ISL62881C Datasheet, PDF (17/37 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62881C, ISL62881D
ISL62881C monitors the VSEN pin voltage and compares
it to 100ns filtered version. When the unfiltered version is
20mV below the filtered version, the controller knows
there is a fast voltage dip due to load insertion, hence
issues an additional master clock signal to deliver a PWM
pulse immediately.
The R^3™ modulator intrinsically has voltage feed
forward. The output voltage is insensitive to a fast slew
rate input voltage change.
Protections
The ISL62881C provides overcurrent, undervoltage,
and overvoltage protections.
The ISL62881C 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 ISL62881C 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. ISL62881C OCP THRESHOLD AND
OVERSHOOT REDUCTION FUNCTION
MIN
(kΩ)
R_comp
NOMINAL
(kΩ)
MAX
(kΩ)
OCP
THRESHOLD
(µA)
OVERSHOOT
REDUCTION
FUNCTION
none
none
60
Disabled
305
400
410
68
205
235
240
62
155
165
170
54
104
120
130
60
Enabled
78
85
90
68
62
66
68
62
45
50
55
54
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 ISL62881C 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 ISL62881C 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 ISL62881C will declare a fault and de-assert
PGOOD.
The ISL62881C takes the same actions for all of the
above fault protections: de-assertion 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 ISL62881C 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
PROTECTION ACTION
FAULT
RESET
Overcurrent
Way-Overcurrent
(2.5xOC)
Overvoltage
+200mV
120µs
<2µs
1ms
PWM tri-state, VR_ON
PGOOD latched toggle or
low
VDD
toggle
Undervoltage -
300mV
Overvoltage 1.55V Immediately
Low-side
VDD
MOSFET on toggle
until V_core
<0.85V, then
PWM tri-state,
PGOOD latched
low.
Current Monitor
The ISL62881C 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)
17
FN7596.0
March 8, 2010