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NCP5425DBR2G Datasheet(PDF) 19 Page - ON Semiconductor

Part # NCP5425DBR2G
Description  Dual Synchronous Buck Controller
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

NCP5425DBR2G Datasheet(HTML) 19 Page - ON Semiconductor

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NCP5425
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19
The values of R2 and R3 can be found by solving two
simultaneous equations:
R2
+ R3 2
R1
+ (R2 * R3) (R2 ) R3)
Solving for R2 and R3 yields:
R2
+ 1.5R1
R3
+ 3R1
Example 2
Assume we have elected to source 66.7% output current
from the Master controller, and 33.3% from the Slave.
Figure 13 shows the configuration of the inductor sense
networks and Slave error amplifier for this case. The ratio of
Master−to−Slave load current is equal to 66.7%/33.3%, or
2:1. Therefore R2 and R3 must be chosen to satisfy two
conditions:
A parallel equivalent resistance equal to R1, and,
A ratio such that the drop across the parasitic resistance of
the Master inductor is 2 times the drop across the parasitic
resistance of the Slave inductor when the inputs to the
Slave error amplifier are equal (assumes the inductors are
identical). The optimum value of R1 is described by the
equation:
R1
+ Lx (C1 * Rx)
The values of R2 and R3 can be found by solving two
simultaneous equations:
R2
+ R3
R1
+ (R2 * R3) (R2 ) R3)
Solving for R2 and R3 yields:
R2
+ 2R1
R3
+ 2R1
Note that the Mode pin must be floating for a two−phase,
single output design. This disables the internal Error
Amplifier Reference clamp, and increases its common mode
range.
No Load Zero Balance
To improve current matching, a low pass filter can be
inserted between the Master controller inductor sensing RC
network and the Slave controller Vref2 input pin (see
Figure 14). This will attenuate the amplitude of the
out−of−phase ripple current signal superimposed on the DC
current signal, providing a smoother Slave Error Amplifier
reference input.
Vin
Q1
L1
R1
C1
R3
R3
R4
C3
+
Vfb1
Q2
Master
Error Amp
Internal
0.8 V Ref.
L2
R2
C2
+
Vfb2
Vref2
RF
Low Pass
Filter
Slave
Error Amp
Q3
Q4
Figure 14. Addition of a Low Pass Filter to the Current Sense Reference Input
Vout
CF
With the value of RF set to approximately two times the
value of R1, Cf can be calculated as follows:
Cf
+ 1 (2p ·f· RF), where :
f = operating frequency of the controller
When a filter is added, the response delay introduced by
the RC time constant must be considered.
Configuring a Dual Output Application
To configure the NCP5425 for a dual output application:
The Mode pin must be grounded
An external voltage reference must be provided for
Controller 2, via the Vref2 pin


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