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FAN2103EMPX Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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FAN2103EMPX Datasheet(HTML) 10 Page - Fairchild Semiconductor |
10 / 14 page © 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN2103 Rev. 1.0.5 10 Circuit Description Initialization Once VCC exceeds the UVLO threshold and EN is HIGH, the IC checks for an open or shorted FB pin before releasing the internal soft-start ramp (SS). If R1 is open (as shown in Figure 1), the error amplifier output (COMP) is forced LOW and no pulses are generated. After the SS ramp times out (T1.0), an under- voltage latched fault occurs. If the parallel combination of R1 and RBIAS is ≤ 1KΩ, the internal SS ramp is not released and the regulator does not start. Soft-Start Once internal SS ramp has charged to 0.8V (T0.8), the output voltage is in regulation. Until SS ramp reaches 1.0V (T1.0), the “Fault Latch” is inhibited. To avoid skipping the soft-start cycle, it is necessary to apply VIN before VCC reaches its UVLO threshold. Soft-start time is a function of oscillator frequency. SS 1.35V FB EN 0.8V T0.8 T1.0 3200 CLKs 4000 CLKs Fault Latch Enable 0.8V 1.0V 2400 CLKs Figure 21. Soft-Start Timing Diagram The regulator does not allow the low-side MOSFET to operate in full synchronous rectification mode until internal SS ramp reaches 95% of VREF (~0.76V). This helps the regulator start against pre-biased outputs (as shown in Figure 16) and ensures that inductor current does not "ratchet" up during the soft-start cycle. VCC UVLO or toggling the EN pin discharges the SS and resets the IC. Bias Supply The FAN2103 requires a 5V supply rail to bias the IC and provide gate-drive energy and controller power. Connect a ≥ 1.0µf X5R or X7R decoupling capacitor between VCC and PGND. Whenever the EN pin is pulled up to VCC, the 5V supply connected to VCC should be turned ON after VIN comes up. If the power supply is turned ON using EN pin with an external control after VCC and VIN come up, the VCC and VIN power sequencing is not relevant. Since VCC is used to drive the internal MOSFET gates, supply current is frequency and voltage dependent. Approximate VCC current (ICC) can be calculated using: )] 128 f ( ) 013 . 0 227 5 V [( 58 . 4 I CC ) mA ( CC − • + − + = (1) where frequency (f) is expressed in KHz. Setting the Output Voltage The output voltage of the regulator can be set from 0.8V to ~80% of VIN by an external resistor divider (R1 and RBIAS in Figure 1). The internal reference is 0.8V with 650nA, sourced from the FB pin to ensure that if the pin is open, the regulator does not start. The external resistor divider is calculated using: nA 650 1 R V 8 . 0 V R V 8 . 0 OUT BIAS + − = (2) Connect RBIAS between FB and AGND. Setting the Frequency Oscillator frequency is determined by an external resistor, RT, connected between the R(T) pin and AGND: 135 ) R 65 ( 10 f T 6 ) KHz ( + • = (3) where RT is expressed in KΩ. 65 135 ) f / 10 ( R 6 ) K ( T − = Ω (4) where frequency (f) is expressed in KHz. The regulator does not start if RT is left open. |
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