ISL8120_15 Datasheet, PDF (34/38 Pages) Intersil Corporation – Dual/n-Phase Buck PWM Controller with Integrated Drivers

ISL8120
VOUT (LOCAL)
VSENSE- (REMOTE)
10
VSENSE+ (REMOTE)
GND (LOCAL)
10
C_SEN
R_OS
R_FB
VCC
VSEN-
VSEN+
Z_FB
VMON
Z_COMP
FB
COMP
PGOOD
400mV
GAIN=1
V_REF
OV/UV
COMP
ERROR AMP
PGOOD
FIGURE 31. SIMPLIFIED REMOTE SENSING IMPLEMENTATION
The output of the remote sense buffer is connected directly to the
internal OV/UV comparator. As a result, a resistor divider should
be placed on the input of the buffer for proper regulation, as
shown in Figure 31. The VMON pin should be connected to the FB
pin by a standard feedback network. The output voltage can be
set by using Equation 17:
V_OUT=
V_r
ef




1
+
R^-R----_O^-F---B-_S^- _{}
(EQ. 17)
In such an event, the VMON pin can be used as an additional
monitor of the output voltage with a resistor divider to protect the
system against single point of failure, which occurs in the system
using the same resistor divider for the UV/OV comparator and the
output regulation. The resistor divider ratio should be the same
as the one for the output regulation so that the correct voltage
information is provided to the OV/UV comparator. Figure 33
shows the differential sense amplifier can be directly used as a
monitor without pulling VSEN- high.
To optimize system accuracy, it is highly recommended to
include this impedance into calculation and use resistor with
resistance as low as possible for the lower leg (R_OS) of the
feedback resistor divider. Note that any RC filter at the inputs of
the differential amplifier, will contribute as a pole to the overall
loop compensation.
VCC
I = VSEN+ + 1µA
40k
VSEN+
20k
1µA
RDIF = -600k
20k
VSEN-
20k
20k
FIGURE 32. EQUIVALENT DIFFERENTIAL AMPLIFER
The differential remote sense buffer has a precision unity gain
resistor matching network, which has a ultra low offset of 1mV.
This true remote sensing scheme helps compensate the droop
due to load on the positive and negative rails and maintain the
high system accuracy of ±0.6%.
As some applications will not need the differential remote sense,
the output of the remote sense buffer can be disabled and be
placed in high impedance by pulling VSEN- within 400mV of VCC.
Internal Reference and System Accuracy
The internal reference is set to 0.6V. Including bandgap variation
and offset of differential and error amplifiers, it has an accuracy
of ±0.6% over commercial temperature range, and 0.9% over
industrial temperature range. While the remote sense is not
used, its offset (V_{OS_DA}) should be included in the tolerance
calculation. Equations 18 and 19 show the worst case of system
accuracy calculation. V_{OS_DA} should set to zero when the
differential amplifier is in the loop, the differential amplifier’s
input impedance (R_DIF) is typically -600k with a tolerance of
20% (RDIF%) and can be neglected when R_OS is less than 100.
To set a precision setpoint, R_OS can be scaled by two paralleled
resistors.
Figure 34 shows the tolerance of various output voltage
regulation for 1%, 0.5%, and 0.1% feedback resistor dividers.
Note that the farther the output voltage setpoint away from the
internal reference voltage, the larger the tolerance; the lower the
resistor tolerance (R%), the tighter the regulation.
%min =
Vr
ef

1
–
R
e
f
%
–
V_OS
_DA


1

+
-^R----^F---_R-^B----_O-^---^_S--¹--_M---–--_A---^R-_X---^%------^{}
R_OSMAX= --_R--------_O----------_S----------_------_----₁₁----------+----------_R----------_%-----------_----+-----_R-¹---------_{D--------I----F---------}------_-----_{1--------1--+----------R---------D}--------_I----_F-------_{%------------}
(EQ. 18)
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FN6641.2
February 9, 2015