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AP1534 Datasheet(PDF) 7 Page - Diodes Incorporated |
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AP1534 Datasheet(HTML) 7 Page - Diodes Incorporated |
7 / 10 page AP1534 PWM CONTROL 2A STEP-DOWN CONVERTER AP1534 Rev. 5 7 of 10 FEBRUARY 2009 www.diodes.com © Diodes Incorporated Functional Description PWM Control The AP1534 is a DC/DC converter that employs pulse width modulation (PWM) scheme. Its pulse width varies in the range of 0% to 99%, based on the output current loading. The output ripple voltage caused by the PWM high frequency switching can easily be reduced through an output filter. Therefore, this converter provides a low ripple output supply over a broad range of input voltage & output current loading Under Voltage Lockout The under voltage lockout circuit of the AP1534 assures that the high-side MOSFET driver remains in the off state whenever the supply voltage drops below 3.3V. Normal operation resumes once VCC rises above 3.5V. Current Limit Protection The current limit threshold is set by external resistor ROCSET connected from VCC supply to OCSET pin. The internal sink current IOCSET (90uA typical) across this resistor sets the voltage at OCSET pin. When the PWM voltage is less than the voltage at OCSET, an over-current condition is triggered. The current limit threshold is given by the following equation: R I R I OCSET OCSET DS(ON) PEAK × = × 2 I) ( I I (MAX) OUT PEAK Δ + > where, IN OUT S OUT IN V V L F V V I × × − = Δ IPEAK is the output peak current; RDS (ON) is the MOSFET ON resistance; FS is the PWM frequency (300KHz typical). Also, the inductor value will affect the ripple current ΔI. The above equation is recommended for input voltage range of 5V to 18V. For input voltage lower than 5V or ambient temperature over 100°C, higher ROCSET is recommended. The recommended minimum ROCSET value is summarized below: VIN (V) VOUT (V) ROCSET (Ω) 4 1 6.8K 5 3.3 5.6K 12 5 3.9K 18 12 4.7K Inductor Selection For most designs, the operates with inductors of 22µH to 33µH. The inductor value can be derived from the following equation: IN OUT S OUT IN V V I f V V L × Δ × − = Where ΔIL is inductor Ripple Current. Large value inductors lower ripple current and small value inductors result in high ripple current. Choose inductor ripple current approximately 15% of the maximum load current 2A, ∆IL=0.3A. The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation (2A+0.15A). Input Capacitor Selection This capacitor should be located close to the IC using short leads and the voltage rating should be approximately 1.5 times the maximum input voltage. The RMS current rating requirement for the input capacitor of a buck regulator is approximately 1⁄2 the DC load current. A low ESR input capacitor sized for maximum RMS current must be used. A 470µF low ESR capacitor for most applications is sufficient. Output Capacitor Selection The output capacitor is required to filter the output voltage and provides regulator loop stability. The important capacitor parameters are the 100KHz Equivalent Series Resistance (ESR), the RMS ripples current rating, voltage rating and capacitance value. For the output capacitor, the ESR value is the most important parameter. The output ripple can be calculated from the following formula. ESR I V L RIPPLE × Δ = The bulk capacitor’s ESR will determine the output ripple voltage and the initial voltage drop after a high slew-rate transient. An aluminum electrolytic capacitor's ESR value is related to the capacitance and its voltage rating. In most case, higher voltage electrolytic capacitors have lower ESR values. Most of the time, capacitors with much higher voltage ratings may be needed to provide the low ESR values required for low output ripple voltage. PCB Layout Guide If you need low TC & TJ or large PD (Power Dissipation), The dual SW pins(5& 6) and Vss pins(7& 8)on the SOP-8L package are internally connected to die pad, The evaluation board should be allowed for maximum copper area at output (SW) pins. 1. Connect FB circuits (RA, RB, CC) as closely as possible and keep away from inductor flux for pure VFB. 2. Connect CVCC to Vcc and Vss pin as closely as possible to get good power filter effect. 3. Connect ROCSET to Vcc and OCSET pin as closely as possible. 4. Connect ground side of the CIN & D1 & COUT as closely as possible and use ground plane for best performance. |
Similar Part No. - AP1534_09 |
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Similar Description - AP1534_09 |
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