ISL6742B Datasheet, PDF (17/20 Pages) Intersil Corporation – Fast current sense to output delay

ISL6742B
Next, determine the effect of the magnetizing current from
Equation 21.
V_CS = 91mV
Using Equation 24, solve for the summing resistor, R₉, from CT to
CS.
R₉ = 13.2kΩ
Determine the new value of R_CS, R’_CS, using Equation 25.
R’_CS = 15.7Ω
Additional slope compensation may be considered for design
margin. This discussion determines the minimum external ramp
that is required. The buffer transistor used to create the external
ramp from CT should have a sufficiently high gain (>200) so as to
minimize the required base current. Whatever base current is
required reduces the charging current into CT and will reduce the
oscillator frequency.
Parallel Operation
Parallel operation of converters using the ISL6742B may be
accomplished using the average current signal, I_OUT. I_OUT
provides a very accurate representation of the output current and
may be used for active current sharing with many sharing
techniques commonly used including master-slave and average
current sharing methods.
Since I_OUT represents the average inductor current (CCM operation),
sharing errors introduced by techniques using peak inductor current
are reduced. In particular, the current sharing error introduced by
mismatched switching frequencies is eliminated.
Figure 18 illustrates a master-slave current sharing method.
VOLTAGE ERROR
AMPLIFIER INVERTING (-)
INPUT
BIAS
U1
1
16
2 ^ISL6742B 15
3
VDD 14
4
13
5
12
6
7 CS
S&H
4x
11
10
8 IOUT
9
R₂
(>>R₁)
C₁
R₁
+
U2A
-
R₃
R₄
(>>R₃)
R₅ (>>R₁)
-
U2B
+
VOUT
OUTPUT
VOLTAGE
FEEDBACK
DIVIDER
Q1
R₇
R₆
In parallel and redundant applications, the ISHARE signals from
each power supply are connected together. Each power supply
produces a voltage proportional to its average output current on
I_OUT, and through limiting resistor R₃, on ISHARE. The unit with
the highest ISHARE signal (and highest output current) sources
current onto the ISHARE Bus, and is identified as the master unit.
The units with lower ISHARE signals do not source current onto
ISHARE, and are identified as slave units. Each slave unit
compares the master’s ISHARE signal with its own, and if there is
sufficient difference, turns Q1 on, which pulls down on the
feedback voltage. Reducing the feedback voltage causes the
output voltage to appear low; the feedback loop compensates by
increasing the output voltage, and the output current increases.
Each slave unit will increase its output voltage until its output
current is nearly equal to that of the master.
The difference between the master’s output current and that of a
slave unit is set by R₁ and R₂. Some difference is required to
prevent undesirable switching of master and slave roles. This
difference also prevents operation of the current sharing circuitry
when a power supply is operating stand alone.
The maximum output voltage that a slave can induce in its output
is controlled by R₆ and the output voltage feedback divider.
Typically, the maximum allowed output voltage increase is limited
to a few percent, but must be greater than the tolerance of the
feedback and reference components and any distribution drops
between units. If remote sensing is used, the adjustment range
must also include the difference in distribution drops between the
power supply outputs and the remote sensing location. The current
limit circuit must limit the voltage change to less than the output
overvoltage threshold or an overvoltage condition can be induced.
Amplifier U2A sets the scaling factor from I_OUT to ISHARE and
increases the current sourcing capability of ISHARE. U2B is a low
bandwidth amplifier that sets the frequency response and gain of
the current share circuitry. The current share bandwidth must be
much lower than the voltage feedback loop bandwidth to ensure
overall stability. The gain is set by R₁ and R₅, and the bandwidth
by R₅ and C₁.
The disconnect in series with ISHARE may be omitted for power
systems that do not require fault isolation. The disconnect switch
is normally implemented with MOSFET or JFET devices.
DISCONNECT IF P/S FAILS
OR IS TURNED OFF
ISHARE
FIGURE 18. MASTER-SLAVE CURRENT SHARING USING AVERAGE
CURRENT
Submit Document Feedback ₁₇
FN8565.1
November 3, 2015