AN013 AIC | Alldatasheet
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figure 2a<Basic Circuits>Figure 2b<Current through Refer to figure 2a and 2b shown above Step1: Switcher Q is turned on with the current flow of IL( =i1 ) and the energy is charged in L. At this moment ( IL=i1) , IL is increased from ILmin (=0) to ILmax in proportion to the on-time period (ton) of switcher Q which is given by ： L tonVI IN ×= (1) That is , as the turn-on time gets longer , the current flow through inductor might get higher . Step2: When Q is off, Scho ttky diode is turned on in order that L maintains IL at I Lmax. That causes the release of current IL ( =i2 ). Step3: IL (=i2) is gradually decreased. And IL reaches to ILmin (=0) in a period (topen) so that the diode is off. Switch transistor selection NPN type bipolar transistor steady-state characteristics Although there are three possible configurations, i.e. CC 、CE and CB. The CE ( Common Emitter) configuration , which is shown in figure 3 for an NPN-transistor , is generally used in switching applications. There are three operating regions of a transistor : cutoff , active , and saturation . In the cutoff region , either the transistor is off or the base current is not enough to turn it on . In the active region , the transistor acts as an amplifier, where the collector current is amplified by a gain and the collector-emitter voltage decreases with the base current . In the saturation region , the base current is sufficiently high so that the collect -emitter voltage is low. And the transistor is considered as a switch. The transfer characteristics, a plot of VCE against IB, are shown in figure3 below . Cutoff Active Saturation IB0 VCE(sat) IBS VCE VCC
0.5 VBE(sat)
The model of an NPN-transistor is shown in Fig. 4 under large -signal dc operation , the relevant current is IE = IC + IB (2) The base current is the input current and the collector current the output current. The ratio of
10% 20% 30% 40% 50% 60% 70% 80% 90% 0 100 200 300 400 500 (mA)MOS si2302ds NPN 2SD1803 NPN 2N2222 NPN 2N3904 Fig 9.Output Voltage vs Output Current The figures t ell us the general purpose of transistors ( e.g. 2N2222, 2N3904 … etc) are not recommended due to its low current capacity , low DC gain and high VBE(SAT) when larger switching current implemented. Characteristics list for transistors Transistor ICmax VBE VCE hFE remark 2SD1803 0.75Vmax@IC=0.15A 0.85Vmax@IC=0.5A 1.35Vmax@IC=3.0A 0.07Vmax 0.22Vmax 50min High current switch 2N2222A 600mA 1.30Vmax@IC=0.15A 2.00Vmax@IC=0.5A 0.30Vmax 1.00Vmax 50min 40min General purpose 2N3904 200mA 0.95Vmax@IC=50mA 0.30Vmax 60min General purpose Fig 10. and Fig 11. also show the load regulation and efficiency for different type of transistor and inductance: (VIN=3.0V , VOUT=5.0V) 50% 60% 70% 80% 90% 100% 0 100 200 300 400 500 (mA) 15uH/MOS 33uH/MOS 15uH/2SD1803 4.6 4.7 4.8 4.9 5.1 5.2 0 100 200 300 400 500 (mA) (V) 33uH/MOS 15uH/MOS 15uH/2SD1803 Fig 11. Output Voltage vs Output Current
Characteristics of N-type MOSFET Si2302DS Transistor IDmax VGS(TH) RDS Remark Si2302DS 10A 0.65Vmin 0.085 @ VGS=4.5V 0.115 @ VGS=2.5V N-Channel 1.25-W 2.5-V MOSFET Layout To insure good noise performance, use the following basic layout practices: 1. Minimize stray inductance by keeping board trace lengths to a minimum 2. Mount the controller IC as close to the load as possible to minimize output impedance. 3. Mount the supply decoupling capacitors as close as possible to the controller IC. Recommended selection As we can s ee, the MOSFET is a quite excellent choice to be the driver transistor of AIC1639 due to its low R DSON and outstanding performance in efficiency. However the threshold voltage should be low . When the output voltage is as low as 2.7V, the circuit operate s only when the MOSFET has the threshold voltage lower than 2.7V. Besides, the transistor that is intended to used as high current switch featuring Ø High collector current ( IC ) Ø Low saturation voltage for VBE and VCE Ø High DC gain hFE Is recommended. By esti mating the switch current from output current to choose a proper peripheral component is the best way to enhance the voltage converter system and save more cost. It would be great for your application! Come on, step it up!