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參數(shù)資料
| 型號： | NCP5424DG |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Dual Synchronous Buck Controller with Input Current Sharing |
| 中文描述： | 1.5 A DUAL SWITCHING CONTROLLER, 750 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | LEAD FREE, SOIC-16 |
| 文件頁數(shù)： | 15/18頁 |
| 文件大?。?/td> | 148K |
| 代理商： | NCP5424DG |
NCP5424
http://onsemi.com
15
capacitor on the Comp1 pin and a 0.01 F capacitor on the
Comp2 pin in suggested.
Selecting Current Sharing Current Limit
In a twophase single output application, there are two
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 CycleByCycle 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.
Exceeding 70 mV between the IS+ and IS pins trips the
current limits. A divided down V
out
signal is used to generate
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
L
time constant, the
voltage across the capacitor will be equal to the voltage drop
across the internal resistance of the inductor.
The resistance of the output choke (LSR) must be known
to calculate the overcurrent trip point. The voltage drop
across the inductor at overcurrent is calculated as follows:
VL
RL· Iout
where:
V
L
= Voltage drop across the inductor,
R
L
= LSR of the inductor,
I
out
= Output current trip point for one phase.
For Hiccup Mode only, both sensing networks should have
the identical values.
If the inductor selected has a 5.0 m LSR and the current
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
out
by 20 mV, thus placing a 20 mV offset across the IS
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:
(eq. 1)
RRC
L
CRC· RL
(eq. 2)
L =
Inductor value, both Controllers should have the
same value.
R
L
= Internal resistance of L, see data sheet.
C
RC
=Chosen value, 0.1 F will make R a reasonable
value.
And the IS divider value can be selected with this equation.
Vout
Vout
R3
Vos
1 · R4
(eq. 3)
where:
V
out
= Output regulated voltage.
V
os
= Offset voltage, example above was 20 mV.
R4 = Chosen value, 10 K is a good choice.
If V
os
is larger than 70 mV, then the current signal from the
output chokes must be divided down. For example, if the
inductor’s LSR is equal to 8.0 m 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
RC
by a factor of 2. This divides the voltage across the
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.
To ensure accuracy, the equivalent parallel resistance of
R1 and R2 should be greater or equal to the value R
RC
, the
resistance value calculated from equation (2). If Hiccup
Mode is used, then both sensing network values must be
equal.
If CyclebyCycle is desired, then equation (1), (2) and
(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.
R1(1
R2
Ratio)
Ratio
(eq. 4)
where:
R1 = Chosen value, 10 K is recommended,
Islave.limit
Imaster.limit
limit ratio.
To ensure greater accuracy, the equivalent parallel
resistance of R1 and R2 should be greater or equal to the
value R
RC
, value calculated from equation (2).
Ratio
,
Master’s and Slave’s current
RRC
R1 · R2
R1
R2
(eq. 5)
Current Sensing
The current supplied to the load can be sensed easily using
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.
Sense Resistor.
A sense resistor can be added in series
with the inductor. When the voltage drop across the sense
resistor exceeds the internal voltage threshold of 70 mV, a
fault condition is set.
The sense resistor is selected according to:
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