LTC3708 LINER [Linear Technology], LTC3708 Datasheet - Page 19

• Manufacturer Part Number: LTC3708
• Description: Fast 2-Phase, No RSENSE Buck Controller with Output Tracking
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Controlled soft-start requires that the timing capacitor, C
be made large enough to guarantee that the output can
track the voltage rise on the RUN/SS pin. The minimum
C
where R1 and R2 are the feedback resistive dividers
(Functional Diagram), C
and R
MOSFET R
be replaced with the worst-case R
0.1μF is more than suffi cient for C
After the controller has been started and given adequate
time to charge the output capacitor, C
circuit timer. After the RUN/SS pin charges above 3V and
if either output voltage falls below 70% of its regulated
value, a short-circuit fault is assumed. A 2μA current then
begins discharging C
until the RUN/SS pin drops to 2.5V, the controller turns
off all power MOSFETs, shutting down both channels. The
RUN/SS pin must be actively pulled down to ground in
order to restart operation.
Overcurrent latchoff operation is not always needed or
desired and can prove annoying during troubleshooting.
This feature can be overridden by adding a pull-up cur-
rent of >5μA to the RUN/SS pin (Figure 7). The additional
current prevents the discharge of C
also shortens the soft-start period.
SS
C
capacitance can be calculated:
SS
SENSE
ON THRESHOLD
>
R
Figure 6. Monotonic Soft-Start Waveforms
DS(ON)
1
is the current sense resistance. When bottom
R
+
1
R
2
is used for current sensing, R
•
30
SS
μ
V = 0.6V
A R
. If the fault condition persists
V
OUT
RNG
•
SENSE
is the output capacitance
DS(ON)(MAX)
SS
•
SS
SS
.
C
OUT
during a fault and
is used as a short-
RUN/SS
V
TIME
TIME
SENSE
OUT1
3708 F06
. Generally,
should
SS
,
Output Voltage Tracking
The LTC3708 allows the user to program how the second
channel output ramps up and down by means of the TRACK2
pin. Through this pin, the second channel output can be
set up to either coincidently or ratiometrically track the
channel 1 output, as shown in Figure 8.
Similar to RUN/SS, the TRACK2 pin acts as a clamp on
channel 2’s reference voltage. V
TRACK2 voltage when the TRACK2 < 0.6V and to the
internal precision reference when TRACK2 > 0.6V.
To implement the tracking in Figure 8a, connect an extra
resistive divider to the output of channel 1 and connect
its midpoint to the TRACK2 pin. The ratio of this divider
should be selected the same as that of channel 2’s feedback
divider (Figure 9a). In this tracking mode, V
be set higher than V
tracking in Figure 8b, no extra divider is needed; simply
connect the TRACK2 pin to the V
By selecting different resistors, the LTC3708 can achieve
different modes of tracking including the two in Figure 8.
So which mode should be programmed? While either
mode in Figure 8 satisfi es most practical applications,
there does exist some trade-off. The ratiometric mode
saves a pair of resistors but the coincident mode offers
better output regulation. This can be better understood
with the help of Figure 10. At the input stage of channel
2’s error amplifi er, two common anode diodes are used to
clamp the equivalent reference voltage and an additional
diode is used to match the shifted common mode voltage.
The top two current sources are of the same amplitude. In
the coincident mode, the TRACK2 voltage is substantially
higher than 0.6V at steady state and effectively turns off
Figure 7. RUN/SS Pin Interfacing with Latchoff Defeated
ON
3.3V OR 5V
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
OUT2
D1
. To implement the ratiometric
V
IN
R
OUT2
SS
FB1
*
RUN/SS
pin (Figure 9b).
is referenced to the
3708 F07
C
LTC3708
SS
OUT1
19
must
3708fb