LTC3708_15 Datasheet, PDF (19/32 Pages) Linear Technology – Fast 2-Phase, No RSENSE Buck Controller with Output Tracking

LTC3708
APPLICATIONS INFORMATION
ON THRESHOLD
$V = 0.6V
RUN/SS
TIME
V_OUT1
3.3V OR 5V
D1
ON
V_IN
RUN/SS
R_SS*
C_SS
3708 F07
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
Figure 7. RUN/SS Pin Interfacing with Latchoff Defeated
TIME
3708 F06
Figure 6. Monotonic Soft-Start Waveforms
Controlled soft-start requires that the timing capacitor, C_SS,
be made large enough to guarantee that the output can
track the voltage rise on the RUN/SS pin. The minimum
C_SS capacitance can be calculated:
C_SS
>
R1+ R2
R1
•
30μA • R_SENSE
V_RNG
•
C_OUT
where R1 and R2 are the feedback resistive dividers
(Functional Diagram), C_OUT is the output capacitance
and R_SENSE is the current sense resistance. When bottom
MOSFET R_DS(ON) is used for current sensing, R_SENSE should
be replaced with the worst-case R_DS(ON)(MAX). Generally,
0.1μF is more than sufficient for C_SS.
After the controller has been started and given adequate
time to charge the output capacitor, C_SS is used as a short-
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_SS. If the fault condition persists
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_SS during a fault and
also shortens the soft-start period.
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_OUT2 is referenced to the
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_OUT1 must
be set higher than V_OUT2. To implement the ratiometric
tracking in Figure 8b, no extra divider is needed; simply
connect the TRACK2 pin to the V_FB1 pin (Figure 9b).
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 satisfies 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 amplifier, 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
3708fb
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