AT MHz, 800mA Synchronous Step-Down Regulator

Transcription

1 FETURES Up to 96% Efficiency 800m Load Current 2.5 to 5.5 Operating nput oltage Range Fixed 2.25MHz Oscillator Frequency at PWM Operation Maximum Duty Cycle up to 00% 0.6 Reference oltage Shutdown Current u Current Mode Operation for Excellent Line and Load Transient Response Over Current Protecton / Thermal Shutdown Buildin Low R DS(ON) / No Schottky Diode Required Power MOSFET switch DESCRPTON The is a highefficiency, synchronous stepdown converter ideally suited for portable systems powered by cell Lion or 3cell NiMH/NiCd batteries. The devices are also suitable to operate from a standard 3.3 to 5 voltage rail. The is available in an adjustable version and fixed output voltages. The output current is up to 800m, the is ideal for powering the low voltage processors used in PDs, pocket PCs, and smart phones. Under nominal load current, the devices operate with a fixed switching frequency of typically 2.25 MHz. The Current Mode operation achieves fast line and load transient response. The full duty cycle allows the works at very low difference voltage. Low output voltages are easily supported with the 0.6 feedback reference voltage. PPLCTON Cell Phone, Smart Phone, PD. WLN Digital Cameras Portable Media Players Personal nformation ppliances The needs only three small external components except the resistive divider. The internal synchronous switch increases efficiency and eliminates the need for an external Schottky diode. TYPCL PPLCTON CRCUTS dj = 2.5 to µh 22 pf =.8/800m C 4.7µF CER FB 200k 00k C 0µF CER <Figure.> High Efficiency StepDown Converter

2 BSOLUTE MXMUM RTGS nput Supply oltage FB oltages oltage oltage Operating Temperature Range Junction Temperature (Note ) Storage Temperature Range Lead Temperature(Soldering, 5sec) PRMETER LUE UNT Stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. ELECTRCL CHRCTERSTCS 0.3 to to 0.3 to ( 0.3) 40 to C C 65 to 50 C 260 C Thermal Resistance Between Junction and mbience SOT TQFN 3x36L 68 C / W Specifications with standard type face are for T = 25 C, and those with boldface type apply over Full Operating Temperature Range. = 3.6, L = 2.2µ H, C = 0 µf unless otherwise specified. 0.3 to Parameter Symbol Conditions Min Typ Max Unit nput oltage Range Quiescent Current Regulated Feedback oltage Regulated Output oltage nput Bias Current FB 2.5 T = 25 T = 0 T 85 T = 40 T 85 = 0.65 FB , 40 T , 40 T 85.8, 40 T Reference oltage Line Regulation(dj) REG LE = 2.5 to 5.5, = FB % / Output oltage Line Regulation(Fixed) = 2.5 to 5.5, Output oltage Load Regulation(Fixed) LOD =0m n 0.5 % Peak nductor Current PK = 3, FB = 0.5 or = 90% Duty cycle 35%.3 Oscillator Frequency F OSC FB = 0.6 or = 00% MHz RDS(ON) of PCH MOSFET R PFET = 300m RDS(ON) of NCH MOSFET R NFET = 300m Leakage L = 0, = 0 or 5, = µ Threshold 40 T 85 Leakage Current Q SHDN Shutdown Current = FB FB REG LE REG LOD = 0m, FB= REF 5% Reference oltage Load Regulation(dj) REG LOD 0.5 % > µ µ % / µ Note : T is calculated from the ambient temperature T and power dissipation P : T =T (P )(220 J D J D 2 /W)

3 ORDERG FORMTON T T 77X.X KE R Shipping R: Tape & Reel FB P CONFGURTONS 5 4 SOT Circuit Type Output oltage dj, ,.8.8 Package KE: SOT25 QCS6: TQFN 3x36L 2 3 djustable ersion (TOP EW) 2 3 Fixed ersion (TOP EW) TQFN 3x36L (TOP EW) NC NC NC TOP EW T FB djustable ersion Fixed ersion (BOTTOM EW) NC NC NC TOP EW T P DESCRPTONS Pin Name Pin Description This pin is the enable control pin of the device. Forcing this pin High to enable the part and low to shutdown the part. Do not leave the Enable pin floating. Should pull up with 00 K Resistor. Ground Pin. Switch Pin, connects to the inductor. FB/ Supply oltage. Connects to the power, 2.5 to 5.5. FB (dj): Feedback nput Pin. Connect FB to the center point of the external resistive divider. (.2/.5/T 77.8): Output oltage Feedback Pin. n internal resistive divider divides the output voltage down for comparison to the internal reference voltage. 3

4 BLOCK DGRM Sawtooth Generater CLK Slope Compensation CurrentSense mplifier * FB/ 0.6 REF Error mplifier PWM Comparator Control Logic Driver ZeroCurrent Comparator CurrentLimit Comparator For DJ version, this pin = FB For Fixed voltage version, this pin = TYPCL OPERTG CHRCTERSTCS (From Figure, Except for the Resistive Divider Resistor alues) Start Up from Shutdown. Start Up LOD Load Current = 600m LOD Load Current = 600m 4

