RT8803A_11 Datasheet, PDF (25/27 Pages) Richtek Technology Corporation – 2/3-Phase PWM Controller for High-Density Power Supply

RT8803A
For example, assuming the negative inductor current is
I_{LX_50%} = −5A at no load, then for
R_CSN 330Ω, R_ADJ = 160Ω, V_OUT = 1.300V
1.3V
R_CSN2
≥
− 5A ×1mΩ
330Ω
R_CSN2 ≤ 85.8kΩ
Choose R_CSN2 = 82kΩ
Figure 10 shows that dead zone of load line at light load
is eliminated by applying this technique.
VR_HOT & VR_FAN Setting
V_CC
5V
R1
V_TSEN
TSEN
R_NTC
+
CMP
Q1
0.39 x V_CC
-
+
CMP
Q2
0.33 x V_CC
-
+
CMP
Q3
0.28 x V_CC
-
Figure 12
V_TSEN
0.39
0.33
0.28
x
x
x
VVV_CCCCCC
V_TSEN is inversely proportional
to Temperature.
VR_FAN
VR_HOT
Temperature
Figure 13. VR_HOT and VR_FAN Signal vs TSEN Voltage
Load Line Setting and Thermal Compensation
V_ADJ = Sum(I_X) x R_ADJ = (DCR x R_ADJ / R_CSN) x I_OUT
= LL x I_OUT
V_OUT = V_DAC − V_ADJ = V_DAC − LL x I_OUT
LL = DCR(PTC) x R_ADJ(NTC) / R_CSN
DCR is the inductor DCR which is a PTC resistance.
If R_ADJ is connected as in Figure 14, R_ADJ = R1 + (R2//
R_NTC), which is a negative temperature correlated
resistance. By properly selecting R1 and R2, the positive
temperature coefficient of DCR can be canceled by the
negative temperature coefficient of R_ADJ. Thus the load
line will be thermally compensated.
ADJ
R_NTC
R1
R_ADJ
R2
Figure 14. R_ADJ Connection for Thernal Compensation
Over Current Protection
Phase current OCP
RT8803A uses an external resistor R_IMAX connected to
IMAX pin to generate a reference current I_IMAX for over
current protection :
I_IMAX
=
V_IMAX
R_IMAX
where V_IMAX is typical 1.0V . OCP comparator compares
each sensed phase current I_X with this reference current
as shown in Figure 15. Equivalently, the maximum phase
current I_LX(MAX) is calculated as below:
1
3
I_X(MAX)
=
1
2
I_IMAX
I_X(MAX)
=
3
2
I_IMAX
=
3 _× ^VIMAX
2 R_IMAX
I_LX(MAX)
=
I_X
×
R_CSNX
DCR _X
=
3 _× ^VIMAX
2 R_IMAX
× ^RCSNX
R_LX
OCP Comparator
+
1/3 I_X
-
1/2 I_IMAX
Figure 15. Over Current Comparator
DS8803A-06 April 2011
www.richtek.com
25