PET innovations SAM DAVIS, Editor in Chief Adaptive On-Time DC-DC Converters Combine Speed and Efficiency 3.3 to 5V BST VIN VOUT= 0.75 V TO 95% LX SC173 VDD FB PGOOD Power Good EMTECH CORP.’S newest EcoSpeed™ dc-dc conEnable/ TON EN/PSV verters are the SC173 and SC174 synchronous Power Save PGND AGND buck regulators. Their dc-dc converter platform incorporates a patented adaptive on-time (AOT) topology intended primarily for point-of-load (POL) applications. The SC173 and SC174 POL regulators satisfy 1. The SC173 is an integrated, synchronous 3A step-down regulator with an input today’s “green” power requirements by efficiently managing voltage range of 3V to 5.5V and a programmable output voltage from 0.75V up both standby and full current demands. Typical applications to 95% x VIN. include office automation, networking and communications equipment; set-top boxes; portable products; power supply on the high-side MOSFET via the flip-flop and gate drive. modules; and other embedded products. The resulting output ripple triggers the one-shot multivibraFigure 1 is a typical configuration of the SC173 regulator, which sends a single pulse to the high-side MOSFET tors. The SC174 configuration is similar, the only difference by way of the flip-flop (F/F) and gate drive. One FB comis their output, the SC173 can supply 3A and the SC174 parator input accepts a percentage of VOUT determined by can supply 4A. Their programmable switching frequency the voltage divider consisting of R1 and R2, its other input range of 200KHz to 1MHz enables circuit optimization for accepts the reference voltage (VREF). minimum board space and optimum efficiency. You can configure the SC174 with various capacitor types, such as electrolytic, tantalum, POS and ceramic. For The SC173 and SC174 converters exhibit advantages over some applications that use low-ESR ceramic capacitors, a conventional constant on-time control topologies, as shown in signal injection circuit may be implemented to provide 10 Table 1 that compares their characteristics. The adaptive onmVp-p, as shown in Fig. 3. The on-time control regulates time topology in common with hysteretic topologies, enables fast transient response while eliminating TABLE 1. CHARACTERISTICS COMPARISON OF CONSTANT the need for external compensation comON-TIME AND ADAPTIVE ON-TIME TOPOLOGIES ponents. This results in simple, spaceand cost-efficient power management Function Constant On-Time Adaptive On-Time solutions. However, compared with conLine Regulation Excellent Excellent stant on-time and hysteretic topologies, Load Regulation Excellent Excellent the adaptive on-time technique provides External Compensation None None for more predictable switching frequenStable Operation with Wide No Yes cies. Range of Load Capacitance Figure 2 shows the simplified circuit Switching Frequency Variable Pseudo-Fixed for pseudo-fixed frequency, adaptive Transient Response Ultra-Fast Ultra-Fast on-time control. When VFB is less than Limited Flexible COUT the internal 750mV reference, the FB Power Save Inherent Inherent Comparator output goes high, turning S 3697E_POW_SEM.indd Sec1:41 10/5/10 2:13 PM PET innovations 2. Simplified circuit of the SC173 and SC174 employs a comparator, one-shot multivibrator, flip-flop, gate drivers and on-chip power MOSFETs. Use of the one-shot provides adaptive on-time control. VIN CIN QHI VFB – TON TON Hi-Side FB Comparator S Q F/F + R VREF Gate Drivers VLX QLO VOUT L VLX R1 COUT Lo-Side ESR FB COUT R2 VFB FB Threshold One-Shot VIN VOUT RTON the valley of the output ripple voltage. This ripple voltage is the sum of the two voltages: one produced by the ESR of the output capacitor, and the other due to capacitive charging and discharging of the output capacitor during the switching cycle. For most applications, the output capacitor dominates total output ripple voltage. In the one-shot multivibrator used in the SC173 and SC174, a current source determined by VIN and RTON charges an internal 2.5pF capacitor. The highside MOSFET turns off when the capacitor voltage hits VOUT, which is derived internally by the heavily filtered LX voltage. Therefore, at the selected frequency, the IC automatically anticipates the on-time needed