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MP2307 Datasheet(PDF) 8 Page - Monolithic Power Systems |
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MP2307 Datasheet(HTML) 8 Page - Monolithic Power Systems |
8 / 11 page MP2307 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER MP2307 Rev. 1.7 www.MonolithicPower.com 8 3/14/2006 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2006 MPS. All Rights Reserved. TM Since the input capacitor (C1) absorbs the input switching current, it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by: ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ × − × = IN OUT IN OUT LOAD 1 C V V 1 V V I I The worst-case condition occurs at VIN = 2VOUT, where IC1 = ILOAD/2. For simplification, use an input capacitor with a RMS current rating greater than half of the maximum load current. The input capacitor can be electrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1µF, should be placed as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. The input voltage ripple for low ESR capacitors can be estimated by: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − × × × = ∆ IN OUT IN OUT S LOAD IN V V 1 V V f 1 C I V Where C1 is the input capacitance value. Output Capacitor The output capacitor (C2) is required to maintain the DC output voltage. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. Low ESR capacitors are preferred to keep the output voltage ripple low. The output voltage ripple can be estimated by: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × × + × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − × × = ∆ 2 C f 8 1 R V V 1 L f V V S ESR IN OUT S OUT OUT Where C2 is the output capacitance value and RESR is the equivalent series resistance (ESR) value of the output capacitor. When using ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance which is the main cause for the output voltage ripple. For simplification, the output voltage ripple can be estimated by: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − × × × × = IN OUT 2 S OUT OUT V V 1 2 C L f 8 V ∆V When using tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching frequency. For simplification, the output ripple can be approximated to: ESR IN OUT S OUT OUT R V V 1 L f V ∆V × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − × × = The characteristics of the output capacitor also affect the stability of the regulation system. The MP2307 can be optimized for a wide range of capacitance and ESR values. Compensation Components MP2307 employs current mode control for easy compensation and fast transient response. The system stability and transient response are controlled through the COMP pin. COMP is the output of the internal transconductance error amplifier. A series capacitor-resistor combination sets a pole-zero combination to govern the characteristics of the control system. The DC gain of the voltage feedback loop is given by: OUT FB EA CS LOAD VDC V V A G R A × × × = Where VFB is the feedback voltage (0.925V), AVEA is the error amplifier voltage gain, GCS is the current sense transconductance and RLOAD is the load resistor value. The system has two poles of importance. One is due to the compensation capacitor (C3) and the output resistor of the error amplifier, and the other is due to the output capacitor and the load resistor. These poles are located at: VEA EA 1 P A 3 C 2 G f × × π = LOAD 2 P R 2 C 2 1 f × × π = Where GEA is the error amplifier transconductance. |
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