RT8867A Datasheet, PDF (16/21 Pages) Richtek Technology Corporation – Advanced 4/3-Phase PWM Controller for CPU Core Power

RT8867A
state-of-the-art transient boost function which detects load
transient by monitoring V_OUT. If V_OUT suddenly drops below
“V_TB” the transient boost signal rises up and the RT8867A
turns on all high side MOSFETs and turns off all low side
MOSFETs. The voltage difference “V_OUT − V_TB” is set by
following equation :
V_OUT − V_TB = 10μA x R_TB.
Sensitivity of the transient boost can be adjusted by varying
the values of C_FB and R_FB. Smaller R_FB and/or larger C_FB
will make transient boost easier to be triggered. Figure 5
shows the circuit and typical waveforms.
V_OUT
VOUT
R_TB
TB
C_TB
Transient
- Boost
+
10µA
Figure 5. (a) Transient Boost Circuit
I_OUT
V_OUT
Transient
Boost
Figure 5. (b) Typical Waveforms
Output Current Sensing
The RT8867A provides a low input offset Current Sense
Amplifier (CSA) to monitor the output current. The output
current of CSA (I_X) is used for load line control, dynamic
phase control and over current protection. In this average
inductor current sensing topology, R_S and C_S must be set
according to the equation below :
Requ = R_X //R_NTC
L ₌ RS × CS
DCR
N
+
R_S
Requ
Where the constant N is a set maximum operation phase
number, not affected by the dynamic phase control
machine. Then, the output current of CSA will follow the
equation below :
I_X
=
I_OUT ×DCR
N× R_CSN
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
16
Figure 6 is the current sense circuit.
PHn
L
DCR ^IOUT
V_CORE
PH1
PH2
L
DCR
L
DCR
R_S
C_S
R_L
C_OUT
(Current Sense
Amplifier)
CSA
CSP
+
-
Load Line
Adjustment
CSN _IX
R_S
R_X
R_S
NTC
R_CSN
Figure 6. Circuit for Current Sensing
Load Line
The RT8867A utilizes inductor DCR current sense
technique for load line control function. The sensed output
current is proportionally mirrored from the I_X signal to the
R_DROOP resistor to establish the voltage of V_DROOP. V_DROOP
subtracted from V_DAC generates V_EAP. The voltage control
loop is shown in Figure 3. Because I_X is a PTC (Positive
Temperature Coefficient) current, an NTC (Negative
Temperature Coefficient) resistor is needed to connect in
parallel with the capacitor C_S. If the NTC resistor is properly
selected to compensate the temperature coefficient of I_X,
the V_DROOP voltage will be proportional to I_OUT without
temperature effect. In the RT8867A, the positive input of
error amplifier is V_EAP and V_OUT will follow “V_DAC −
V_DROOP”. Thus, the output voltage which decreases
linearly with I_OUT is obtained. The load line is defined as :
LL(Load Line) =
ΔV_OUT
ΔI_OUT
=
ΔV_DROOP
ΔI_OUT
=
DCR × R_DROOP
N × R_CSN
×
4
Basically, the resistance of R_DROOP sets the resistance of
the load line. The temperature coefficient of R_DROOP
compensates the temperature effect of the load line.
Connecting VRSEL pin to GND enables load line
adjustment function. When load line adjustment function
is enabled, the current I_X is decreased by 10mV/R_CSN.
Note that the minimum output current sensing range is
also reduced by 10mV/R_CSN when load line adjustment
function is enabled.
is a registered trademark of Richtek Technology Corporation.
DS8867A-01 August 2012