RT8802A
For example, assuming the negative inductor current is
If RADJ is connected as in Figure 14, RADJ = R1 + (R2//
RNTC), 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 RADJ. Thus the load
ILX_50% = −5A at no load, then for
RCSN 330Ω, RADJ = 160Ω, VOUT = 1.300V
1.3V
RCSN2
−5A×1mΩ
330Ω
≥
line will be thermally compensated.
RCSN2 ≤ 85.8kΩ
ADJ
Choose RCSN2 = 82kΩ
R1
Figure 10 shows that dead zone of load line at light load
is eliminated by applying this technique.
R
ADJ
R
R2
NTC
VR_HOT & VR_FAN Setting
V
CC
5V
Figure 14. RADJ Connection for Thermal Compensation
+
R1
Q1
Q2
Q3
CMP
Over Current Protection
-
0.39 x V
0.33 x V
CC
TSEN
V
TSEN
+
Thermally compensated total current OCP
CMP
-
CC
VTCOC is compared with VADJ. If VADJ > VTCOC then OCP
R
NTC
+
is triggered.
CMP
-
ADJ
0.28 x V
CC
Figure 12
IMAX(1V)
R1
V
TSEN
+
TCOC
V
is inversely proportional
OC
CMP
TSEN
-
to Temperature.
R2
0.39 x V
0.33 x V
0.28 x V
CC
CC
CC
Figure 15
Phase Current OCP
VR_FAN
VR_HOT
RT8802A uses an external resistor RIMAX connected to
IMAX pin to generate a reference current IIMAX for over
current protection :
Temperature
V
IMAX
IIMAX
=
Figure 13. VR_HOT and VR_FANSignal vs TSENVoltage
RIMAX
where VIMAX is typical 1.0V. OCP comparator compares
each sensed phase current IX with this reference current
as shown in Figure 16. Equivalently, the maximum phase
current ILX(MAX) is calculated as below :
Load Line Setting and Thermal Compensation
VADJ = Sum(IX) x RADJ = (DCR x RADJ / RCSN) x IOUT
1
3
1
2
= LL x IOUT
IX(MAX)
=
IIMAX
VOUT = VDAC − VADJ = VDAC − LL x IOUT
LL = DCR(PTC) x RADJ(NTC) / RCSN
V
IMAX
RIMAX
3
2
3
2
=
IIMAX
=
×
IX(MAX)
RCSNX
RLX
RCSNX
DCRX
V
IMAX
RIMAX
3
2
=
×
×
ILX(MAX) IX
=
×
DCR is the inductor DCR which is a PTC resistance.
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
All brandname or trademark belong to their owner respectively.
www.richtek.com
26
DS8802A-09 March 2012
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
