AN013 Optimized external driver for AIC1639 Introduction AIC1639 is a member of AIC1638 PFM （ Pulse Frequency Modulation ） controller IC family, for step-up DC/DC converter featuring high efficiency and low ripple voltage, which is exactly the same as AIC1638 except the external driver with higher output current comparing to the built-in edition ( AIC1638 ). The figures are shown below ： The key electrical characteristics of AIC1639 ü PFM based control ü 75% ( Typ. ) oscillator duty cycle , 100KHz ( Typ. ) switching frequency ü Low start up voltage Figure 1b Principle of step-up DC/DC converter operation A step-up DC/DC converter charges energy in the inductor when the switch is on and discharges the energy with additional energy from Input Power Source thereto. Therefore a higher voltage output, than input, is obtained. The operation will be explained with reference to the following diagrams ： Figure 1a July 2000 1 AN013 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 , figure 2a<Basic Circuits>Figure 2b<Current through either the transistor is off or the base current is not L> enough to turn it on . In the active region , the transistor acts as an amplifier, where the collector Refer to figure 2a and 2b shown above current is amplified by a gain and the collector-emitter voltage decreases with the base Step1: Switcher Q is turned on with the current current . In the saturation region , the base current is flow of IL( =i1 ) and the energy is charged in sufficiently high so that the collect-emitter voltage is L. At this moment (IL=i1) , IL is increased low. And the transistor is considered as a switch. from ILmin (=0) to ILmax in proportion to the on-time period (ton) of switcher Q which is given by ： The transfer characteristics, a plot of VCE against IB, are shown in figure3 below . VCE I= VIN × ton L VCC Cutoff Active Saturation (1) That is , as the turn-on time gets longer , the current flow through inductor might get higher . Step2: When Q is off, Schottky diode is turned on VCE(sat) in order that L maintains IL at ILmax. That IB 0 IBS causes the release of current IL ( =i2 ). 0 Step3: VBE 0.5 IL (=i2) is gradually decreased. And IL reaches to ILmin (=0) in a period (topen) so VBE(sat) Figure 3 that the diode is off. The model of an NPN-transistor is shown in Fig. 4 under large-signal dc operation, the relevant Switch transistor selection current is IE = IC + IB (2) NPN type bipolar transistor steady-state characteristics The base current is the input current and the Although there are three possible configurations, i.e. collector current the output current. The ratio of 2 AN013 collector current, IC, to base current, IB, is known as the current gain, hFE: and the corresponding value of the base current is I BM = IC hFE = IB I CM h FE . (7) If the base current exceeds IBM, VBE and the (3) collector current increase and the VCE falls below C VBE. This will continue until the CB junction is IC forward biased with VBC of about 0.4 to 0.5V. The transistor then goes into saturation . The transistor saturation may be defined as the point above. Any IB increases in the base current do not increase the B collector current significantly . IE In the saturation , the collector current remains E almost constant . If the collector-emitter saturation Figure 4 Model of NPN transistor voltage is VCE(sat), the collector current is ICS = VCC VCC − VCE( sat ) RC (8) Ic and the corresponding value of base current is RC RB VB Ib + VBE _ I BS = + VCE _ (9) The total power loss in the two junctions is PT = VBEIB + VCEIC (10) Ie Figure 5 Transistor switch A high value of overdrive factor will not reduce the Let us take a look at the circuit of Fig. 5, where the collector-emitter voltage significantly. However, VBE will increase due to increased base current , transistor is operated as a switch. resulting in increasing power loss in BE junction VB − VBE RB (4) VCB = VCE − VBE (5) IB = I CS h FE