ISL62882HRTZ View Datasheet(PDF) - Intersil
Part Name
Description
MFG CO.
'ISL62882HRTZ' PDF : 42 Pages View PDF
ISL62882, ISL62882B
Substitution of Equation 27 into Equation 1 gives
Equation 28:
Idroop
=
--2---
Ri
×
R-----s---e---n--
N
×
Io
(EQ.28)
Therefore
Ri
=
-2---R-----s---e---n-----×----I--o--
N × Idroop
(EQ.29)
Substitution of Equation 29 and application of the OCP
condition in Equation 25 gives Equation 30:
Ri = -2N---R---×--s---Ie--d-n--r--o-×--o--I--po---m-m----aa---x-x-
(EQ.30)
where Iomax is the full load current, Idroopmax is the
corresponding droop current. For example, given N = 2,
Rsen = 1mΩ, Iomax = 51A and Idroopmax = 34.3µA,
Equation 30 gives Ri = 1.487kΩ.
A resistor from COMP to GND can adjust the internal OCP
threshold, providing another dimension of fine-tune
flexibility. Table 4 shows the detail. It is recommended to
scale Idroop such that the default OCP threshold gives
approximately the desired OCP level, then use Rcomp to
fine tune the OCP level if necessary.
Load Line Slope
Refer to Figure 12.
For inductor DCR sensing, substitution of Equation 24
into Equation 2 gives the load line slope expression:
LL
=
V-----d---r--o----o---p-
Io
=
-2---R-----d---r--o----o---p-
Ri
×
------------R----n---t--c---n----e---t-----------
R
ntc
n
et
+
-R----s---u---m---
N
×
D-----C-----R---
N
(EQ.31)
For resistor sensing, substitution of Equation 28 into
Equation 2 gives the load line slope expression:
LL
=
-V----d---r--o----o---p-
Io
=
-2---R-----s---e---n-----×----R-----d---r---o---o---p-
N × Ri
(EQ.32)
Substitution of Equation 25 and rewriting Equation 31,
or substitution of Equation 29 and rewriting Equation 32
give the same result in Equation 33:
Rdroop
=
-------I-o-------- × LL
Idroop
(EQ. 33)
One can use the full load condition to calculate Rdroop.
For example, given Iomax = 51A, Idroopmax = 34.3µA
and LL = 1.9mΩ, Equation 33 gives Rdroop = 2.825kΩ.
It is recommended to start with the Rdroop value
calculated by Equation 33, and fine tune it on the actual
board to get accurate load line slope. One should record
the output voltage readings at no load and at full load for
load line slope calculation. Reading the output voltage at
lighter load instead of full load will increase the
measurement error.
Current Monitor
Refer to Equation 13 for the IMON pin current
expression.
Refer to Figures 1 and 2, the IMON pin current flows
through Rimon. The voltage across Rimon is expressed in
Equation 34:
VRimon = 3 × Idroop × Rimon
(EQ.34)
Rewriting Equation 33 gives Equation 35:
Idroop
=
--------I--o--------
Rdroop
×
LL
(EQ.35)
Substitution of Equation 35 into Equation 34 gives
Equation 36:
VRimon
=
3----I--o-----×----L----L--
Rdroop
×
Rim
on
(EQ.36)
Rewriting Equation 36 and application of full load
condition gives Equation 37:
Rimon
=
V-----R----i--m----o----n----×-----R----d----r--o---o----p-
3Io × LL
(EQ.37)
For example, given LL = 1.9mΩ, Rdroop = 2.825kΩ,
VRimon = 963mV at Iomax = 51A, Equation 37 gives
Rimon = 9.358kΩ.
A capacitor Cimon can be paralleled with Rimon to filter
the IMON pin voltage. The RimonCimon time constant is
the user’s choice. It is recommended to have a time
constant long enough such that switching frequency
ripples are removed.
Compensator
Figure 18 shows the desired load transient response
waveforms. Figure 24 shows the equivalent circuit of a
voltage regulator (VR) with the droop function. A VR is
equivalent to a voltage source (= VID) and output
impedance Zout(s). If Zout(s) is equal to the load line
slope LL, i.e. constant output impedance, in the entire
frequency range, Vo will have square response when Io
has a square change.
Zout(s) = LL
io
VID
VR
LOAD Vo
FIGURE 24. VOLTAGE REGULATOR EQUIVALENT
Intersil provides a Microsoft Excel-based spreadsheet to
help design the compensator and the current sensing
network, so the VR achieves constant output impedance
as a stable system. Figure 27 shows a screenshot of the
spreadsheet.
24
FN6890.2
April 29, 2010
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