ISL95870BHRZ-T 데이터 시트보기 (PDF) - Intersil
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ISL95870BHRZ-T Datasheet PDF : 29 Pages
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ISL95870, ISL95870A, ISL95870B
Where:
- IVS is the ±85µA setpoint voltage-step current;
positive when VNEW > VOLD, negative when VNEW
< VOLD
- VNEW is the new setpoint voltage selected by the
VID inputs
- VOLD is the setpoint voltage that VNEW is changing
from
- RT is the sum of the RSET programming resistors
Choosing the CSOFT capacitor to meet the requirements
of a particular soft-start delay tSS is calculated with
Equation 6, which is written as:
CSOFT
=
-----------------------------–---t--S----S-------------------------------
⎛
⎜
⎝
RT
⋅
L N(1
–
-V----SI--S-T---SA----R-⋅---TR-----T-U---P-- )⎠⎟⎞
(EQ. 6)
Where:
- tSS is the soft-start delay
- ISS is the soft-start current source at the 17µA
limit
- VSTART-UP is the setpoint reference voltage
selected by the state of the VID inputs at the time
EN is asserted
- RT is the sum of the RSET programming resistors
Choosing the CSOFT capacitor to meet the requirements
of a particular voltage-step delay tVS is calculated with
Equation 7, which is written as:
CSOFT
=
----------------------------------–---t--V----S----------------------------------
⎛
⎜
⎝
RT
⋅
L N(1
–
-V----N----EI--V--W--S----–-⋅---R-V----TO-----L---D--)⎠⎟⎞
(EQ. 7)
Where:
- tVS is the voltage-step delay
- VNEW is the new setpoint voltage
- VOLD is the setpoint voltage that VNEW is changing
from
- IVS is the ±85µA setpoint voltage-step current;
positive when VNEW > VOLD, negative when VNEW
< VOLD
- RT is the sum of the RSET programming resistors
Output Voltage Programming for ISL95870
The ISL95870 has a fixed 0.5V reference voltage
(VSREF). As shown in Figure 9, the output voltage is the
reference voltage if RFB is shorted and ROFS is open. A
resistor divider consisting of ROFS and RFB allows the
user to scale the output voltage between 0.5V and 5V.
The relation between the output voltage and the
reference voltage is given in Equation 8:
VOUT
=
VS
R
E
F
⋅
R-----F----B-----+-----R----O-----F---S--
ROFS
(EQ. 8)
VOUT RFB
FB
SREF
−
EA
+
VCOMP
+
VSET
−
VREF
FIGURE 9. ISL95870 VOLTAGE PROGRAMMING
CIRCUIT
Output Voltage Programming for ISL95870A
The ISL95870A allows the user to select four different
reference voltages, thus four different output voltages,
by voltage identification pins VID1 and VID0. The
maximum reference voltage cannot be designed higher
than 1.5V. The implementation scheme is shown in
Figure 10. The setpoint reference voltages are
programmed with resistors that use the naming
convention RSET(x) where (x) is the first, second, or
third programming resistor connected in series starting
at the SREF pin and ending at the GND pin. As shown in
Table 1, different combinations of VID1 and VID0 closes
different switches and leaves other switches open. For
example, for the case of VID1 = 1 and VID0 = 0, switch
SW1 closes and all the other three switches SW0, SW2
and SW3 are open. For one combination of VID1 and
VID0, the internal switch 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 9 gives the general form of the gain equation
for the VSET amplifier:
VSETX
=
VREF
⋅
⎛
⎜1
⎝
+
R--R---I-F-N--⎠⎟⎞
(EQ. 9)
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 1. ISL95870A VID TRUTH TABLE
VID STATE
RESULT
VID1
1
1
0
VID0
1
0
1
CLOSE
SW0
SW1
SW2
VSREF
VSET1
VSET2
VSET3
VOUT
VOUT1
VOUT2
VOUT3
16
FN6899.0
December 22, 2009
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