www.ti.com SLVS485 − AUGUST 2003 FEATURES D Step-Down Controller for Applications With up to 95% Efficiency D 1.8-V to 6.5-V Operating Input Voltage Range D Adjustable Output Voltage Range From 1.2 V D to VI High Efficiency Over a Wide Load Current Range Low Power DSP Supply Digital Cameras Hard Disk Drives Portable Audio Players DESCRIPTION The TPS6420x are nonsynchronous step-down controllers that are ideally suited for systems powered from a 5-V or 3.3-V bus or for applications powered from a 1-cell Li-Ion battery or from a 2- to 4-cell NiCd, NiMH, or alkaline battery. These step-down controllers drive an external P-channel MOSFET allowing design flexibility. To achieve highest efficiency over a wide load current range, this controller uses a minimum on time, minimum off time control scheme and consumes only 20-µA quiescent current. The minimum on time of typically 600 ns (TPS64203) allows the use of small inductors and capacitors. When disabled, the current consumption is reduced to less than 1 µA. The TPS6420x is available in the 6-pin SOT23 (DBV) package and operates over a free air temperature range of −40°C to 85°C. D 100% Maximum Duty Cycle for Lowest Dropout D D D D D D D D Internal Softstart 20-µA Quiescent Current (Typical) Overcurrent Protected Available in a SOT23 Package APPLICATIONS D USB Powered Peripherals D Organizers, PDAs, and Handheld PCs TYPICAL APPLICATION CIRCUIT TPS64200 EFFICIENCY vs LOAD CURRENT 5V 100 Rs = 33 mΩ 10 µF 90 TPS64200 SW 2 EN GND 6 VIN 5 3 FB ISENSE 4 ZHCS2000 Si5447DC 10 µH 80 R1 620 kΩ R2 360 kΩ 47 µF PosCap 6TPA47M VI = 4.2 V 70 3.3 V / 2 A Efficiency − % 1 60 50 40 30 20 10 TA = 25°C, VO = 3.3 V 0 0.0001 0.001 0.01 0.1 IO − Load Current − A 1 10 Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. !"# $"%&! '#( '"! ! $#!! $# )# # #* "# '' +,( '"! $!#- '# #!#&, !&"'# #- && $##( Copyright 2003, Texas Instruments Incorporated www.ti.com SLVS485 − AUGUST 2003 These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam during storage or handling to prevent electrostatic damage to the MOS gates. ORDERING INFORMATION PLASTIC SOT23-6(1) (2) (DBV) OUTPUT VOLTAGE MINIMUM ON-TIME MINIMUM OFF-TIME MARKING Adjustable 1.2 V to VI Adjustable 1.2 V to VI ON time = 1.6 µs OFF time = 600 ns PJAI Variable minimum on time OFF time = 600 ns PJBI OFF time = 300 ns PJCI OFF time = 600 ns PJDI TPS64200DBVR TPS64201DBVR Adjustable Variable minimum on time 1.2 V to VI Adjustable TPS64203DBVR ON time = 600 ns 1.2 V to VI (1) The R suffix indicates shipment in tape and reel with 3000 units per reel. (2) The T suffix indicates a mini reel with 250 units per reel. TPS64202DBVR ABSOLUTE MAXIMUM RATINGS over operating free-air temperature range unless otherwise noted(1) Supply voltage, VIN −0.3 V to 7 V Voltage at EN, SW, ISENSE −0.3 V to VIN Voltage at FB −0.3 V to 3.3 V Maximum junction temperature, TJ 