to regulate VOUT based on the existing VIN. This method automatically produces an on-time proportional to VOUT and inversely proportional to VIN. Under steady-state operation, you can program the switching frequency from 200kHz to 1MHz, according to: f sw = VOUT TON × VIN able frequency methods. • Reduced component count by eliminating the error amplifier and external compensation components. • Reduced component count by removing the need to sense and control inductor current. • Fast transient response, controlled by a fast comparator rather than a typically slow error amplifier used in traditional regulator circuits. Figures 4a and 4b illustrate the transient response of an SC174 during a rising and falling load current. • Operates with ceramic input and output capacitors. • Reduced output capacitance due to fast transient response LOW STANDBY POWER Very low power during standby mode is increasingly important to comply with emerging “green” initiatives. VIN (1) Q1 The SC173 and SC174 use external resistor RTON to set the on-time, which indirectly sets the frequency. You can determine RTON from the desired switching frequency: R TON = 1 25pF × fsw CFF RL (2) The switcher output voltage (VOUT) is regulated by comparing the internal 750mV reference voltage with the percentage of VOUT determined by the R1 and R2 voltage divider at FB. Adjusting the values of the voltage divider controls VOUT. Advantages of adaptive on-time control are: • Predictable operating frequency compared to other vari- 3697E_POW_SEM.indd Sec1:42 L VLX R1 COUT Q2 CC CL R2 3. Signal injection circuit provides 10 mVp-p at FB. 10/5/10 2:13 PM PET innovations Load Transient Response – Load Rising (SC 174) VIN = 5V, VOUT = 1.2 V Load Transient Response – Load Falling (SC 174) VIN = 5V, VOUT = 1.2 V Output Voltage Output Voltage 50 mV 50 mV 5V 5V Switching Node Waveform Switching Node Waveform 2A 2A Output Load Current Output Load Current 20 μs 20μs (a) Output voltage, output current, and switching node waveforms for rising load with VIN = 5V and VOUT = 1.2V. (b) Output voltage, output current, and switching node waveforms for falling load with VIN = 5V and VOUT = 1.2V. 4. Transient response for the SC174 However, most electronic products also require fast “wakeup” capability, which demands fast recovery from low-power standby to full-power steady state mode. The SC173 and SC174 converters solve this with an ultrasonic power-save (UPSAVE) mode. UPSAVE reduces the switching frequency to meet low-power standby requirements, which cuts power MOSFET switching losses and results in up to 95% peak standby efficiency. Then, when the load changes from standby to steady state, the EcoSpeed technology cycle-bycycle response quickly exits UPSAVE mode. To keep the switching frequency from exceeding 25 kHz (high end of the audible range) on-times are prevented from occurring at intervals greater than 40μs. You can see the effect of power-save in Figures 5 and 6 that show the SC173 and SC174 efficiency vs. output current, respectively. The red curve is with power-save and the blue curve is without power-save. 6. Efficiency vs. output current for the SC174. Blue line is forced conduction mode efficiency. Red line is efficiency with Power-Save. Efficiencyy vs. Output p Current 100.0 0 95.0 0 90.0 0 90.0 0 85.0 0 VIN = 5 V VO = 3.3 V 80.0 0 LOUT: DS86C-B992AS-2RON COUT = 22μFx2 75.0 0 RTON = 80.6 kOhm Red: PSAVE Mode 70.0 0 Blue: Forced Continuous PWM Mode 0.75 1.50 2.25 Output Current (A) 3.0 Efficiency (%) 95.0 0 65.0 0 0.0 3697E_POW_SEM.indd Sec1:43 Active loads may leak current from a higher voltage into the switcher output. Under light load conditions with power-save enabled, this can force VOUT to slowly rise and reach the over-voltage threshold, resulting in a hard shutdown. Smart Power Save prevents this condition. When the FB voltage exceeds 10% above nominal (825mV), the ICs immediately disable power-save, and turn on the low-side MOSFET. This draws current from VOUT through the inductor and causes VOUT to fall. When VFB drops back to the 750mV trip point, a normal TON switching cycle begins. This method prevents a hard over-voltage protection (OVP) shutdown and also cycles energy from VOUT back