ADP3210 Datasheet, PDF (26/38 Pages) ON Semiconductor – 7-Bit Programmable Multiphase Mobile CPU Synchronous

ADP3210
inductance and 15% DCR (at room temperature) are reasonable
assumptions that most manufacturers can meet.
Power Inductor Manufacturers
The following companies provide surface mount power
inductors optimized for high power applications upon request:
• Vishay Dale Electronics, Inc.
http://www.vishay.com
• Panasonic
http://www.panasonic.com
• Sumida Corporation
http://www.sumida.com
• NEC Tokin Corporation
http://www.nec-tokin.com
Output Droop Resistance
The inductor design requires that the regulator output voltage
measured at the CPU pins drops when the output current
increases. The specified voltage drop corresponds to a dc output
resistance (R_O).
The output current is measured by summing the currents of the
resistors monitoring the voltage across each inductor and by
passing the signal through a low-pass filter. This summer-filter
is implemented by the CS amplifier that is configured with
resistors R_PH(X) (summer), and R_CS and C_CS (filter). The output
resistance of the regulator is set by the following equations,
where R_L is the DCR of the output inductors:
R_O
=
R_CS
R_{PH( X)}
× R_L
(7)
C_CS
=
R_L
L
× R_CS
(8)
Users have the flexibility of choosing either R_CS or R_PH(X). Due to
the current drive ability of the CSCOMP pin, the R_CS resistance
should be larger than 100 kΩ. For example, users should
initially select R_CS to be equal to 220 kΩ, then use Equation 8 to
solve for C_CS
360 nH
C_CS = _{0.89mΩ × 220kΩ} = 1.84 nF
Because C_CS is not the standard capacitance, it is implemented
with two standard capacitors in parallel: 1.8 nF and 47 pF. For
the best accuracy, C_CS should be a 5% NPO capacitor.
Next, solve R_PH(X) by rearranging Equation 7.
R_{PH ( X )}
≥
0.89 mΩ
2.1mΩ
× 220 kΩ
=
93.2 kΩ
The standard 1% resistor for RPH(X) is 93.1 kΩ.
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 R_CS 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, R_CS1 and R_CS2 (see Figure 25) are
needed to linearize the NTC and produce the desired
temperature coefficient tracking.
Figure 25. Temperature Compensation Circuit Values
The following procedure and equations yield values for
R_CS1, R_CS2, and R_TH (the thermistor value at 25°C) for a given
R_CS 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
R_TH(50°C)/R_TH(25°C)) and Resistance Value B (B is
R_TH(90°C)/R_TH(25°C)). Note that the relative value of NTC
is always 1 at 25°C.
3. Next, find the relative value of R_CS that is required for each
of these temperatures. This is based on the percentage of
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