150°C Operating free−air temperature, TA −40°C to 85°C Storage temperature, Tsgt −65°C to 150°C Lead temperature 1,6 mm (1/16 inch) from case for 10 seconds 300°C (1) Stresses beyond those listed under “absolute maximum ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under “recommended operating conditions” is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability. PACKAGE DISSIPATION RATINGS PACKAGE TA ≤255C POWER RATING DERATING FACTOR ABOVE TA = 25°C 4 mW/°C TA = 705C POWER RATING SOT23−6 400 mW : NOTE The thermal resistance junction to ambient of the 6−pin SOT23 package is 250°C/W. TA = 855C POWER RATING 220 mW 180 mW RECOMMENDED OPERATING CONDITIONS MIN NOM MAX UNIT Supply voltage at VIN 1.8 6.5 V Operating junction temperature −40 125 °C 2 www.ti.com SLVS485 − AUGUST 2003 ELECTRICAL CHARACTERISTICS VIN = 3.8 V, VO = 3.3 V, EN = VIN, TA = −40°C to 85°C (unless otherwise noted) SUPPLY CURRENT PARAMETER VI I(Q) Operating quiescent current I(SD) Shutdown current OUTPUT/CURRENT LIMIT VO VFB TEST CONDITIONS Input voltage range MIN IO = 0 mA EN = VI Adjustable output voltage range MAX V 20 35 µA 0.1 1 µA VI V 0.01 0.2 µA +2 % 105 120 mV 0.01 0.2 µA 1.213 Feedback voltage tolerance −2 V(ISENSE) Reference voltage for current limit ISENSE leakage current 90 UNIT 6.5 VFB Feedback voltage Feedback leakage current η TYP 1.8 V Line regulation Measured with circuit according to Figure 1 0.6 %/V Load regulation Measured with circuit according to Figure 1 VI = 3.8 V 0.6 %/A Measured with circuit according to Figure 1 VI = 3.8 V, VO = 3.3 V, IO = 1000 mA 94% Measured with circuit according to Figure 1 VI = 3.8 V, VO = 1.2 V, IO = 800 mA 80% IO = 0 mA, Time from active EN to VO, CO = 47 µF 0.25 Efficiency Start-up time ms GATE DRIVER (SW-PIN) P-channel MOSFET on-resistance VI ≥ 2.5 V VI = 1.8 V 4 rDS(ON) N-channel MOSFET on-resistance VI ≥ 2.5 V VI = 1.8 V 4 rDS(ON) IO ENABLE Maximum gate drive output current, SW VIH VIL EN high level input voltage Device is off EN low level input voltage Device is operating EN input leakage current Ω 6 150 1.3 115 EN = GND or VIN 0.01 Undervoltage lockout threshold mA V 0.3 EN trip point hysteresis Ilkg V(UVLO) Ω 6 V mV 0.2 1.7 µA V ON TIME and OFF TIME ton toff Minimum on time TPS64200, TPS64201, TPS64202 1.36 1.6 1.84 TPS64203 only 0.56 0.65 0.74 µss Reduced on time 1 TPS64201,TPS64202 0.80 µs Reduced on time 2 TPS64201,TPS64202 0.40 µs Reduced on time 3 TPS64201,TPS64202 Minimum off time µs 0.20 TPS64200,TPS64201, TPS64203 0.44 0.55 0.66 TPS64202 only 0.24 0.3 0.36 µss 3 www.ti.com SLVS485 − AUGUST 2003 PIN ASSIGNMENTS DBV PACKAGE (TOP VIEW) EN 1 6 SW GND 2 5 VIN FB 3 4 ISENSE Terminal Functions TERMINAL NAME NO. I/O DESCRIPTION EN 1 I Enable. A logic low enables the converter, logic high forces the device into shutdown mode reducing the supply current to less than 1 µA. FB 3 I Feedback