Datasheet MP2307 (Monolithic Power Systems) - 8

MP2307 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER
Since the input capacitor (C1) absorbs the input When using tantalum or electrolytic capacitors, switching current, it requires an adequate ripple the ESR dominates the impedance at the current rating. The RMS current in the input switching frequency. For simplification, the capacitor can be estimated by: output ripple can be approximated to: V ⎛ V ⎞ V ⎛ V ⎞ OUT ⎜ OUT ⎟ I = I × OUT OUT ∆V = × 1− × 1 C LOAD 1 ⎜ × − ⎟ OUT R ⎜⎜ ⎟⎟ ESR × IN V ⎝ IN V ⎠ f L ⎝ VIN S ⎠ The worst-case condition occurs at V The characteristics of the output capacitor also IN = 2VOUT, where I affect the stability of the regulation system. The C1 = ILOAD/2. For simplification, use an input capacitor with a RMS current rating MP2307 can be optimized for a wide range of greater than half of the maximum load current. capacitance and ESR values. The input capacitor can be electrolytic, tantalum
Compensation Components
or ceramic. When using electrolytic or tantalum MP2307 employs current mode control for easy capacitors, a small, high quality ceramic compensation and fast transient response. The capacitor, i.e. 0.1µF, should be placed as close system stability and transient response are to the IC as possible. When using ceramic controlled through the COMP pin. COMP is the capacitors, make sure that they have enough output of the internal transconductance error capacitance to provide sufficient charge to amplifier. A series capacitor-resistor prevent excessive voltage ripple at input. The combination sets a pole-zero combination to input voltage ripple for low ESR capacitors can govern the characteristics of the control system. be estimated by: The DC gain of the voltage feedback loop is I V ⎛ V ⎞ given by: ∆V = LOAD × OUT × OUT IN ⎜⎜1 − ⎟⎟ 1 C × f VIN S ⎝ IN V ⎠ FB V A = × × × VDC RLOAD GCS AEA Where C1 is the input capacitance value. OUT V MP2393
Output Capacitor
Where VFB is the feedback voltage (0.925V), The output capacitor (C2) is required to AVEA is the error amplifier voltage gain, GCS is maintain the DC output voltage. Ceramic, the current sense transconductance and RLOAD O tantalum, or low ESR electrolytic capacitors are is the load resistor value. recommended. Low ESR capacitors are The system has two poles of importance. One preferred to keep the output voltage ripple low. is due to the compensation capacitor (C3) and The output voltage ripple can be estimated by: the output resistor of the error amplifier, and the other is due to the output capacitor and the load RECOMMENDED FOR V ⎛ V ⎞ ⎛ OUT OUT 1 ⎞ ∆V = × 1− × OUT ⎜⎜ ⎟⎟ ⎜⎜R + ESR ⎟⎟ resistor. These poles are located at: NEW DESIGNS f × L S ⎝ VIN ⎠ ⎝ 8 × f × 2 C S ⎠ GEA Where C2 is the output capacitance value and f = 1 P 2π × 3 C × A R VEA ESR is the equivalent series resistance (ESR) value of the output capacitor. 1 f = P2 NOT When using ceramic capacitors, the impedance 2π × 2 C × RLOAD at the switching frequency is dominated by the Where G REFER T capacitance which is the main cause for the EA is the error amplifier transconductance. output voltage ripple. For simplification, the output voltage ripple can be estimated by: V ⎛ V ⎞ ∆V = OUT × OUT OUT 1 2 ⎜⎜ − ⎟⎟ 8 × f × L × 2 C V S ⎝ IN ⎠ MP2307 Rev. 1.9 www.MonolithicPower.com
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5/28/2008 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved.