RT8841
RT8841 provides wide range no-load positive offset for over-
clocking applications. The IOFSPcapability can supply from
30uA to 640uA, which means in Equation (5), ROFS can
range from 240kΩ to 10kΩ. Other resistances of ROFS
exceeding this range can also provide no-load positive
offset but cannot be guaranteed by Equation (5).
Then the output current of CSA will follow the equation
below :
IX = [IL x DCR − VOFS-CSA + 235n x (RCSP − RCSN)]/RCSN
235nA is typical value of CSA input offset current.
VOFS-CSA is the input offset voltage of CSA. VOFS-CSA of
RT8841 is smaller than +/- 1mV. Usually, “VOFS-CSA
+
235n x (RCSP − RCSN)” is negligible except at very light
load and the equation can be simplified as the equation
below :
Load Transient Quick Response
C
FB
C2
I
OUT
C1
R
R1
FB
IX = IL xDCR/RCSN
V
OUT
FB
V
OUT
FB
COMP
-
+
-
FB
= V
EAP
L
S
EAP
DCR
QR
EAP = V - V
+
QR
ADJ
= V
- V
QR
C
S
R
EAP - V
QR
QR
CSA: Current Sense Amplifier
235nA
R
R
CSP
CSN
ISP
ISN
+
-
+
Figure 7. Load Transient Quick Response
V
OFS_CSA
I
X
-
235nA
In steady state, the voltage of VFB is controlled to be very
close to VEAP. While a load step transient from light load
to heavy load could cause VFB lower than VEAP by several
tens of mV. In prior design, owing to limited control
bandwidth, controller is hard to prevent VOUT undershoot
during quick load transient from light load to heavy load.
Figure 8. Circuit for Channel Current Sensing
Loadline
RT8841 detects load transient by comparing VFB and VEAP
.
Output current of CSA is summed and averaged in
RT8841. Then 0.5Σ(IX[n]) is sent to ADJ pin. Because
ΣIX[n] is a PTC (Positive Temperature Coefficient) current,
an NTC (Negative Temperature Coefficient) resistor is
needed to connectADJ pin toGND. If theNTC resistor is
properly selected to compensate the temperature
coefficient of IX[n], the voltage on ADJ pin will be
proportional to IOUT without temperature effect. In RT8841,
the positive input of ErrorAmp is “VDAC − VADJ”. VOUT will
follow “VDAC − VADJ”, too. Thus, the output voltage
decreasing linearly with IOUT is obtained. The loadline is
defined as
If VFB suddenly drops below “VEAP − VQR”, VQR is a
predetermined voltage. The quick response indicator QR
rises up. When QR = H, RT8841 turns on all high side
MOSFETs and turn off all low side MOSFETs. The
sensitivity of quick response can be adjusted by the values
of CFB and RFB. Smaller RFB and/or larger CFB will make
QR easier to be triggered. Figure 7 is the circuit and typical
waveforms.
Output Current Sensing
The RT8841 provides low input offset current-sense
amplifier (CSA) to monitor the output current of every
channel. Output current of CSA(IX[n]) is used for channel
current balance and active voltage position. In this inductor
current sensing topology, RS and CS must be set according
to the equation below :
LL(loadline) = ΔVOUT/ΔIOUT= ΔVADJ/ΔIOUT
= 0.5 x DCR x RADJ/RCSN
Briefly, the resistance of RADJ sets the resistance of
loadline. The temperature coefficient of RADJ compensates
the temperature effect of loadline.
L/DCR = RS x CS
DS8841-01 April 2011
www.richtek.com
27
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
