ISL95870BHRZ-T View Datasheet(PDF) - Intersil
Part Name
Description
MFG CO.
ISL95870BHRZ-T Datasheet PDF : 29 Pages
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ISL95870, ISL95870A, ISL95870B
connects the inverting input of the VSET amplifier to a
specific node among the string of RSET programming
resistors. All the resistors between that node and the
SREF pin serve as the feedback impedance RF of the
VSET amplifier. Likewise, all the resistors between that
node and the GND pin serve as the input impedance RIN
of the VSET amplifier. Equation 20 gives the general form
of the gain equation for the VSET amplifier:
VSETX
=
VR
E
F
⋅
⎛
⎜
⎝
1
+
R--R---I-F-N--⎠⎟⎞
(EQ. 20)
Where:
- VREF is the 0.5V internal reference of the IC
- VSETx is the resulting setpoint reference voltage
that appears at the SREF pin
TABLE 2. ISL95870B VID TRUTH TABLE
VID STATE
RESULT
VID1
1
1
0
0
VID0
1
0
1
0
CLOSE
SW0
SW1
SW2
SW3
VSREF
VSET1
VSET2
VSET3
VSET4
VOUT
VOUT1
VOUT2
VOUT3
VOUT4
Equations 21, 22, 23 and 24 give the specific VSET
equations for the ISL95870B setpoint reference voltages.
The ISL95870B VSET1 setpoint is written as Equation 21:
VSET1 = VREF
(EQ. 21)
The ISL95870B VSET2 setpoint is written as Equation 22:
VSET2
=
VREF
⋅
⎛
⎜1
⎝
+
R-----S----E----T----2----+-----RR----SS----EE----TT----13----+-----R-----S----E---T----4- ⎠⎟⎞
(EQ. 22)
The ISL95870B VSET3 setpoint is written as Equation 23:
VSET3
=
VREF
⋅
⎛
⎜1
⎝
+
RR-----SS----EE----TT----13----++-----RR----SS----EE----TT----24-⎠⎟⎞
(EQ. 23)
The ISL95870B VSET4 setpoint is written as Equation 24:
VSET4
=
VR
E
F
⋅
⎛
⎜
⎝
1
+
-R----S----E----T----1----+-----RR----SS----EE----TT----24----+-----R-----S----E---T----3- ⎠⎟⎞
(EQ. 24)
The VSET1 is fixed at 0.5V because it corresponds to the
closure of internal switch SW0 that configures the VSET
amplifier as a unity-gain voltage follower for the 0.5V
voltage reference VREF. The setpoint reference voltages
use the naming convention VSET(x) where (x) is the first,
second, third, or fourth setpoint reference voltage
where:
- VSET1 < VSET2 < VSET3 < VSET4 Thus,
- VOUT1 < VOUT2 < VOUT3 < VOUT4
For given four user selected reference voltages VSETx,
the programmed resistors RSET1, RSET2, RSET3 and
RSET4 are designed in the following way. First, assign an
initial value to RSET4 of approximately 100kΩ then
calculate RSET1, RSET2 and RSET3 using Equations 25,
26, and 27 respectively.
RSET1 = R-----S----E----T----4----⋅---V---V-S---R-E---E-T---F-4----⋅⋅---V(--V--S---S-E---E-T--T--2--2-----–----V----R----E----F----)
(EQ. 25)
RSET2
=
-R----S----E----T----4----⋅---V----S----E----T----4----⋅---(--V-----S----E---T----3-----–----V----S----E----T----2---)
VSET2 ⋅ VSET3
(EQ. 26)
RSET3
=
R-----S----E----T----4----⋅---(--V-----S----E---T----4----–-----V----S----E----T----3---)
VSET3
(EQ. 27)
The sum of all the programming resistors should be
approximately 300kΩ, as shown in Equation 28,
otherwise adjust the value of RSET4 and repeat the
calculations.
RSET1 + RSET2 + RSET3 + RSET4 ≅ 300kΩ
(EQ. 28)
VOUT RFB
FB
SREF
SET0
SET1
SET2
−
EA
+
VCOMP
+
VSET
−
VREF
0.5V
SW0
SW1
SW2
SW3
FIGURE 11. ISL95870B VOLTAGE PROGRAMMING
CIRCUIT
If the output voltage is in the range of 0.5V to 1.5V, the
external resistor-divider is not necessary. The output
voltage is equal to one of the reference voltages
depending on the status of VID1 and VID0. The external
resistor divider consisting of RFB and ROFS allows the
user to program the output voltage in the range of 1.5V
to 5V. The relation between the output voltage and the
reference is given in Equation 29:
VOUT
=
VS
R
E
F
⋅
R-----F----B-----+-----R----O-----F---S--
ROFS
=
VSREF ⋅ k
(EQ. 29)
In this case, the four output voltages are equal to each of
the corresponding reference voltages multiplying the
factor k.
VOUTx = VSETx ⋅ k
(EQ. 30)
18
FN6899.0
December 22, 2009
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