UCC28180
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SLUSBQ5A –NOVEMBER 2013–REVISED NOVEMBER 2013
The individual loop factors, M1 which is the current loop gain factor, and M2 which is the voltage loop PWM ramp
slope, are calculated using the following conditions:
The M1 non-linear current loop gain factor follows the following identities:
M1 = 0.068
if VCOMP < 1 V
(81)
(82)
(83)
(84)
M1 = 0.156 ´ VCOMP - 0.088
if 1 V < VCOMP < 2 V
if 2 V < VCOMP < 4.5 V
M1 = 0.313 ´ VCOMP - 0.401
M1 = 1.007
if 4.5 V < VCOMP < 5 V
In this example, according to the chart in Figure 32, VCOMP is approximately equal to 3 V, so M1 is calculated
to be approximately equal to 0.538:
M1 = 0.313 ´ 2.45 - 0.401= 0.366
(85)
The M2 non-linear PWM ramp slope will obey the following relationships:
V
M2 = 0
ms
if VCOMP ≤ 0.5 V
(86)
(87)
(88)
fSW
V
M2 =
M2 =
´0.1223 ´(VCOMP - 0.5)2
65kHz
fSW
ms
if 0.5 V ≤ VCOMP ≤ 4.6 V
V
´ 2.056
65kHz
ms
if 4.6 V ≤ VCOMP ≤ 5 V
In this example, with VCOMP approximately equal to 3 V, M2 equals 1.388 V/µs:
118kHz
V
V
M2 =
´0.1223 ´(3 - 0.5)2
= 1.388
65kHz
ms
ms
(89)
Verify that the product of the individual gain factors, M1 and M2, is approximately equal to the M1M2 factor
determined above, if not, iterate the VCOMP value and recalculate M1M2
V
V
M1 ´M2 = 0.538 ´1.388
= 0.747
ms
ms
(90)
The product of M1 and M2 is within 1% of the M1M2 factor previously calculated:
M1 ´M2 @ M1M2
(91)
(92)
V
V
0.747
@ 0.751
ms
ms
If more accuracy was desired, iteration results in a VCOMP value of 3.004 V where M1M2 and M1 x M2 are both
equal to 0.751 V/µs.
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