ISL62883HRTZ-T Datasheet, PDF (19/37 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs

ISL62883, ISL62883B
comparator threshold voltage accordingly in iterative steps such
that the low-side MOSFET body diode conducts for approximately
40ns to minimize the body diode-related loss.
Overshoot Reduction Function
The ISL62883 has an optional overshoot reduction function.
Using R_BIAS = 47kΩ enables this function and using
R_BIAS = 147kΩ disables this function.
When a load release occurs, the energy stored in the inductors
will dump to the output capacitor, causing output voltage
overshoot. The inductor current freewheels through the low-side
MOSFET during this period of time. The overshoot reduction
function turns off the low-side MOSFET during the output voltage
overshoot, forcing the inductor current to freewheel through the
low-side MOSFET body diode. Since the body diode voltage drop
is much higher than MOSFET R_dson voltage drop, more energy is
dissipated on the low-side MOSFET therefore the output voltage
overshoot is lower.
If the overshoot reduction function is enabled, the ISL62883
monitors the COMP pin voltage to determine the output voltage
overshoot condition. The COMP voltage will fall and hit the clamp
voltage when the output voltage overshoots. The ISL62883 will
turn off LGATE1 and LGATE2, and tri-state PWM3 when COMP is
being clamped. All the low-side MOSFETs in the power stage will
be turned off. When the output voltage has reached its peak and
starts to come down, the COMP voltage starts to rise and is no
longer clamped. The ISL62883 will resume normal PWM
operation.
When PSI# is low, indicating a low power state of the CPU, the
controller will disable the overshoot reduction function as large
magnitude transient event is not expected and overshoot is not a
concern.
While the overshoot reduction function reduces the output voltage
overshoot, energy is dissipated on the low-side MOSFET, causing
additional power loss. The more frequent transient event, the more
power loss dissipated on the low-side MOSFET. The MOSFET may
face severe thermal stress when transient events happen at a high
repetitive rate. User discretion is advised when this function is
enabled.
Key Component Selection
R_BIAS
The ISL62883 uses a resistor (1% or better tolerance is
recommended) from the RBIAS pin to GND to establish highly
accurate reference current sources inside the IC. Using
R_BIAS = 47kΩ enables the overshoot reduction function and using
R_BIAS = 147kΩ disables this function. Do not connect any other
components to this pin. Do not connect any capacitor to the RBIAS
pin as it will create instability.
Care should be taken in layout that the resistor is placed very close
to the RBIAS pin and that a good quality signal ground is
connected to the opposite side of the R_BIAS resistor.
R_is and C_is
As Figures 1 and 2, show, the ISL62883 needs the R_is - C_is
network across the ISUM+ and the ISUM- pins to stabilize the
droop amplifier. The preferred values are R_is = 82.5Ω and
C_is = 0.01µF. Slight deviations from the recommended values are
acceptable. Large deviations may result in instability.
Inductor DCR Current-Sensing Network
Phase1 Phase2 Phase3
Rsum
Rsum
Rsum
ISUM+
L
L
L
DCR DCR
DCR
Rntcs
Rp
Rntc
Ro
Ro
Ro
Cn Vcn
Ri ^ISUM-
Io
FIGURE 14. DCR CURRENT-SENSING NETWORK
Figure 14 shows the inductor DCR current-sensing network for a
3-phase solution. An inductor current flows through the DCR and
creates a voltage drop. Each inductor has two resistors in R_sum
and R_o connected to the pads to accurately sense the inductor
current by sensing the DCR voltage drop. The R_sum and R_o
resistors are connected in a summing network as shown, and feed
the total current information to the NTC network (consisting of
R_ntcs, R_ntc and R_p) and capacitor C_n. R_ntc is a negative
temperature coefficient (NTC) thermistor, used to
temperature-compensate the inductor DCR change.
The inductor output side pads are electrically shorted in the
schematic, but have some parasitic impedance in actual board
layout, which is why one cannot simply short them together for the
current-sensing summing network. It is recommended to use
1Ω~10Ω R_o to create quality signals. Since R_o value is much
smaller than the rest of the current sensing circuit, the following
analysis will ignore it for simplicity.
The summed inductor current information is presented to the
capacitor C_n. Equations 19 thru 23 describe the
frequency-domain relationship between inductor total current
I_o(s) and C_n voltage V_Cn(s):
⎛
⎞
V_Cn(s)
=
⎜
⎜
⎜
⎝
-----------^R----ⁿ---^t---^c--ⁿ----^e---^t-----------
R_ntcn
e
t
+
-^R----^s--^u----^m---
N
×
D_-----NC----R_{--⎟⎟⎟}
⎠
× I_o(s) × A_cs(s)
(EQ. 19)
R_ntcnet
=
(_---R_{----n---t--c---s----+-----}R_{----n---t--c---})_----×_-----R_----p-
R_ntcs + R_ntc + R_p
A_cs(s)
=
---¹-----+-----^{ω------s----L-----}
1
+
------^s------
ω_sns
(EQ. 20)
(EQ. 21)
where N is the number of phases.
19
FN6891.4
June 21, 2011