LTC3706_15 Datasheet, PDF (16/22 Pages) Linear Technology – Secondary-Side Synchronous Forward Controller with PolyPhase Capability

LTC3706
APPLICATIONS INFORMATION
current (high side or low side) is generally less noisy but
dissipates more power than sensing the switch current
(Figures 3a and 3b). High side inductor current sensing
provides a more convenient layout than low side (no split
ground plane), but can only be used for output voltages
up to 5.5V, due to the common mode limitations of the
current sense inputs (I_S+ and I_S–). For most applications,
low side switch current sensing will be a good solution
(Figure 3c).
For high current applications where efficiency (power dis-
sipation) is very important, a current sense transformer
may be used. As shown in Figure 3d, the I_S– pin should
be tied off to V_CC when a current sense transformer is
used. This causes the I_S+ pin to become a single ended
(nondifferential) current sense input with a maximum
current sense voltage of 1.28V. Figure 3d shows a typical
application circuit using a current transformer.
••
I_S+
LTC3706
I_S–
78mV MAX
3706 F03a
Figure 3a. High Side Inductor:
Easier Layout, Low Noise, Accurate
••
I_S+
LTC3706
I_S–
78mV MAX
3706 F03b
Figure 3b. Low Side Inductor:
Accurate, Low Noise, High V_OUT Capable
••
I_S+
LTC3706
I_S–
78mV MAX
3706 F03c
Figure 3c. Switch Current Sensing: Easy Layout,
Accurate, Higher Efficiency, High V_OUT Capable
••
•
5W TO
• ^50Ω
1.28V MAX
TRIP _IS+
LTC3706
V_CC I_S–
3706 F03d
Figure 3d. Current Transformer:
Highest Efficiency, High V_OUT Capable
Figure 3. Current Sensing Techniques
3706fd
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