5 TYPCL OPERTG CHRCTERSTCS (continued) (From Figure, Except for the Resistive Divider Resistor alues) =00m =0m Output Error (%) =3.6 =2.7 Efficiency (%) =600m =m =.8 Oscillator Frequency Deviation (%) Switch On Resistance (m ) Sync. Switch Main Switch =2.7 =2.7 =3.3 =3.3 =4.2 Load Current (m) 5

6 TYPCL OPERTG CHRCTERSTCS (continued) (From Figure, Except for the Resistive Divider Resistor alues) =2.7 =2.7 =3.3 =4.2 =3.3 =4.2 Load Current (m) Load Current (m) =2.7 =2.5 =3.3 =4.2 =.2 =.8 Load Current (m) 6

7 PPLCTON FORMTON OPERTON The is a monolithic switching mode Step Down DCDC converter. t utilizes internal MOSFETs to achieve high efficiency and can generate very low output voltage by using internal reference at 0.6. t operates at a fixed switching frequency, and uses the slope compensated current mode architecture. Current Mode PWM Control Slope compensated current mode PWM control provides stable switching and cyclebycycle current limit for excellent load and line responses and protection of the internal main switch (PCh MOSFET) and synchronous rectifier (NCh MOSFET). During normal operation, the internal P Ch MOSFET is turned on for a certain time to ramp the inductor current at each rising edge of the internal oscillator, and switched off when the peak inductor current is above the error voltage. The current comparator, limits the peak inductor current. When the main switch is off, the synchronous rectifier will be turned on immediately and stay on until either the inductor current starts to reverse, as indicated by the current reversal comparator, ZERO, or the beginning of the next clock cycle. Dropout Operation When the input voltage decreases toward the value of the output voltage, the allows the main switch to remain on for more than one switching cycle and increases the duty cycle until it reaches 00%. The output voltage then is the input voltage minus the voltage drop across the main switch and the inductor. t low input supply voltage, the RDS(ON) of the PCh MOSFET increases, and the efficiency of the converter decreases. Caution must be exercised to ensure the heat dissipated not to exceed the maximum junction temperature of the C. ShortCircuit Protection current limit (.3) circuit is equipped in the, to protect the when the output pin is shorted to pin. This current limit will suppress the output current so that the inductor current has enough time to decay. pplication Circuit Figure 2 below shows the Basic pplication Circuit with adjustable versions. = 2.5 to 5.5 Figure 2. Basic pplication Circuit with adjustable output versions Setting the Output oltage Figure 2 above shows the basic application circuit with adjustable output version. R should not be greater than 00 k for reasons of stability. The external resistor sets the output voltage according to the following equation: C 4.7µF FB L 2.2 µh C2 2.2pF R2 200k R 00k R2 = 0.6( ) R R = 00k for all outputs R2 = 00k for =.2 R2 = 50k for =.5 R2 = 200k for =.8 R2 = 36k for = 2.5 R2 = 450k for = 3.3 =.8 C3 0µF Note 4.The duty cycle D of a stepdown converter is defined as: D = T f 00% 00% ON OSC where T ON is the main switch on time, and f OSC is the oscillator frequency (2.25Mhz). Maximum Load Current The will operate with input supply voltage as low as 2.5, however, the maximum load current decreases at lower input due to large R drop on the main switch and synchronous rectifier. The slope compensation signal reduces the peak inductor current as a function of the duty cycle to prevent subharmonic oscillations at duty cycles greater than 50%. Conversely the current limit increases as the duty cycle decreases. nput Capacitor Selection The input capacitor reduces the surge current drawn from the input and switching noise from the device. The input capacitor impedance at the switching frequency shall be less than input source impedance to prevent high frequency switching current passing to the input. low ESR input capacitor sized for maximum RMS current must be used. Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR and small temperature coefficients. 4.7 µf ceramic capacitor for most applications is sufficient. 7

8 PPLCTON FORMTON (continued) Output Capacitor Selection The output capacitor is required to keep the output voltage ripple small and to ensure regulation loop stability. The output capacitor must have low impedance at the switching frequency. Ceramic capacitors with X5R or X7R dielectrics are recommended due to their low ESR and high ripple current. The output ripple is determined by ( ) (ESR ) f L 8 f C3 OSC OSC nductor Selection must work with inductor of 2.2 µh. For optimum voltagepositioning load transients, choose an inductor with DC series resistance in the 50m to 50m range. For higher efficiency at heavy loads (above 200m), or minimal load regulation (but some transient overshoot), the resistance should be kept below 00m. The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. 8

9 SOT25 PCKGE LE DMSONS b C L2 E E L (REF.) L (REF.) (MX.) 2 e D e D e TQFN 3x36L PCKGE LE DMSONS (TOP EW) Symbol Dimensions n Millimeters Min Max.45 MX C D 2.90 BSC. E 2.80 BSC..60 BSC. L L BSC. L2 0.25BSC. 0 0 b e e 0.95BSC..90BSC. (BOTTOM EW) D E 6 Pin Dot By Marking 5 5 C 6 S E2 4 L 4 P b Pin ndentification 2 Unit :mm Symbol Dimensions in Millimeters M. MON. MX b C 0.25REF D D2.7 REF. E E2 L P S REF REF