ZL2006_14 Datasheet, PDF (24/45 Pages) Intersil Corporation – Adaptive Digital DC-DC Controller with Drivers and Current Sharing

ZL2006
5.8.9 Current Sensing Components
Once the current sense method has been selected
(Refer to Section 5.9, “Current Limit Threshold
Selection,”), the components are selected as follows.
When using the inductor DCR sensing method, the
user must also select an R/C network comprised of R1
and CL (see Figure 16).
Figure 16. DCR Current Sensing
For the voltage across C_L to reflect the voltage across
the DCR of the inductor, the time constant of the
inductor must match the time constant of the RC
network. That is:
τ _RC = τ _{L / DCR}
R₁ ⋅ C_L
=
L
DCR
For L, use the average of the nominal value and the
minimum value. Include the effects of tolerance, DC
Bias and switching frequency on the inductance when
determining the minimum value of L. Use the typical
value for DCR.
The value of R₁ should be as small as feasible and no
greater than 5 kΩ for best signal-to-noise ratio. The
designer should make sure the resistor package size is
appropriate for the power dissipated and include this
loss in efficiency calculations. In calculating the
minimum value of R₁, the average voltage across C_L
(which is the average I_OUT·DCR product) is small and
can be neglected. Therefore, the minimum value of R₁
may be approximated by the following equation:
( ) ( ) _R1−min
=
D V_{IN −max}
− V_OUT ² + 1 − D
δ P ⋅ _{R1 pkg−max P}
⋅V_OUT ²
,
where P_R1pkg-max is the maximum power dissipation
specification for the resistor package and δ_P is the
derating factor for the same parameter (eg.: P_R1pkg-max =
0.0625W for 0603 package, δ_P = 50% @ 85°C). Once
R_1-min has been calculated, solve for the maximum
value of C_L from
C _{L − max}
=
L
R_1−min ⋅ DCR
and choose the next-lowest readily available value (eg.:
For C_L-max = 1.86uF, C_L = 1.5uF is a good choice).
Then substitute the chosen value into the same
equation and re-calculate the value of R₁. Choose the
1% resistor standard value closest to this re-calculated
value of R₁. The error due to the mismatch of the two
time constants is
ε_τ
= _{⎜⎜⎝⎛}1 −
R₁
⋅
C_L ⋅ DCR
L_avg
⎟⎟⎠⎞
⋅
100%
The value of R₂ should be simply five times that of R₁:
R₂ = 5 ⋅ R₁
For the R_DS(ON) current sensing method, the external
low side MOSFET will act as the sensing element as
indicated in Figure 17.
5.9 Current Limit Threshold Selection
It is recommended that the user include a current
limiting mechanism in their design to protect the power
supply from damage and prevent excessive current
from being drawn from the input supply in the event
that the output is shorted to ground or an overload
condition is imposed on the output. Current limiting is
accomplished by sensing the current through the circuit
during a portion of the duty cycle.
Output current sensing can be accomplished by
measuring the voltage across a series resistive sensing
element according to the following equation:
V_LIM = I _LIM × R_SENSE
Where:
I_LIM is the desired maximum current that should
flow in the circuit
R_SENSE is the resistance of the sensing element
V_LIM is the voltage across the sensing element at the
point the circuit should start limiting the output
current.
24
FN6850.1
December 15, 2010