ISL62881_14 Datasheet, PDF (18/35 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

ISL62881, ISL62881B
Overshoot Reduction Function
The ISL62881 has an optional overshoot reduction function,
enabled or disabled by the resistor from the COMP pin to GND, as
shown in Table 3.
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_DS(ON) 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 ISL62881
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 ISL62881 will
turn off LGATE when COMP is being clamped. The low-side
MOSFET 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 ISL62881 will
resume normal PWM operation.
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 the transient
event, the more power loss is 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 ISL62881 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 = 147kΩ sets the controller for CPU core application and
using R_BIAS = 47kΩ sets the controller for GPU core application.
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 ISL62881 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
PHASE
RSUM
ISUM+
L
DCR
RNTCS
RP
RNTC
+
CN VCN
-
RI
ISUM-
IO
FIGURE 13. DCR CURRENT-SENSING NETWORK
Figure 13 shows the inductor DCR current-sensing network for a
2-phase solution. An inductor current flows through the DCR and
creates a voltage drop. The inductor has a resistors in R_sum
connected to the phase-node-side pad and a PCB trace
connected to the output-side pad to accurately sense the
inductor current by sensing the DCR voltage drop. The sensed
current information is fed 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
current information is presented to the capacitor C_n. Equations 7
through 11 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_ntcnet + R_sum
×
D
C
⎞
R⎟
⎠
× I_o(s) × A_cs(s)
(EQ. 7)
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 + -_ω----_s-^s--_n---_s-
(EQ. 8)
(EQ. 9)
ω_L
=
D_-----C_----R_--
L
(EQ. 10)
ω_sns
=
--------------------------¹---------------------------
-^R----ⁿ---^t--^c---ⁿ---^e---^t---^×-----^R----^s---^u---^m---
R_ntcnet + R_sum
×
C_n
(EQ. 11)
Transfer function A_cs(s) always has unity gain at DC. The inductor
DCR value increases as the winding temperature increases,
giving higher reading of the inductor DC current. The NTC R_ntc
values decreases as its temperature decreases. Proper
selections of R_sum, R_ntcs, R_p and R_ntc parameters ensure that
V_Cn represents the inductor total DC current over the
temperature range of interest.
18
FN6924.3
June 16, 2011