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ADP3207C Datasheet(PDF) 24 Page - ON Semiconductor

Part No. ADP3207C
Description  7-Bit Programmable, Multi-Phase Mobile, CPU Synchronous Buck Controller
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

ADP3207C Datasheet(HTML) 24 Page - ON Semiconductor

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ADP3207C
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24
by passing the signal through a low−pass filter. This
summer−filter is implemented by the CS amplifier that is
configured with resistors RPH(X) (summer), and RCS and CCS
(filter). The output resistance of the regulator is set by the
following equations:
RO +
RCS
RPH(x)
RL
(eq. 5)
CCS +
L
RL @ RCS
(eq. 6)
where RL is the DCR of the output inductors.
Users have the flexibility of choosing either RCS or RPH(X).
Due to the current drive ability of the CSCOMP pin, the RCS
resistance should be larger than 100 k
W. For example, users
should initially select RCS to be equal to 220 kW, then use
Equation 6 to solve for CCS.
CCS +
360 nH
0.89 mW
220 kW
+ 1.84 nF
Because CCS is not the standard capacitance, it is
implemented with two standard capacitors in parallel: 1.8 nF
and 47 pF. For the best accuracy, CCS should be a 5% NPO
capacitor. Next, solve RPH(X) by rearranging Equation 5.
RPH(X) w
0.89 mW
2.1 mW
@ 220 kW + 93.2 kW
The standard 1% resistor for RPH(X) is 93.1 kW.
To prevent the saturation of the current sense amplifier
when multiple phases turn on together, it is recommended to
keep RPH(X) > 90 kW in the 2−phase application and RPH(X)
> 133 k
W in the 3−phase application.
To avoid high frequency noise coupling across the RPH
resistors, the size of the RPH resistors should not be smaller
than the 0603 size.
Inductor DCR Temperature Correction
With the inductor DCR used as a sense element, and copper
wire being the source of the DCR, users need to compensate
for temperature changes in the inductor’s winding. Fortunately,
copper has a well−known temperature coefficient (TC) of
0.39%/
°C.
If RCS is designed to have an opposite sign but equal
percentage change in resistance, then it cancels the temperature
variation of the inductor DCR. Due to the nonlinear nature of
NTC thermistors, series resistors, RCS1 and RCS2 (see
Figure 29) are needed to linearize the NTC and produce the
desired TC tracking.
Figure 29. Temperature Compensation Circuit Values
+
RTH
19
18
17
CSCMOP
CSSUM
CSREF
RCS1
RCS2
RPH1
RPH2
RPH3
To VOUT
Sense
CCS
To Switch Nodes
Keep This Path as Short as
Possible and Well Away
from Switch Node Lines
ADP3207C
Place as close as possible
to the nearest inductor
The following procedure and equations yield values for
RCS1, RCS2, and RTH (the thermistor value at 25°C) for a
given RCS value:
1. Select an NTC to be used based on type and value.
Because there is no value yet, start with a thermistor
with a value close to RCS. The NTC should also
have an initial tolerance of better than 5%.
2. Based on the type of NTC, find its relative resistance
value at two temperatures. Temperatures that work
well are 50
°C and 90°C. These are called Resistance
Value A (A is RTH(50°C)/RTH(25°C)) and
Resistance Value B (B is RTH(90°C)/RTH(25°C)).
Note the relative value of NTC is always 1 at 25
°C.
3. Next, find the relative value of RCS that is required
for each of these temperatures. This is based on the
percentage of change needed, which is initially
0.39%/
°C. These are called r1 and r2.
r1 +
1
1 ) TC @ (T1 * 25)
(eq. 7)
r2 +
1
1 ) TC @ (T2 * 25)
Where:
TC = 0.0039
T1 = 50°C
T2 = 90°C
4. Compute the relative values for rCS1, rCS2, and rTH
using:
(A * B)
r1 r2 * A (1 * B) r2 ) B (1 * A) r1
A
(1 * B)
r1 * B (1 * A) r2 * (A * B)
(eq. 8)
rCS2 +
rCS1 +
(1 * A)
1
1*rCS2
*
1
r1*rCS2
rTH +
1
1
1*rCS2
* 1
rCS1
5. Calculate RTH = RTH x RCS, then select the closest
value of thermistor that is available. Also, compute
a scaling factor k based on the ratio of the actual
thermistor value relative to the computed one.
k +
RTH(ACTUAL)
RTH(CALCULATED)
(eq. 9)
6. Finally, calculate values for RCS1 and RCS2 using:
RCS1 + RCS k rCS1
(eq. 10)
RCS2 + RCS ((1 * k) ) (k rCS2))
This example starts with a thermistor value of 100 k
W and
uses a Vishay NTHS0603N04 NTC thermistor (a 0603 size
thermistor) with A = 0.3359 and B = 0.0771. From this data,
rCS1 = 0.359, rCS2 = 0.729, and rTH = 1.094. Solving for RTH
yields 240 k
W, so 220 kW is chosen, making k = 0.914.
Finally, RCS1and RCS2 are 72.3 kW and 166 kW. Choosing the
closest 1% resistor values yields a choice of 71.5 k
W and
165 k
W.


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