LTC3726 Datasheet, PDF (13/16 Pages) Linear Technology – Secondary-Side Synchronous Forward Controller

LTC3726
APPLICATIO S I FOR ATIO
the LTC3726) can be used as the input to the peak charge
circuit of Figure 2. For lower output voltages, however, it
is normally necessary to use a dedicated bias winding to
generate adequate bias voltage for the LTC3726.
Current Sensing
The LTC3726 provides considerable flexibility in cur-
rent sensing techniques. It supports two main meth-
ods: 1) resistive current sensing and 2) current trans-
former current sensing. Resistive current sensing is
generally simpler, smaller and less expensive, while
current transformer sensing is more efficient and gen-
erally appropriate for higher (>20A) output currents.
For resistive current sensing, the sense resistor may
be placed in any one of three different locations: high
side inductor, low side inductor or low side switch, as
shown in Figure 3. Sensing the inductor 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 pro-
vides a more convenient layout than low side (no split
ground plane), but can only be used for output volt-
ages 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
dissipation) is very important, a current sense trans-
former 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+
LTC3726
I_S–
78mV MAX
3726 F03a
Figure 3a. High Side Inductor:
Easier Layout, Low Noise, Accurate
••
I_S+
LTC3726
I_S–
78mV MAX
3726 F03b
Figure 3b. Low Side Inductor:
Accurate, Low Noise, High V_OUT Capable
••
I_S+
LTC3726
I_S–
78mV MAX
3726 F03c
Figure 3c. Switch Current Sensing: Easy Layout, Accurate,
Higher Efficiency, High V_OUT Capable
••
•
5Ω TO
• ^50Ω
1.28V MAX
TRIP _IS+
LTC3726
V_CC I_S–
3726 F03d
Figure 3d. Current Transformer:
Highest Efficiency, High V_OUT Capable
Figure 3. Current Sensing Techniques
3726fb
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