LT3688 LINER [Linear Technology], LT3688 Datasheet - Page 19

• Manufacturer Part Number: LT3688
• Description: Dual 800mA Step-Down Switching Regulator with Power-On Reset
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
age source. However, the full benefi t 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 V
For lower start-up voltage, the boost diode can be tied to
V
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 V
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 V
the startup of V
from working on V
programmed value, increasing the required startup volt-
age. Using circuit in Figure 6b for V
startup voltage to its normal value. An alternative is to tie
BIAS to V
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
IN
; however, this restricts the input range to one-half of the
OUT1
, if it is greater than 2.8V.
OUT1
OUT1
. This will prevent the boost circuit
until it has risen to 90% of its
OUT2
IN
is ramping very slowly.
, it will be low during
OUT1
OUT
will reduce the
. At higher load
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
Confi guration 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 V
switch current and therefore input current during start up.
A good value for the soft-start capacitor is C
where C
The RUN/SS pins can be left fl oating 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
Figure 7. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
OUT
7.5
6.5
5.5
4.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
is the value of the output capacitor.
8
7
6
5
4
1
1
V
V
OUT
OUT
= 3.3V
= 5V
TO RUN
TO RUN
10
10
TO START
LOAD (mA)
LOAD (mA)
TO START
100
100
C
pin, limiting the peak
3688 F07a
3688 F07b
1000
1000
LT3688
OUT
/10,000,
19
3688f