AT mA One Cell Synchronous Boost Regulator

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1 FEATURES DESCRIPTION Synchronous converter saves the use of Schottky diode Low start-up voltage, 0.85 at 1mA True load disconnect during shutdown Output voltage discharge during shutdown Anti-Ringing in Discontinuous Conduction Mode Small package for SOT-23-6 High efficiency up to 85% 1.5 to 5 output voltage range Logic control shutdown The AT720 is a one cell synchronous DC/DC boost converter suitable for single AA cell operations. Only three external components are needed for the fixed voltage version. The output voltage can be ranged from 1.5 to 5 with only 2% error. Once the operation is started, the special design on the input power can drain the battery down to very low voltage, which makes the best use of the battery and extends battery life. Unlike most DC/DC boost APPLICATION Pagers Mp3 player PDA Camcorders Handheld instruments Power source for battery-powered equipment Power source for applications, which require higher voltage than that of batteries used in the appliances converters, the AT720 realizes true load disconnection during shutdown, which completely separates battery from load. Moreover, the anti-ringing design during DCM can suppress the EMI caused by the inductor in many step-up applications. The AT720 is available in tiny SOT23-6 package. ORDER INFORMATION PIN CONFIGURATIONS (TOP IEW) AT 720- A KF R IAT Circuit Type Output oltage: A=ADJ Shipping: R: Tape & Reel KF: SOT-26 1

2 PIN DESCRIPTIONS Pin Name IN GND EN FB OUT SW Pin Description Input oltage. Reference ground. Enable Input. Pulling this pin below 0.4 turns the regulator off, reducing the quiescent current to below 0.1uA. Connecting this pin higher than 1 will start normal operation. This pin can not be left floated. Regulator feedback pin. This pin senses a positive output voltage and is referenced to 1.2. The pin is the output of the device. This pin connects internally to N and P channel switches. TYPICAL APPLICATION CIRCUITS 10µH SW OUT Output Single AA cell + IN 4.7µF FB 10µF EN GND R2 R1 Efficiency Inductor=10µH (DCR=0.2), Cout =10µF (ESR=0.1) 2

3 BLOCK DIAGRAM ABSOLUTE MAXIMUM RATINGS (Note 1) Parameter Symbol Max alue Unit Operating Temperature Range (Note 2) T OPR -20 ~ 85 C Storage Temperature Range T STORAGE -40 ~ 125 C Lead Temperature (Soldering, 10 secretary ) T LEAD 260 C Power Dissipation SOT 23-6 P D 300 mw ESD Rating HBM MM ABSOLUTE MAXIMUM RATINGS (Note 3) Pin Name Pin Symbol MAX MIN Unit Power Supply oltage IN Switch Input SW Enable oltage EN oltage Feedback Input FB Output oltage OUT

4 ELECTRICAL CHARACTERISTICS IN =1.5, OUT =3.3, T A =25 C, unless other wise specified. Function Parameter Test Conditions Min Typ Max Unit Start-up oltage Range R OUT =1k Minimum Operating oltage EN = IN 0.7 Switch current limit 280 ma Iq (non_switching) IN = 1.5, OUT =3.3, FB =1.5(on IN ) 1 IN = 1.5, OUT =3.3, FB =1.5(on OUT ) 85 µa Shutdown Current IN = 1.5, EN =0 1 µa Output voltage CO frequency IN = 1.5 when startup 800 khz Switch off time 1200 ns Efficiency IN = 2 85 % Shutdown Supply Current EN = µa EN Logic Low Level 0.4 EN Logic High Level 1 Feedback voltage Feedback Input Current 0.1 µa NMOS Rds (on) I=100mA 0.25 ohm PMOS Rds (on) I=100mA 0.65 ohm Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The AT720 is guaranteed to meet performance specifications from 0 C to70 C. Specifications over the - 30 C to 85 C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: The device is not guaranteed to function outside its operating rating. 4

5 TYPICAL CHARACTERISTICS _start vs I OUT L=10µH, C IN = 4.7µF, C OUT =10µF OUT vs I OUT L=10µH, C IN = 4.7µF, C OUT =10µF Figure1. Figure2. IN vs OUT L=10µH, C IN = 4.7µF, C OUT =10µF Rds_N vs OUT Figure 3. Figure 4. Enable, CH1= IN, CH2= EN, CH3= OUT = 3 Disable Figure 5. Figure 6. 5

