ISL62883_14 Datasheet, PDF (18/37 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs

ISL62883, ISL62883B
value by +200mV for 1ms, the ISL62883 will declare a fault and
dessert PGOOD.
The ISL62883 takes the same actions for all of the above fault
protections: desertion of PGOOD and turn-off of the high-side and
low-side power MOSFETs. Any residual inductor current will decay
through the MOSFET body diodes. These fault conditions can be
reset by bringing VR_ON low or by bringing V_DD below the POR
threshold. When VR_ON and V_DD return to their high operating
levels, a soft-start will occur.
The second level of overvoltage protection is different. If the
output voltage exceeds 1.55V, the ISL62883 will immediately
declare an OV fault, dessert PGOOD, and turn on the low-side
power MOSFETs. The low-side power MOSFETs remain on until
the output voltage is pulled down below 0.85V when all power
MOSFETs are turned off. If the output voltage rises above 1.55V
again, the protection process is repeated. This behavior provides
the maximum amount of protection against shorted high-side
power MOSFETs while preventing output ringing below ground.
Resetting VR_ON cannot clear the 1.55V OVP. Only resetting V_DD
will clear it. The 1.55V OVP is active all the time when the
controller is enabled, even if one of the other faults have been
declared. This ensures that the processor is protected against
high-side power MOSFET leakage while the MOSFETs are
commanded off.
The ISL62883 has a thermal throttling feature. If the voltage on
the NTC pin goes below the 1.18V OT threshold, the VR_TT# pin is
pulled low indicating the need for thermal throttling to the
system. No other action is taken within the ISL62883 in response
to NTC pin voltage.
Table 4 summarizes the fault protections.
.
TABLE 4. FAULT PROTECTION SUMMARY
FAULT TYPE
FAULT DURATION
BEFORE
PROTECTION
PROTECTION
ACTION
FAULT
RESET
Overcurrent
Way-Overcurrent
(2.5xOC)
120µs
<2µs
PWM tri-state, VR_ON
PGOOD latched toggle or
low
VDD toggle
Overvoltage +200mV
1ms
Undervoltage -300mV
Phase Current
Unbalance
Overvoltage 1.55V
Immediately
Low-side MOSFET VDD toggle
on until V_core
<0.85V, then
PWM tri-state,
PGOOD latched
low.
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
18
FN6891.4
June 21, 2011