Therefore, it is necessary that the setting of the input/output conditions and the selections of Equation (5) indicates that as long as VCE ≧ VBE, the peripheral components transistor will be in the active region . The maximum consideration of ILmax. collector current (ICM) in the active region, which can Besides, there are also several key features to be be obtained by setting VCB = 0 and VBE = VCE, is ICM = VCC − VCE VCC − VBE = RC RC (6) are made with the considered ,i.e., Collector to Emitter Saturation voltage VCE(sat) Collector current: IC DC current gain hFE 3 AN013 N type MOSFET transistor ID = k( VGS − VT )2 steady-state (11) characteristics The VT here is usually between 2V~4V , and k is The figure1b. shows the circuit example which is usually 0.2 or 0.3 . using N-MOSFET. resistance , RDS , which is defined as : RDS = and ∆VDS ∆ID the output (12) ID is normally very high in the pitch-off region , typically on the order of megaohms (MΩ) and is very small in the linear region , typicallyon the 0 VGS VT n- Since the ON resistor of the MOS FET might affect the output ability as well as the efficiency, CH Figure 6 order of milliohms (mΩ) the threshold voltage should be low. When the Transfer Characteristic of N-Channel MOSFET output voltage is as low as 2.7V, which is the same as AIC1639-27, the circuit operates only VDD when the MOSFET has the threshold voltage ID lower than 2.7V RD D Fig 8. and Fig 9. show the difference between 4 transistors (3 NPN-type transistor and 1 N-type G +VGS _ S MOSFET) implemented for its load regulation and efficiency : (VIN=3.0V , VOUT=5.0V L=15µH) Figure 7 N-MOS Transistor Switch 5.4 NPN 2SD1803 Figure 6 shows the output characteristics of an 5.2 N-channel MOSFET. There are three regions of 5 operation: (1) cutoff region, where VGS ≦ VT; (2) 4.8 (V) MOS si2302ds NPN 2N2222 pinch-off or saturation region , whereVDS≧VGS-VT ; 4.6 (3) linear region , where VDS≦VGS-VT , due to high 4.4 drain current and low drain voltage , the power 4.2 MOSFETs are operated in the linear region for 4 NPN 2N3904 0 switching actions. The drain current ID is given by: 100 200 300 400 500 (mA) Fig 8.Efficiency vs Output Current 4 AN013 The figures tell us the general purpose of 90% MOS si2302ds transistors (e.g. 2N2222, 2N3904… etc) are not 80% 70% recommended due to its low current capacity, low NPN 2SD1803 60% DC gain and high VBE(SAT) when larger switching 50% current implemented. 40% NPN 2N2222 30% 20% NPN 2N3904 10% 0% 0 100 200 300 400 500 (mA) Fig 9.Output Voltage vs Output Current Characteristics list for transistors Transistor ICmax 2SD1803 5A VBE VCE hFE remark 0.75Vmax@IC=0.15A 2N2222A 600mA 2N3904 200mA High current 0.85Vmax@IC=0.5A 0.07Vmax switch 1.35Vmax@IC=3.0A 0.22Vmax 50min 1.30Vmax@IC=0.15A 0.30Vmax 50min 2.00Vmax@IC=0.5A 1.00Vmax 40min 0.95Vmax@IC=50mA 0.30Vmax 60min General purpose 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) 5.2 100% 5.1 90% 33uH/MOS 15uH/MOS 5 15uH/2SD1803 80% 70% (V) 15uH/MOS 33uH/MOS 4.9 15uH/2SD1803 4.8 60% 4.7 4.6 50% 0 100 200 300 400 500 0 100 200 300 400 500 (mA) (mA) Fig 11. Output Voltage vs Output Current 5 AN013 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 low . When the output voltage is as low as 2.7V, the To insure good noise performance, use the following circuit operates only when the MOSFET has the basic layout practices: threshold voltage lower than 2.7V. 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. 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 estimating the switch current from output Recommended selection current to choose a proper peripheral component As we can see, the MOSFET is a quite excellent is the best way to enhance the voltage converter choice to be the driver transistor of AIC1639 due to system and save more cost. its low RDSON and outstanding performance in It would be great for your application! Come on, efficiency. However the threshold voltage should be step it up! 6