LTC3708_15 Datasheet, PDF (21/32 Pages) Linear Technology – Fast 2-Phase, No RSENSE Buck Controller with Output Tracking

LTC3708
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.
V_OUT1
TO TRACK2 PIN
TO V_FB1 PIN
V_OUT2
R1
R4
TO V_FB2 PIN
R2
R5
R3
3708 F11
Figure 11. Alternative Setup for Coincident Tracking
⎛
⎝⎜
R1+ R2
R3
=
V_OUT1
0.6
–
1,
R1
R2 + R3
=
R4
R5
=
V_OUT2
0.6
−
⎞
1_⎠⎟
Efficiency Considerations
The percent efficiency 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 efficiency 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 I²R Losses. These arise from the resistances of the
MOSFETs, inductor and PC board traces and cause the
efficiency to drop at high output currents. In continuous
mode, the average output current flows 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 I²R
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.
V_OUT1
TO V_CC
R4 R5 R1
R2 R2 R2
TRACK1 TRACK2
LTC3708
“MASTER”
V_FB1
RUN/SS
V_FB2
C_SS
V_OUT2
R3
R2
V_OUT1
V_OUT3
V_OUT3
TO V_CC
R4 100k
R2
TRACK1 TRACK2
LTC3708
“SLAVE”
V_FB1
RUN/SS
V_FB2
V_OUT4
R5
R2
(12a) Circuit Setup
V_OUT4
V_OUT2
TIME
3708 F12
(12b) Output Voltage
Figure 12. Four Outputs with Tracking and Ratiometric Sequencing
⎛ R1
⎝⎜ R2
=
V_OUT1
0.6
− 1,
R3
R2
=
V_OUT2
0.6
− 1,
R4
R2
=
V_OUT3
0.6
− 1,
R5
R2
=
V_OUT4
0.6
⎞
− 1_⎠⎟
3708fb
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