, the channel will begin to switch

If a capacitor is tied from the RUN/SS pin to ground, then

the internal pull-up current will generate a voltage ramp on

this pin. This voltage clamps the VC pin, limiting the peak

switch current and therefore input current during start up.

A good value for the soft-start capacitor is COUT/10,000,

where COUT is the value of the output capacitor.

The RUN/SS pins can be left ﬂoating if the Soft-Start feature

is not used. They can also be tied together with a single

capacitor providing soft-start. The internal current sources

APPLICATIONS INFORMATION

LOAD (mA)

TO START

VOUT = 5V

TO RUN

100

1000

7.5

6.5

4.5

5.5

3688 F07a

LOAD (mA)

100

1000

3.0

6.0

6.5

5.0

5.5

3.5

4.0

4.5

7.0

3688 F07b

TO START

TO RUN

VOUT = 3.3V

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