6 DETAIL DESCRIPTION The AT720 is a DC-DC step up converter that is guaranteed to start up with input voltage as low as Once the chip is enabled, it can work until the input voltage down to very low voltage. This special design is best for one cell battery applications. The AT720 includes a built-in synchronous rectifier which can reduce cost and board area, an anti-ringing switch which can suppress EMI, and the true load disconnect function which can completely shutdown the device. Anti-Ringing in Discontinuous Conduction Mode once the chip is enabled. Then the N channel and P channel FET continue working like other synchronous boost converters. The output voltage is increased gradually, after the output voltage is high enough to operate the reference voltage circuit, the free running oscillator stops and the PFM control scheme follows. PFM Operation A forced discontinuous, current limited pulse-frequency-modulation is the key feature of AT720. The N channel FET is on first to allow the current to flow through the inductor up to about 280mA, following each on cycle, a 1200ns off time which enables the P channel FET to release the inductor energy to charge the output capacitor. When the error comparator senses the output voltage has reached the designated output value, this on-off cycle stops and the inductor current must ramp to close to zero current before the P channel FET is completely off. When the output Figure7. Anti-Ringing in Discontinuous Conduction Mode voltage has fallen below the designated output voltage value, another cycle may start. The AT720 include an internal Anti-Ringing switch (Figure 7) to minimize ringing at SW and reduce EMI. When the energy in the inductor is insufficient to supply current to the output, the capacitance and inductance at SW form a resonant circuit that causes ringing. The Anti-Ringing switch supplies a path to quickly dissipate this energy suppressing the ringing at SW. this does not reduce the output ripple, but does reduce EMI with minimal impact on efficiency. Start up Operation A low voltage free running oscillator is active first 6

7 DETAIL DESCRIPTION (CONTINUED) Synchronous Rectification There is a dead time between N and P channel switches on-off operation. The P channel switch must be off before N channel switch is on to store the inductor energy. The N channel switch must be off before P channel switch is on to release the inductor energy to output capacitor. The periods when both switches are off can cause voltage spikes on switch waveform. An external RC snubber circuit which connects a 2 ohm resistor and a 1nF capacitor in series from SW pin to GND can reduce the voltage spikes significantly. An optional Schottky diode can also be connected from SW pin to OUT pin to further increase efficiency, but this Schottky diode can cause the true load disconnect feature disabled. 7

8 APPLICATION INFORMATION The AT720 is a current limited constant off time PFM control scheme. This PFM control scheme is inherently stable. The selections of input capacitor, output capacitor and inductor do not cause the stability issues. OUT = FB R1 + R ( R 2 Where FB is the 1.2 feedback voltage. 2 ) Inductor selection A 10uH inductor value performs well in most applications. Typically, a small inductance offers smaller physical size. A larger inductance can provide higher efficiency, less ripple but smaller output current. Choosing inductors with low ESR decrease power losses and improve the efficiency. Capacitor selection Choose low ESR capacitors can reduce the output voltage ripple, higher output capacitance provides less output voltage ripple. Typically, a 4.7uF input capacitor and a 10uF output capacitor are sufficient for most applications. Maximum Output Current The maximum output current depends on the peak inductor current, input voltage, output voltage and efficiency. Using the following equation, I OUT(MAX) 1 ( I 2 PEAK ) ( IN OUT )η Where η is the overall efficiency and I PEAK is about 330mA. Setting the Output oltage The output voltage can be adjusted from 1.5 to 5 range. Setting he output voltage using the following equation, 8

9 PACKAGE OUTLINE DIMENSIONS Note : Information provided by IAT is believed to be accurate and reliable. However, we cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an IAT product; nor for any infringement of patents or other rights of third parties that may result from its use. We reserve the right to change the circuitry and specifications without notice. Life Support Policy: IAT does not authorize any IAT product for use in life support devices and/or systems. Life support devices or systems are devices or systems which, (I) are intended for surgical implant into the body or (II) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. Typical numbers are at 25 C and represent the most likely norm. 9