XC9201CCCAL datasheet(13/24 Pages) TOREX

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XC9201

Series

5. FB Voltage and CFB

With regard to the XC9201D series, the output voltage is set by attaching externally divided resistors. The output voltage is

determined by the equation shown below according to the values of RFB1 and RFB2. In general, the sum of RFB1 and RFB2

should be 1 MΩor less.

VOUT = 0.9 x (RFB1+ RFB2) / RFB2

The value of CFB (phase compensation capacitor) is approximated by the following equation according to the values of RFB1

and fzfb. The value of fzfb should be 10 kHz, as a general rule.

CFB = 1/(2 x π x RFB1 x fzfb)

Example: When RFB1 = 455 kΩ and RFB2 = 100 kΩ : VOUT = 0.9 x (455 k + 100 k)/100 k = 4.995 V

: CFB= 1/(2 x π x 455 k x 10 k) = 34.98 pF.

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APPLICATION NOTES

1. The XC9201 series are designed for use with an output ceramic capacitor. If, however, the potential difference between

input and output is too large, a ceramic capacitor may fail to absorb the resulting high switching energy and oscillation

could occur on the output side. If the input-output potential difference is large, connect an electrolytic capacitor in parallel

to compensate for insufficient capacitance.

2. The EXT pin of the XC9201 series is designed to minimize the through current that occurs in the internal circuitry.

However, the gate drive of external PMOS has a low impedance for the sake of speed. Therefore, if the input voltage is

high and the bypass capacitor is attached away from the IC, the charge/discharge current to the external PMOS may lead

to unstable operations due to switching operation of the EXT pin.

As a solution to this problem, place the bypass capacitor as close to the IC as possible, so that voltage variations at the VIN

and VSS pins caused by switching are minimized. If this is not effective, insert a resistor of several to several tens of

ohms between the EXT pin and PMOS gate. Remember that the insertion of a resistor slows down the switching speed

and may result in reduced efficiency.

3. A PNP transistor can be used in place of PMOS. If using a PNP transistor, insert a resistor (RB) and capacitor (CB)

between the EXT pin and the base of the PNP transistor in order to limit the base current without slowing the switching

speed. Adjust RB in a range of 500Ω to 1kΩ according to the load and hFE of the transistor. Use a ceramic capacitor

for CB, complying with CB ≦ 1/ ( 2 x π x RB x Fosc x 0.7), as a rule.

4. This IC incorporates a limit comparator to monitor the voltage produces across the RSEN resistor at the current peak of the

coil. It functions as a limiter when, for example, the output is short-circuited. In such a case, the limit comparator

senses that the voltage across the RSEN resistor has reached a current-limiting voltage (typically 150mV) and outputs a

signal to turn off the external transistor. After sensing a current-limit voltage, the limit comparator typically takes 200nsec

(TYP.) before it turns off the external resistor. During this time, the voltage across the RSEN resistor can exceed the

current-limit voltage, especially when the difference between the input voltage and the output voltage is large and the coil

inductance is small. Therefore, exercise great care in selecting absolute maximum ratings of the external transistor, coil,

and Schottky diode.

5. If the difference between the input voltage and the output voltage is large or small, the switching ON time or OFF time of

this IC becomes short and actual operation can be critically influenced by values of peripheral components 'inductance of

coil, resistance of CLK connection, capacitance of capacitor, etc.) Before use, it is recommended to evaluate this IC

thoroughly with an actual unit.

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OPERATIONAL EXPLANATION (Continued)

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Functional Settings (Continued)

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