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RT9709 Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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RT9709 Datasheet(HTML) 9 Page - Richtek Technology Corporation |
9 / 12 page RT9709 9 DS9709-02 April 2011 www.richtek.com Figure 1. Derating Curves for RT9709/A Package OCP ILIM ILIM K 25000 I = = RR Selecting Capacitors To get the better performance of the RT9709, it is very important to select peripherally appropriate capacitors. These capacitors determine some parameters such as input inrush current and input over shoot voltage. Generally, it is necessary to increase the input capacitance CIN for reducing the input over shoot voltage. However, this will increase the inrush current of input. There are two scenarios that can cause the input over shoot voltage. The first one is that when the AC adapter is hot-plugged and the second one is when the RT9709 has a step-down change. The cable between the AC adapter output and the handheld system input has a parasitic inductor and resistor causing the input over shoot voltage.Generally, the input over shoot voltage range is 1.5 to 2 times the input voltage. It is recommended to use 1uF CIN and COUT capacitance and the CIN rated voltage should be at 1.5 to 2 time of the input voltage. Thermal Considerations For continuous operation, do not exceed absolute maximum operation junction temperature. The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surroundings airflow and temperature difference between junction to ambient. The maximum power dissipation can be calculated by following formula : PD(MAX) = ( TJ(MAX) − TA ) / θJA Where TJ(MAX) is the maximum operation junction temperature 125 °C, TAis the ambient temperature and the θJAis the junction to ambient thermal resistance. For recommended operating conditions specification of RT9709, where TJ(MAX) is the maximum junction temperature of the die (125 °C) and TA is the maximum ambient temperature. The junction to ambient thermal resistance θJA is layout dependent. The thermal resistance θJA for the WDFN-12L 4x3 packages is 60°C/W and the WDFN-10L 3x3 packages is 70 °C/W on the standard JEDEC 51-7 four layers thermal test board. The maximum power dissipation at TA = 25 °C can be calculated by following formula : PD(MAX) =(125 °C − 25°C) / (60°C/W) = 1.429W for WDFN 3x3 packages PD(MAX) =(125 °C − 25°C) / (60°C/W) = 1.667W for WDFN 4x3 packages The maximum power dissipation depends on operating ambient temperature for fixed TJ(MAX) and thermal resistance θJA. For the RT9709 packages, the Figure 1 of derating curves allows the designer to see the effect of rising ambient temperature on the maximum power allowed. OCP (Over Current Protection) Setting The OCP (Over Current protection) threshold can be set by the ILIM pin. The resistor is connected between the ILIM pin and GND to set the OCP threshold. The OCP threshold can be calculated using the following equation : 0 0.3 0.6 0.9 1.2 1.5 1.8 0 25 50 75 100 125 Ambient Temperature (°C) WDFN-10L 3x3 Four Layers PCB WDFN-12L 4x3 ( °C) Layout Consideration The RT9709 is a protection device. So, a careful PCB layout is necessary. For best performance, place all peripheral components as close to the IC as possible. Place CIN, COUT, RVB, and RILIM near to VIN, VOUT, VB, ILIM and GND pin respectively. A short connection is highly recommended. The following guidelines should be strictly followed when designing a PCB layout for the RT9709. The exposed pad, GND, must be soldered to a large ground plane for heat sinking and noise prevention. The through-hole vias located at the exposed pad is connected to ground plane of internal layer. |
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