to VIN. These ICs feature fixed current limiting that employs the RDS(ON) of the lower MOSFET for current sensing. While the low-side MOSFET is on, the inductor current flows through it and creates a voltage across its RDS(ON). During Efficiencyy vs. Output p Current 100.0 0 Efficiency (%) 5. Efficiency vs. output current for the SC173. Blue line is forced conduction mode efficiency. Red line is efficiency with Power-Save. CIRCUIT PROTECTION 85.0 0 VIN = 5 V VO = 3.3 V 80.0 0 LOUT: DS86C-B992AS-2RON COUT = 22μFx2 75.0 0 RTON = 80.6 kOhm Red: PSAVE Mode 70.0 0 Blue: Forced Continuous PWM Mode 0 65.0 0 1 3 2 Output Current (A) 4 10/5/10 2:13 PM PET innovations this time, the voltage across the MOSFET is negative with respect to ground. If this MOSFET voltage drop exceeds the internal reference voltage, it activates the current limit. The current limit then keeps the low-side MOSFET on and will not allow another high side on-time, until the current in the low-side MOSFET reduces enough to drop below the internal reference voltage once more. ADDITIONAL PROTECTION FEATURES The SC173 and SC174 synchronous buck regulators incorporate a full range of protection features, including: • Over-Voltage Protection (OVP) • Over-Temperature Protection (OTP) • Output short circuit protection • Under-voltage lock-out (UVLO) • Soft-Start To indicate that the SC173 and SC174 are operating properly, the regulators provide a power good (PGOOD) output, with open-drain that requires a pull-up resistor. When the output voltage is 10% below the nominal voltage, PGOOD goes low and stays low until the output voltage returns to the nominal voltage. PGOOD is held low during soft start and activated approximately 1ms after VOUT reaches regulation. The total PGOOD delay is typically 2ms. PGOOD will transition low if VFB exceeds +20% of nominal, which is also the over-voltage shutdown threshold (900mV). PGOOD also pulls low if the EN/PSV pin is low when VDD is present. SUMMARY Designers wishing to implement point-of-load power circuits now have an energy-saving alternative in the SC173 and SC174 synchronous buck regulators. The parts efficiently manage current in standby and full current modes, using an advanced adaptive on-time topology to provide fast transient response, reduced output capacitance, and predictable switching frequencies. ■ ECOSPEED FAMILY PORTRAIT THE SC173 AND SC174 are the newest introductions of Semtech’s EcoSpeed family. lists the other members of the family that will follow. Some of these ICs have internal Power MOSFETs, whereas others will require external MOSFETs. Those intended for external MOSFETs have an asterisk in the IOUT (max) column of Table 2. Similar to the other AOT converters, the SC493 is also a synchronous buck power supply controller, except that it has an I2C interface. This can be used to program output voltage offset, power-on delay time, soft-start time, power save operating mode, and can enable/disable the controller. Also, a status register provides information on device state and faults. TABLE 2. SEMTECH’S ECOSPEED FAMILY OF DC-DC CONVERTERS Part No. Type On-Chip LDO VIN Range (V) VDD Range (V) VOUT Range (V) IOUT (max.) (A) PGOOD Enable Selectable PSAVE Package (mm) SC173 Regulator No 3.0-5.5 3.0-5.5 0.75 to 95% VIN 3 Yes Yes Ultrasonic MLPD-10 (3 x 3 x1) SC174 Regulator No 3.0-5.5 3.0-5.5 0.75 to 95% VIN 4 Yes Yes Ultrasonic MLPD-10 (3 x 3 x1) SC414 Regulator Yes 3.0-5.5 3.0-5.5 0.75 to 5.25 6 Yes Yes Ultrasonic MLPQ-28 (4 x 4 x1) MLPQ-32 (5 x 5 x1) SC417 Regulator Yes 3.0-28 4.5-5.5 0.5 to 5.25 10 Yes Yes Ultrasonic MLPQ-32 (5 x 5 x1) SC418 PWM Controller Yes 3.0-28 3.0-5.5 0.5 to 5.25 30* Yes Yes Programmable Ultrasonic or Regular MLPQ-UT-20 (3 x 3 x0.6) SC419 PWM Controller No 3.0-28 4.5-5.5 0.5 to 5.25 30* Yes Yes Programmable Ultrasonic or Regular MLPQ-UT-20 (3 x 3 x1) SC424 Regulator Yes 3.0-28 3.0-5.5 0.75 to 5.25 6 Yes Yes Regular MLPQ-28 (4 x 4 x1) SC427 Regulator Yes 3.0-28 4.5-5.5 0.5 to 5.25 10 Yes Yes Regular MLPQ-32 (5 x 5 x1) SC493 I2C Controller No 3.0-28 3.0-5.5 0.5 to 3.3 30* Yes Yes Programmable Ultrasonic or Regular MLPQ-UT-20 (3x 3 x0.6) Copyright © 2010 by Penton Media, Inc. 3697E_POW_SEM.indd Sec1:44 10/5/10 2:13 PM