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AN-9720 Datasheet(PDF) 2 Page - Fairchild Semiconductor |
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AN-9720 Datasheet(HTML) 2 Page - Fairchild Semiconductor |
2 / 7 page AN-9720 APPLICATION NOTE © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • 12/23/10 2 PWM MODULATOR Q1 Q2 Q1A Q1B ISNS PMID SW PGND PMID L1 VBAT CMID + Battery CSIN 1 H 4.7 F CBAT SYSTEM LOAD VREF SDA SCL OTG/USB# VCC VBUS CBUS 1 F STAT I2C INTERFACE LOGIC AND CONTROL POWER OUTPUT STAGE PMID OSC 30mA COUT 0.1 F RSENSE Q3 CHARGE PUMP VBUS OVP I_IN CONTROL VREG CREG 1 F DAC DISABLE 1.8V / PMID REG Figure 2. FAN5400 Block Diagram System and Battery Power Sharing Power sharing between the system and the battery means that power can be steered or prioritized to go to the system in the case that the input power is not sufficient to power both the system and charge the battery. The typical configuration for the FAN5400, as shown in Figure 3, is one where the system is connected in parallel to the battery. The way in which this configuration can steer power similar to power path is sometimes confusing, so scenarios based on real-world battery capacity and input power numbers are provided below. Figure 3. Typical Application Circuit, System Parallel with Battery Example 1: 1500mAh Battery (1C Maximum Charge Current Capability of Battery is 1500mA), Input Power Source USB 2.0, 5V 500mA Scenario A) Partially charged battery at 3.6V and system load turns on at 400mA Before the system load turns on, the charger is already in CC mode. Because the input power source is 5V 500mA and the battery is at 3.6V, there is ~632mA of current available to charge the battery. This is computed by accounting for the charger conversion efficiency and also the output current multiplication factor achieved when bucking down a voltage: Efficiency I V V I IN OUT IN OUTMAX ∗ ∗ = / (1) Using the values in this example reveals that 5V/3.6V•500mA•91%=632mA. The 91% efficiency data point can be found in Figure 4. 84% 86% 88% 90% 92% 94% 2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 Battery Voltage, VBAT (V) 4.5VBUS 5.0VBUS 5.5VBUS Figure 4. FAN5400 Conversion Efficiency vs. Battery Voltage vs. VBUS Voltage |
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