Datasheet BL8531 (Belling) - 5

BL8531 SELECTION THE EXTERNAL COMPONENTS Thus it can be seen, the inductor and schottky An inductor value of 3uH to 1mH works well in diode affect the conversion efficiency greatly. The most applications. If DC-DC converter delivers inductor and the capacitor also have great large output current (for example: output current influence on the output voltage ripple of the is great than 50mA), large inductor value is converter. So it is necessary to choose a suitable recommended in order to improve efficiency. If inductor, a capacitor and a right schottky diode, DC-DC must output very large current at low to obtain high efficiency, low ripple and low noise. input supply voltage, small inductor value is Before discussion，we define recommended. Vout − Vin D ≡ . The ESR of inductor will affect efficiency greatly. Vout Suppose ESR value of inductor is rL，Rload is load INDUCTOR SELECTION resistor，then the energy can be calculated by following expression: Above all, we should define the minimum value rL ∆η ≈ of the inductor that can ensure the boost DC-DC 2 R 1 ( − D) load to operate in the continuous current-mode For example: input 1.5V, output is 3.0V, condition. Rload=20Ω, rL=0.5Ω, The energy loss is 10%. D 1 ( D)2 − R L L min ≥ Consider all above，inductor value of 47uH、 2 f ESR<0.5Ω is recommended in most applications. The above expression is got under conditions of Large value is recommended in high efficiency continuous current mode, neglect Schottky applications and smaller value is recommended. diode’s voltage, ESR of both inductor and capacitor. The actual value is greater that it. If CAPACITOR SELECTION inductor’s value is less than Lmin，the efficiency of DC-DC converter will drop greatly, and the DC- Ignore ESR of capacitor，the ripple of output DC circuit will not be stable. voltage is: Vout ∆ D r = = Secondly, consider the ripple of the output Vout R Cf load voltage, So large value capacitor is needed to reduce D •Vin I ∆ = ripple. But too large capacitor value will slow Lf down system reaction and cost will improve. So Vin DVin 100uF capacitor is recommended. Larger Im ax = + capacitor value will be used in large output 1 ( − D)2 R 2Lf L current system. If output current is small (<10mA), If inductor value is too small, the current ripple small value is needed. through it will be great. Then the current through diode and power switch will be great. Because Consider ESR of capacitor，ripple will increase： the power switch on chip is not ideal switch, the ax • R energy of switch will improve. The efficiency will r = Im ' r ESR + fall. Vout When current is large, ripple caused by ESR will Thirdly ， in general, smaller inductor values be main factor. It may be greater than 100mV。 supply more output current while larger values The ESR will affects efficiency and increase start up with lower input voltage and acquire energy loss. So low-ESR capacitor (for example: high efficiency. tantalum capacitor) is recommend or connect two or more filter capacitors in parallel. www.belling.com.cn 5