RT8015AGQW

RT8015A 3A, 2MHz, Synchronous Step-Down Converter General Description Features The RT8015A is a high efficiency synchronous, step-down DC/DC converter. Its input voltage range is from 2.6V to 5.5V and provides an adjustable regulated output voltage from 0.8V to 5V while delivering up to 3A of output current. z The internal synchronous low on-resistance power switches increase efficiency and eliminate the need for an external Schottky diode. The switching frequency is set by an external resistor or can be synchronized to an external clock. The 100% duty cycle provides low dropout operation extending battery life in portable systems. Current mode operation with external compensation allows the transient response to be optimized over a wide range of loads and output capacitors. The RT8015A is operated in forced continuous PWM Mode which minimizes ripple voltage and reduces the noise and RF interference. The 100% duty cycle in Low Dropout Operation further maximize battery life. z z z z z z z High Efficiency : Up to 95% Low RDS(ON) Internal Switches : 110mΩ Ω Programmable Frequency : 300kHz to 2MHz No Schottky Diode Required 0.8V Reference Allows for Low Output Voltage Forced Continuous Mode Operation Low Dropout Operation : 100% Duty Cycle RoHS Compliant and 100% Lead (Pb)-Free Applications z z z z z z Portable Instruments Battery-Powered Equipment Notebook Computers Distributed Power Systems IP Phones Digital Cameras Pin Configurations (TOP VIEW) The RT8015A is available in the WDFN-10L 3x3 package. 1 2 3 4 5 10 9 8 7 11 9 Ordering Information SHDN/RT GND LX LX PGND COMP FB VDD PVDD PVDD RT8015A Package Type QW : WDFN-10L 3x3 Lead Plating System P : Pb Free G : Green (Halogen Free and Pb Free) Note : Richtek products are : ` RoHS compliant and compatible with the current require- ` Suitable for use in SnPb or Pb-free soldering processes. WDFN-10L 3x3 Marking Information For marking information, contact our sales representative directly or through a Richtek distributor located in your area. ments of IPC/JEDEC J-STD-020. DS8015A-04 March 2011 www.richtek.com 1 RT8015A Typical Application Circuit VIN 5V ROSC 332k RCOMP 30k RT8015A 1 SHDN/RT 2 3, 4 5 COMP GND FB LX PGND VDD PVDD 10 CCOMP 1000pF R2 240k 9 8 6, 7 C1 CIN 22pF 22µF R1 510k L1 2µH COUT 22µFx2 VOUT 2.5V/3A Note : Using all Ceramic Capacitors Functional Pin Description Pin No. Pin Name 1 SHDN/RT 2 GND Signal Ground. All small-signal components and compensation components should connect to this ground, which in turn connects to PGND at one point. LX Internal Power MOSFET Switches Output. Connect this pin to the inductor. 5 PGND Power Ground. Connect this pin close to the negative terminal of CIN and COUT. 6, 7 PVDD Power Input Supply. Decouple this pin to PGND with a capacitor. 8 VDD Signal Input Supply. Decouple this pin to GND with a capacitor. Normally V DD is equal to PVDD. 9 FB 3, 4 Pin Function Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the switching frequency. Forcing this pin to V DD causes the device to be shut down. Feedback Pin. This pin Receives the feedback voltage from a resistive divider connected across the output. Error Amplifier Compensation Point. The current comparator threshold increases 10 COMP 11 (Exposed Pad) NC www.richtek.com 2 with this control voltage. Connect external compensation elements to this pin to stabilize the control loop. No Internal Connection. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. DS8015A-04 March 2011 RT8015A Function Block Diagram SHDN/RT SD PVDD ISEN Slope Com OSC COMP 0.8V Output Clamp EA FB OC Limit Driver Int-SS LX 0.9V Control Logic 0.7V NISEN POR PGND NMOS I Limit 0.4V VREF OTP GND VDD Layout Guide CIN must be placed between VDD and GND as closer as possible VIN CIN Output capacitor must be near RT8015A GND VOUT COUT LX should be connected to Inductor by wide and short trace, keep sensitive L1 components away from this trace RT8015A PVDD PVDD VDD FB COMP CF R1 R2 6 5 7 4 8 3 9 2 10 1 PGND LX LX GND SHDN/RT ROSC RCOMP CCOMP VOUT GND Connect the FB pin directly to feedback resistors. The resistor divider must be connected between VOUT and GND. DS8015A-04 March 2011 www.richtek.com 3 RT8015A Operation Main Control Loop The RT8015A is a monolithic, constant-frequency, current mode step-down DC/DC converter. During normal operation, the internal top power switch (P-Channel MOSFET) is turned on at the beginning of each clock cycle. Current in the inductor increases until the peak inductor current reach the value defined by the voltage on the COMP pin. The error amplifier adjusts the voltage on the COMP pin by comparing the feedback signal from a resistor divider on the FB pin with an internal 0.8V reference. When the load current increases, it causes a reduction in the feedback voltage relative to the reference. The error amplifier raises the COMP voltage until the average inductor current matches the new load current. When the top power MOSFET shuts off, the synchronous power switch (N-MOSFET) turns on until either the bottom current limit is reached or the beginning of the next clock cycle. The operating frequency is set by an external resistor connected between the RT pin and ground. The practical switching frequency can range from 300kHz to 2MHz. In an over-voltage condition, the top power MOSFET is turned off and the bottom power MOSFET is switched on until either the over voltage condition clears or the bottom MOSFET's current limit is reached. RT8015A is used at 100% duty cycle with low input voltages to ensure that thermal limits are not exceeded. Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant frequency architectures by preventing sub-harmonic oscillations at duty cycles greater than 50%. It is accomplished internally by adding a compensating ramp to the inductor current signal. Normally, the maximum inductor peak current is reduced when slope compensation is added. In the RT8015A, however, separated inductor current signals are used to monitor over current condition. This keeps the maximum output current relatively constant regardless of duty cycle. Short Circuit Protection When the output is shorted to ground, the inductor current decays very slowly during a single switching cycle. A current runaway detector is used to monitor inductor current. As current increasing beyond the control of current loop, switching cycles will be skipped to prevent current runaway from occurring. Dropout Operation When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle eventually reaching 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the internal P-Channel MOSFET and the inductor. Low Supply Operation The RT8015A is designed to operate down to an input supply voltage of 2.6V. One important consideration at low input supply voltages is that the R DS(ON) of the P-Channel and N-Channel power switches increases. The user should calculate the power dissipation when the www.richtek.com 4 DS8015A-04 March 2011 RT8015A Absolute Maximum Ratings (Note 1) Supply Input Voltage, VDD, PVDD ---------------------------------------------------------------------------- −0.3V to 6V LX Pin Switch Voltage -------------------------------------------------------------------------------------------- −0.3V to (PVDD + 0.3V)