Datasheet

LTC3766
28
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
The damping inductor L
D
does not carry the DC input
current. However, to ensure adequate attenuation during
large transients, choose an inductor whose saturation
current is at least:
I
SAT LD
( )
≥0.6
V
OUT
I
OUT(MAX)
V
IN(MIN)
⎛
⎝
⎜
⎞
⎠
⎟
Output Capacitor Selection
The selection of C
OUT
is driven by the effective series
resistance (ESR) and the resulting output voltage ripple.
Typically, once the ESR requirement is satisfied, the
capacitance is adequate for filtering. The output ripple
(∆V
OUT
) is approximated by:
ΔV
OUT
≈ ΔI
L
ESR+
1
8f
SW
C
OUT
⎛
⎝
⎜
⎞
⎠
⎟
where f
SW
is the operating frequency, C
OUT
is the output
capacitance and ∆I
L
is the ripple current in the inductor.
The output ripple is highest at maximum input voltage
since ∆I
L
increases with input voltage.
Current Sensing and Average Current Limit
The LTC3766 supports current sensing either with a cur
-
rent sense resistor or with an isolated current transformer.
A current transformer is generally more efficient and has
the advantage of sensing current on the primary side in
isolated applications. This can be important because it
provides an additional safeguard against saturating the
main transformer during load transients. In addition, a
current transformer can generate a much larger current
sense signal than a sense resistor, resulting in a vastly
superior signal to noise ratio. This eases board layout
concerns for noise pickup and reduces jitter as well. Also,
the accuracy of LTC3766 current limit is significantly better
in current transformer mode than in current sense mode.
Compared to a current transformer, a current sense resistor
is less expensive and somewhat simpler to apply than a
current transformer. When current sensing on the sec
-
ondary side of an active clamp forward converter, direct
flux limit is required to prevent transformer saturation
and possible damage of the primary-side MOSFET.
This is because the current loop does not see the mag-
netizing current, and will not provide its own safeguard
against saturation. Note that in nonisolated applications,
however, the current sense resistor is placed in series
with the primar
y-side switch, so the current loop will be
monitoring magnetizing current.
When using a current sense resistor, the I
S
+
and I
S
–
pins
operate differentially and the maximum peak current thresh-
old is approximately 75mV. Normally, the current sense
resistor is placed in the sour
ce of the for
ward MOSFET,
as shown in Figure 11. Depending on PCB layout and the
shielding of the traces going to the I
S
+
and I
S
–
pins, it is
sometimes necessary to add a small amount of filtering
as shown in Figure 11. Typically, values of R
FL
= 100Ω
and C
FL
= 200pF to 1nF will provide adequate filtering of
noise pickup without substantially affecting the current
loop response.
R
FL
R
FL
R
SENSE
C
FL
FORWARD
MOSFET
FG
LTC3766
I
S
–
I
S
+
3766 F11
Figure 11. Using a Current Sense Resistor
This filter is also helpful in correcting for the effect of the
ESL (parasitic inductance) of the sense resistor, which can
be important for R
SENSE
values less than 2mΩ. The effect
of the ESL is cancelled if the RC filter is chosen so that:
R
FL
C
FL
=
ESL
R
SENSE
Since the LTC3766 implements an average current limit
architecture, choose the value of R
SENSE
based upon the
desired average current limit:
R
SENSE
=
55mV
I
LIM(AVG)
Alternatively, if a current transformer is used to sense
the primary-side switch current, then the I
S
–
pin should
be tied to V
CC
and the I
S
+
pin to the output of the current