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MP2355 Datasheet(PDF) 8 Page - Monolithic Power Systems |
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MP2355 Datasheet(HTML) 8 Page - Monolithic Power Systems |
8 / 10 page MP2355 – 3A, 23V, 380KHz STEP-DOWN CONVERTER MP2355 Rev. 1.5 www.MonolithicPower.com 8 5/1/2006 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2006 MPS. All Rights Reserved. TM Output Capacitor The output capacitor 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 L is the inductor value, C2 is the output capacitance value, and RESR is the equivalent series resistance (ESR) value of the output capacitor. In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output voltage ripple is mainly caused by the capacitance. 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 In the case of 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 MP2355 can be optimized for a wide range of capacitance and ESR values. Compensation Components MP2355 employs current mode control for easy compensation and fast transient response. The system stability and transient response are controlled through the COMP pin. COMP pin is the output of the internal transconductance error amplifier. A series capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system. The DC gain of the voltage feedback loop is given by: OUT FB VEA CS LOAD VDC V V A G R A × × × = Where AVEA is the error amplifier voltage gain, 400V/V; GCS is the current sense transconductance, 3.8A/V; 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 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, 800µA/V. The system has one zero of importance, due to the compensation capacitor (C3) and the compensation resistor (R3). This zero is located at: 3 R 3 C 2 1 f 1 Z × × π = The system may have another zero of importance, if the output capacitor has a large capacitance and/or a high ESR value. The zero, due to the ESR and capacitance of the output capacitor, is located at: ESR ESR R 2 C 2 1 f × × π = In this case (as shown in Figure 2), a third pole set by the compensation capacitor (C6) and the compensation resistor (R3) is used to compensate the effect of the ESR zero on the loop gain. This pole is located at: 3 R 6 C 2 1 f 3 P × × π = |
Similar Part No. - MP2355_06 |
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Similar Description - MP2355_06 |
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