ncp5424adr2 ON Semiconductor, ncp5424adr2 Datasheet - Page 15

• Manufacturer Part Number: ncp5424adr2
• Description: Dual Synchronous Buck Controller With Input Current Sharing
• Manufacturer: ON Semiconductor

capacitor on the Comp1 pin and a 0.01 mF capacitor on the
Comp2 pin in suggested.
Selecting Current Sharing Current Limit
different current limit options. The Master (Controller One)
current limit can be set equal to the Slave (Controller Two)
which brings both controllers into Hiccup Mode during an
overcurrent condition. The second option is to set Slave
current limit lower than that of the Master, which limits the
Slave’s input power when its limit is reached, while the
output voltage remains in regulation. Both Master and Slave
will go into hiccup mode if the Master’s limit is reached.
During Cycle−By−Cycle current limit, the Slave’s operating
frequency will decrease in half, due to pulse skipping,
resulting in phase overlap. This overlap will increase the
output voltage ripple.
current limits. A divided down V
the IS− reference, and inductor sensing of the controllers
output chokes provide the output current information to
IS+X pin. The inductor sensing is achieved by placing a
series RC in parallel with the output choke. With the RC time
constant selected to equal the L/R
voltage across the capacitor will be equal to the voltage drop
across the internal resistance of the inductor.
to calculate the overcurrent trip point. The voltage drop
across the inductor at overcurrent is calculated as follows:
where:
For Hiccup Mode only, both sensing networks should have
the identical values.
limit is 10 A through one of the phases, then the analog
signal will be 50 mV. Since this value is less than 70 mV,
then the IS− divider, R3 and R4 in Figure 1, must scale down
the V
and IS+x pin at no load and allowing the Controllers to trip
into current limit with only 50 mV across the inductor. In
this case, the RC values are calculated using the following
equation:
And the IS− divider value can be selected with this equation.
In a two−phase single output application, there are two
Exceeding 70 mV between the IS+ and IS− pins trips the
The resistance of the output choke (LSR) must be known
V
R
I
If the inductor selected has a 5.0 mW LSR and the current
L =
R
C
out
L
L
RC
L
out
= LSR of the inductor,
= Internal resistance of L, see data sheet.
= Voltage drop across the inductor,
= Output current trip point for one phase.
= Chosen value, 0.1 mF will make R a reasonable
by 20 mV, thus placing a 20 mV offset across the IS−
Inductor value, both Controllers should have the
same value.
value.
R3 +
R RC +
V L + R L · I out
V out * V os
V out
C RC · R L
out
L
* 1 · R4
signal is used to generate
L
time constant, the
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(eq. 1)
(eq. 2)
(eq. 3)
NCP5424
15
where:
output chokes must be divided down. For example, if the
inductor’s LSR is equal to 8.0 mW and the current limit is
15 A, then the current signal is 120 mV, which is almost
twice the comparator’s offset (70 mV). This signal can be
divided down by adding a resistor (R1) in parallel with the
capacitor (C6) in the inductor sensing network, see Figure 1.
The divider R1 and R2 can be set to equal value to divide the
current signal in half and equation (3) should be used to
select the proper voltage divider. Notice that the divider R1
and R2, divides down the voltage applied to the capacitor
C
output inductor’s LSR by a factor of two and results in twice
the current limit. This scaling technique is another way the
current limit may be set so that virtually any current limit
may be obtained.
R1 and R2 should be greater or equal to the value R
resistance value calculated from equation (2). If Hiccup
Mode is used, then both sensing network values must be
equal.
(3) should be used to select the Slave’s inductor sensing
network for the desired current limit and equation (4) should
be used to raise the Master’s current limit, Hiccup Mode,
above the Slave’s limit.
where:
resistance of R1 and R2 should be greater or equal to the
value R
Current Sensing
the IS+ and IS− pins for the output. These pins sense a
voltage, proportional to the output current, and compare it to
a fixed internal voltage threshold. When the differential
voltage exceeds 70 mV, the internal overcurrent protection
system goes into hiccup mode. Two methods for sensing the
current are available.
with the inductor. When the voltage drop across the sense
resistor exceeds the internal voltage threshold of 70 mV, a
fault condition is set.
RC
V
V
R4 = Chosen value, 10 KW is a good choice.
If V
To ensure accuracy, the equivalent parallel resistance of
If Cycle−by−Cycle is desired, then equation (1), (2) and
R1 = Chosen value, 10 KW is recommended,
limit ratio.
To ensure greater accuracy, the equivalent parallel
The current supplied to the load can be sensed easily using
Sense Resistor.
The sense resistor is selected according to:
Ratio +
out
os
by a factor of 2. This divides the voltage across the
os
= Offset voltage, example above was 20 mV.
= Output regulated voltage.
RC
is larger than 70 mV, then the current signal from the
, value calculated from equation (2).
I master.limit
I slave.limit
R2 +
A sense resistor can be added in series
R RC v R1 · R2
,
R1(1 * Ratio)
Master’s and Slave’s current
R1 ) R2
Ratio
RC
(eq. 4)
(eq. 5)
, the