M. recalculate r, An368 – Cirrus Logic AN368 User Manual
Page 58
AN368
58
AN368REV2
Once the current sense resistor R
Sense
value is determined, the target output current CH1CUR for channel 1
is calculated using:
where,
CH1CUR corresponds to a 9-bit OTP value for channel 1 and is programmed by setting bit CH1CURMSB in
register Config8 at Address 40 to a ‘1’ plus bits CH1CUR[7:0] at Address 41 to ‘00010001’
The target output current CH2CUR for channel 2 is calculated:
where,
CH2CUR corresponds to a 9-bit OTP value for channel 2 and is programmed by setting bit CH2CURMSB in
register Config10 at Address 42 to a ‘0’ plus bits CH2CUR[7:0] at Address 43 to ‘01110111’
Step 4) Tune Second-stage Performance and Limiting Parameters
a. PID Feedback Controller
The maximum coefficient for the second-stage PID integrator is configured using the PID register at Address
45 and set to ‘00000010’. Set bits RSHIFT[3:0] in register Config8 at Address 40 to ‘1100’, setting the number
of right shifts performed on the second-stage PID integrator value to 12.
b. Leading-edge Blanking
The leading-edge blanking time was configured based on the noise observed on the FBSENSE pin. The
duration of the leading-edge blanking time T
LEB
is set using bits LEB[3:0] in register Config18 at Address 50.
Bits LEB[3:0] are set to ‘0100’. T
LEB
can be calculated using Equation 38:
c. Trailing-edge Blanking
The trailing-edge blanking time was configured based on the noise observed on the FBSENSE pin. The
duration of the trailing-edge blanking time T
TEB
is set using the bits TEB[3:0] in register Config18 at Address
50. Bits TEB[3:0] are set to ‘1111’. T
TEB
can be calculated using Equation 39:
d. Maximum Gate Drive Duration
The maximum gate drive duration was configured to prevent the primary winding inductor from saturating. The
approximate maximum duration is 1.5 times the nominal gate on time. Bits in register GD_DUR at Address 33
are set to ‘00010110’. T1
max
can be calculated using Equation 40:
CH1CUR
511 2 R
Sense
I
CH1
N V
Sense
-------------------------------------------------------
=
[Eq. 118]
511 2 4.28
488mA
5.57 1.4V
---------------------------------------------------------------
=
273
=
CH2CUR
511 2 R
Sense
I
CH2
N V
Sense
-------------------------------------------------------
=
[Eq. 119]
511 2 4.28
213mA
5.57 1.4V
---------------------------------------------------------------
=
119
=
[Eq. 120]
T
LEB
LEB[3:0]
=
2
50ns
4 2 50ns
400ns
=
=
[Eq. 121]
T
TEB
TEB[3:0]
=
2
50ns
15 2 50ns
1500ns
=
=
[Eq. 122]
T1
max
GD_DUR 8
7
+
=
50
ns
22 8
7
+
50ns
9.15
s
=
=