Datasheet MAX889 (Maxim) - 7

High-Frequency, Regulated, 200mA, Inverting Charge Pump MAX889
Adjust the output voltage to a negative voltage from -2.5V to -VIN with external resistors R1 and R2 as 15.5  R1  shown in Figures 1 and 4. FB servos to GND. Choose C ≥ I OUT OUT  f   R1 + R2 R1 to be 100kΩ or less. Calculate R2 for the desired MIN output voltage: where COUT is the output capacitor value, and fMIN is the VOUT = -VREF (R2 / R1) minimum oscillator frequency in the Electrical R2 = R1 (VOUT / -VREF) Characteristics table. where VREF can be either VIN or a positive reference To ensure stability for regulated output mode, suitable source. output capacitor ESR should be determined by the follow- ing equation: Typically, choose a voltage-divider current of at least 30µA to minimize the effect of FB input current and capacitance: 19.2 x 10-3   R2  R ≤   ESR 1 R1 ≤ VREF / 30µA  I  +  R1   OUT  R2 < -VOUT / 30µA
Power Dissipation Capacitor Selection
The power dissipated in the MAX889 depends on the The appropriate capacitors used with the MAX889 input voltage, output voltage, and output current. Device depend on the switching frequency. Table 1 provides power dissipation is accurately described by: suggested values for CIN, CFLY, and COUT. P Surface-mount ceramic capacitors are preferred for DISS = IOUT (VIN - (-VOUT)) + (IQ ✕ VIN) CIN, COUT, and CFLY due to their small size, low cost, where IQ is the device quiescent current. PDISS must be and low ESR. To ensure proper operation over the less than the package dissipation rating (see Absolute entire temperature range, choose ceramic capacitors Maximum Ratings). Pay particular attention to power dis- with X7R (or equivalent) low-temperature-coefficient sipation limits when generating small negative voltages (tempco) dielectrics. See Table 2 for a list of suggested from large positive input voltages. capacitor suppliers.
Layout Considerations
The output capacitor stores the charge transferred from the flying capacitor and services the load between The MAX889’s high oscillator frequencies demand oscillator cycles. A good general rule is to make the good layout techniques that ensure stability and help output capacitance at least five-times greater than the maintain the output voltage under heavy loads. Take flying capacitor. the following steps to ensure optimum layout: Output voltage ripple is largely dependent on C 1) Mount all components as close together as possible. OUT. Choosing a low-ESR capacitor of sufficient value is impor- 2) Place the feedback resistors R1 and R2 close to the tant in minimizing the peak-to-peak output voltage ripple, FB pin, and minimize the PC trace length at the FB which is approximated by the following equation: circuit node. 3) Keep traces short to minimize parasitic inductance I V = OUT + and capacitance. RIPPLE 2 x f C OSC OUT 4) Use a ground plane with CIN and COUT placed in a 2 x I ESR OUT COUT star ground configuration (see the MAX889SEVKIT layout). where COUT is the output capacitor value, ESRCOUT is the output capacitor’s ESR, and fOSC is the MAX889 switching frequency. Ceramic capacitors have the lowest ESR and are recommended for COUT. Where larger capacitance at low cost is desired, a low-ESR tantalum capacitor may be used for COUT. See Table 2 for a list of suggested capacitor suppliers. To ensure stability over the entire operating temperature range, choose a low-ESR output capacitor using the fol- lowing equation:
_______________________________________________________________________________________ 7