Datasheet EL7515 (Renesas) - 7

link to page 7 link to page 7 link to page 7 link to page 7 link to page 7 link to page 7 link to page 7 EL7515 At very low load, the MOSFET will skip pulses sometimes. The inductor has peak and average current decided by This is normal. Equations 4 and 5: I
Current Limit
I L (EQ. 4) LPK = ILAVG + ---- 2 The MOSFET current limit is nominally 1.4A and guaranteed 1A. This restricts the maximum output current IOMAX based I I O = ------- (EQ. 5) on Equation 1: LAVG 1 – D I I  L = – ----  VIN  ----- (EQ. 1) The inductor should be chosen to be able to handle this OMAX 1  2  VO current. Furthermore, due to the fixed internal compensation, it is recommended that maximum inductance where: of 10µH and 15µH to be used in the 5V and 12V or higher • IL is the inductor peak-to-peak current ripple and is output voltage, respectively. decided by Equation 2: The output diode has an average current of IO, and peak VIN D current the same as the inductor's peak current. A Schottky I (EQ. 2) L = -----  --- L fS diode is recommended and it should be able to handle those currents. • D is the MOSFET turn-on ratio and is decided by Equation 3: The output voltage ripple can be calculated as Equation 6: V – I  D O VIN = ------------ (EQ. 3) O D (EQ. 6) V V = ----------- + I  O O F LPK ESR S  CO • fS is the switching frequency Where: The following table gives typical values: • CO is the output capacitance.
TABLE 1. MAX CONTINUOUS OUTPUT CURRENTS
• The ESR is the output capacitor ESR value.
VIN VO L FS IOMAX
Low ESR capacitors should be used to minimize the output
(V) (V) (ΜH) (kHz) (mA)
voltage ripple. Multilayer ceramic capacitors (X5R and X7R) 2 5 10 1000 360 are preferred for the output capacitors since they have a low 2 9 10 1000 190 ESR and small packages. Tantalum capacitors also can be used, but they take more board space and have higher ESR. 2 12 10 1000 140 A minimum of 22µF output capacitor is sufficient for high 3.3 5 10 1000 600 output current application. For lower output current, the output capacitor can be smaller, like 4.7µF. The capacitor 3.3 9 10 1000 310 should always have enough voltage rating. In addition to the 3.3 12 10 1000 230 voltage rating, the output capacitor should also be able to 5 9 10 1000 470 handle the RMS current which is given by Equation 7: 5 12 10 1000 340  I 2   L 1  (EQ. 7) 9 12 10 1000 630 ICORMS = 1 – D  D + ----------   ---  I 12 LAVG  I 2 LAVG  12 15 10 100 670
Output Voltage Component Considerations
An external resistor divider is required to divide the output It is recommended that CIN is larger than 10µF. voltage down to the nominal reference voltage. The current Theoretically, the input capacitor has a ripple current of IL. drawn by the resistor network should be limited to maintain Due to high-frequency noise in the circuit, the input current the overall converter efficiency. The maximum value of the ripple may exceed the theoretical value. A larger capacitor resistor network is limited by the feedback input bias current will reduce the ripple further. and the potential for noise being coupled into the feedback pin. A resistor network less than 300k is recommended. FN7120 Rev 2.00 Page 7 of 9 August 10, 2007