Datasheet
LTC3708
21
3708fb
APPLICATIONS INFORMATION
connected to a soft-start capacitor. All other LTC3708s
should have their RUN/SS pins pulled up to V
CC
with a
resistor between 50k and 300k. Figure 12 shows the circuit
with four outputs. Three of them are programmed in the
coincident mode while the fourth one tracks ratiometrically.
If output tracking is not needed, connect the TRACK pins
to V
CC
. Do Not Float These Pins.
Effi ciency Considerations
The percent effi ciency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the effi ciency and which change would
produce the most improvement.
Although all dissipative elements in the circuit produce
losses, four main sources account for most of the losses
in LTC3708 circuits:
1. DC I
2
R Losses. These arise from the resistances of the
MOSFETs, inductor and PC board traces and cause the
effi ciency to drop at high output currents. In continuous
mode, the average output current fl ows through L, but is
chopped between the top and bottom MOSFETs. If the two
MOSFETs have approximately the same R
DS(ON)
, then the
resistance of one MOSFET can simply be summed with the
resistances of L and the board traces to obtain the DC I
2
R
loss. For example, if R
DS(ON)
= 0.01Ω and R
L
= 0.005Ω,
the loss will range from 15mW up to 1.5W as the output
current varies from 1A to 10A.
R1 R4
V
OUT1
V
OUT2
TO TRACK2 PIN
TO V
FB1
PIN
TO V
FB2
PIN
R2 R5
R3
3708 F11
Figure 11. Alternative Setup for Coincident Tracking
RR
R
V
R
RR
R
R
V
OUT OUT
12
306
1
1
23
4
506
1
12
+
=
+
== −
⎛
.
–,
.
⎝⎝
⎜
⎞
⎠
⎟
TRACK1
V
FB1
C
SS
R3R1
R2
R5
V
OUT1
R2
R4
R2
R2
V
FB2
TO V
CC
TO V
CC
V
OUT2
R5
R2
V
OUT4
R4
100k
R2
V
OUT3
RUN/SS
TRACK2
LTC3708
“MASTER”
TRACK1
V
FB1
V
FB2
RUN/SS
TRACK2
LTC3708
“SLAVE”
(12a) Circuit Setup
TIME
3708 F12
(12b) Output Voltage
V
OUT1
V
OUT3
V
OUT4
V
OUT2
OUTPUT VOLTAGE
Figure 12. Four Outputs with Tracking and Ratiometric Sequencing
R
R
V
R
R
V
R
R
V
OUT OUT OUT
1
206
1
3
206
1
4
20
12 3
=−=−=
.
,
.
,
..
,
.6
1
5
206
1
4
−= −
⎛
⎝
⎜
⎞
⎠
⎟
R
R
V
OUT