ISL62883 Datasheet, PDF (19/39 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

ISL62883, ISL62883B
.
TABLE 4. FAULT PROTECTION SUMMARY
FAULT TYPE
FAULT
DURATION
BEFORE PROTECTION
PROTECTION ACTION
FAULT
RESET
Overcurrent
Way-Overcurrent
(2.5xOC)
Overvoltage
+200mV
120µs
<2µs
1ms
PWM tri-state,
PGOOD
latched low
VR_ON
toggle or
VDD
toggle
Undervoltage -
300mV
Phase Current
Unbalance
Overvoltage 1.55V
Immediately
Low-side
MOSFET on
until V_core
<0.85V, then
PWM tri-state,
PGOOD
latched low.
VDD
toggle
Over-Temperature
1ms
N/A
Current Monitor
The ISL62883 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 18:
I_IMON = 3 × I_droop
(EQ. 18)
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 VSSSENSE.
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 ISL62883 is
in 2-phase configuration, when pin 9 serves the FB2
function instead of the ISEN3 function.
CONTROLLER IN
2-PHASE MODE
C1 _R2
C3.1
CONTROLLER IN
1-PHASE MODE
C1 _R2
C3.1
C2 _{R3 FB2 C3.2}
C2 _{R3 FB2 C3.2}
VSEN
R1
VSEN
FB E/A
Vref
COMP
R1
FB E/A
Vref
COMP
FIGURE 13. FB2 FUNCTION IN 2-PHASE MODE
Figure 13 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_dson 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 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
19
FN6891.2
February 25, 2010