ISL62881 Datasheet, PDF (16/35 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62881, ISL62881B
Protections
The ISL62881 provides overcurrent, undervoltage, and
overvoltage protections.
The ISL62881 determines overcurrent protection (OCP)
by comparing the average value of the droop current
I_droop with an internal current source threshold. It
declares OCP when I_droop is above the threshold for
120µs. A resistor R_comp from the COMP pin to GND
programs the OCP current source threshold, as well as
the overshoot reduction function (to be discussed in
later sections), as Table 3 shows. It is recommended to
use the nominal R_comp value. The ISL62881 detects the
R_comp value at the beginning of start-up, and sets the
internal OCP threshold accordingly. It remembers the
R_comp value until the VR_ON signal drops below the
POR threshold.
TABLE 3. ISL62881 OCP THRESHOLD AND
OVERSHOOT REDUCTION FUNCTION
MIN
(kΩ)
R_comp
NOMINAL
(kΩ)
MAX
(kΩ)
OCP
THRESHOLD
(µA)
OVERSHOOT
REDUCTION
FUNCTION
none
none
20
Disabled
305
400
410
22.67
205
235
240
20.67
155
165
170
18
104
120
130
20
78
85
90
22.67
62
66
68
20.67
45
50
55
18
Enabled
The default OCP threshold is the value when R_comp is
not populated. It is recommended to scale the droop
current I_droop such that the default OCP threshold gives
approximately the desired OCP level, then use R_comp to
fine tune the OCP level if necessary.
For overcurrent condition above 2.5x the OCP level, the
PWM output will immediately shut off and PGOOD will go
low to maximize protection. This protection is also
referred to as way-overcurrent protection or fast-
overcurrent protection, for short-circuit protections.
The ISL62881 will declare undervoltage (UV) fault and
latch off if the output voltage is less than the VID set
value by 300mV or more for 1ms. It’ll turn off the PWM
output and de-assert PGOOD.
The ISL62881 has two levels of overvoltage protections.
The first level of overvoltage protection is referred to as
PGOOD overvoltage protection. If the output voltage
exceeds the VID set value by +200mV for 1ms, the
ISL62881 will declare a fault and de-assert PGOOD.
The ISL62881 takes the same actions for all of the above
fault protections: deassertion 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 ISL62881 will
immediately declare an OV fault, de-assert 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.
Table 4 summarizes the fault protections.
TABLE 4. FAULT PROTECTION SUMMARY
FAULT TYPE
FAULT
DURATION
BEFORE
PROTECTION
PROTECTI
ON
ACTION
FAULT
RESET
Overcurrent
120µs
Way-Overcurrent
(2.5xOC)
<2µs
Overvoltage +200mV 1ms
Undervoltage -300mV
Overvoltage 1.55V Immediately
PWM tri-
state,
PGOOD
latched low
VR_ON
toggle or
VDD
toggle
Low-side VDD
MOSFET on toggle
until V_core
<0.85V,
then PWM
tri-state,
PGOOD
latched low.
Current Monitor
The ISL62881 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 as shown by Equation 6.
I_IMON = 3 × I_droop
(EQ. 6)
As Figures 1 and 2 show, a resistor R_imon is connected to
the IMON pin to convert the IMON pin current to voltage.
vAocltaapgaeciitnofrorcmanatbioenp. ^{aTrhaelleIlMedVPw-i6th.5R™im}sp^onectioficfailtteiornthe
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.
16
FN6924.0
October 26, 2009