ISL62882_14 Datasheet, PDF (20/42 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62882, ISL62882B
Current Monitor
The ISL62882 provides the current monitor function. The IMON
pin outputs a high-speed analog current source that is 3 times of
the droop current flowing out of the FB pin. Thus Equation 13:
I_IMON = 3 × I_droop
(EQ. 13)
As Figures 1 and 2 show, a resistor R_imon is connected to the
IMON pin to convert the IMON pin current to voltage. A capacitor
can be paralleled with R_imon to filter the voltage information. The
IMVP-6.5™ specification requires that the IMON voltage
information be referenced to VSS_SENSE.
The IMON pin voltage range is 0V to 1.1V. A clamp circuit
prevents the IMON pin voltage from going above 1.1V.
FB2 Function
The FB2 function is only available when the ISL62882 is in 2-
phase configuration.
CONTROLLER IN
2-PHASE MODE
C2 _R3
C1 _R2
C3.1
FB2 ^C3.2
CONTROLLER IN
1-PHASE MODE
C2 _R3
C1 _R2
C3.1
FB2 ^C3.2
VSEN
R1
VSEN
FB E/A
VREF
COMP
R1
FB E/A
VREF
COMP
FIGURE 16. FB2 FUNCTION IN 2-PHASE MODE
Figure 16 shows the FB2 function. A switch (called FB2 switch)
turns on to short the FB and the FB2 pins when the controller is in
2-phase mode. Capacitors C3.1 and C3.2 are in parallel, serving
as part of the compensator. When the controller enters 1-phase
mode, the FB2 switch turns off, removing C3.2 and leaving only
C3.1 in the compensator. The compensator gain will increase
with the removal of C3.2. By properly sizing C3.1 and C3.2, the
compensator cab be optimal for both 2-phase mode and 1-phase
mode.
When the FB2 switch is off, C3.2 is disconnected from the FB pin.
However, the controller still actively drives the FB2 pin voltage to
follow the FB pin voltage such that C3.2 voltage always follows
C3.1 voltage. When the controller turns on the FB2 switch, C3.2
will be reconnected to the compensator smoothly.
The FB2 function ensures excellent transient response in both
2-phase mode and 1-phase mode. If one decides not to use the
FB2 function, simply populate C3.1 only.
Adaptive Body Diode Conduction Time
Reduction
In DCM, the controller turns off the low-side MOSFET when the
inductor current approaches zero. During on-time of the low-side
MOSFET, phase voltage is negative and the amount is the
MOSFET r_DS(ON) voltage drop, which is proportional to the
inductor current. A phase comparator inside the controller
monitors the phase voltage during on-time of the low-side
MOSFET and compares it with a threshold to determine the
zero-crossing point of the inductor current. If the inductor current
has not reached zero when the low-side MOSFET turns off, it’ll
flow through the low-side MOSFET body diode, causing the phase
node to have a larger voltage drop until it decays to zero. If the
inductor current has crossed zero and reversed the direction
when the low-side MOSFET turns off, it’ll flow through the
high-side MOSFET body diode, causing the phase node to have a
spike until it decays to zero. The controller continues monitoring
the phase voltage after turning off the low-side MOSFET and
adjusts the phase 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 ISL62882 has an optional overshoot reduction function.
Tables 2 and 4 show to enable and disable it.
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 ISL62882
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 ISL62882 will
turn off LGATE1 and LGATE2 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 ISL62882 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 ISL62882 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. Refer to Table 2
to select the resistance according to desired configuration. Do not
connect any other components to this pin. Do not connect any
capacitor to the RBIAS pin as it will create instability.
20
FN6890.4
June 21, 2011