pin. Connect an external voltage divider to this pin to set the output voltage. GND 2 I Ground SW 6 O This pin connects to the gate of an external P-channel MOSFET. ISENSE 4 I Current sense input. Connect the current sense resistor between VIN and ISENSE. (optional) VIN 5 I Supply voltage input FUNCTIONAL BLOCK DIAGRAM VIN EN 105 mV ISense + _ Overcurrent Comparator Softstart FB + _ Regulation Comparator Vref GND 4 Minimum ton Timer (0.2 µs, 0.4 µs, 0.8 µs, 1.6 µs) Minimum toff Timer (0.6 µs, 0.3 µs,) ton Regulation Timer (3 µs, 15 µs, 16 µs) Logic M U X Driver R Q S ton Regulator SW www.ti.com SLVS485 − AUGUST 2003 TYPICAL CHARACTERISTICS All graphs were generated using the circuit as shown unless otherwise noted. For output voltages other than 3.3 V, the output voltage divider was changed accordingly. Graphs for the TPS64203 were taken using the application circuit shown in Figure 25. VI R(ISENSE) = 33 mΩ CI 10 µF X7R TPS6420x 1 2 3 EN SW GND VIN FB ISENSE 6 Si5447DC 5 CDRH 103R−100 4 VO 10 µH MBRM120LT3 R1 620 kΩ Cff 4.7 pF Co 47 µF PosCap 6TPA47M R2 360 kΩ Figure 1. Basic Application Circuit For a 2-A Step-Down Converter TABLE OF GRAPHS FIGURE η Efficiency vs Load current 2−5 Output voltage vs Output current 6−9 Switching frequency vs Output current 10 − 13 Operating quiescent current vs Input voltage Output voltage ripple 14 15 Line transient response Using circuit according to Figure 1 16 Load transient response Using circuit according to Figure 1 17 Start-up timing Using circuit according to Figure 1 18 5 www.ti.com SLVS485 − AUGUST 2003 100 TPS64200 TPS64201 EFFICIENCY vs LOAD CURRENT EFFICIENCY vs LOAD CURRENT 100 VI = 3.6 V VI = 4.2 V 80 Efficiency − % Efficiency − % 70 60 50 40 VI = 6 V 50 40 30 20 20 TA = 25°C, VO = 3.3 V 0.001 0.01 0.1 IO − Load Current − A 1 VI = 5 V 60 30 10 VI = 4.2 V 80 VI = 5 V VI = 6 V 70 0 0.0001 VI = 3.6 V 90 90 TA = 25°C, VO = 3.3 V 10 0 0.0001 10 0.001 Figure 2 TPS64202 TPS64203 EFFICIENCY vs LOAD CURRENT EFFICIENCY vs LOAD CURRENT 90 90 80 VI = 1.8 V VI = 2.5 V Efficiency − % 70 VI = 6 V 60 TA = 25°C, VO = 1.2 V 80 VI = 4.2 V VI = 5 V 70 Efficiency − % 10 100 VI = 3.6 V 50 40 VI = 3.6 V 60 VI = 6 V 50 VI = 5 V 40 30 30 20 20 TA = 25°C, VO = 3.3 V 10 0.001 0.01 0.1 IO − Load Current − A Figure 4 6 1 Figure 3 100 0 0.0001 0.01 0.1 IO − Load Current − A 1 10 10 0 0.0001 0.001 0.01 0.1 IO − Load Current − A Figure 5 1 10 www.ti.com SLVS485 − AUGUST 2003 TPS64200 TPS64201 OUTPUT VOLTAGE vs OUTPUT CURRENT OUTPUT VOLTAGE vs OUTPUT CURRENT 3.40 3.40 TA = 25°C, VO = 3.3 V 3.38 VO − Output Voltage − V VI = 4.2 V VI = 6 V 3.34 3.32 3.30 VI = 3.6 V 3.28 3.26 10 VI = 3.6 V 3.28 3.26 3.22 1 VI = 4.2 V 3.30 3.22 0.01 0.1 IO − Output Current − A VI = 5 V 3.32 3.24 0.001 VI = 6 V 3.34 3.24 3.20 0.0001 TA = 25°C, VO = 3.3 V 3.36 VI = 5 V 3.36 VO − Output Voltage − V 3.38 3.20 0.0001 0.001 0.01 0.1 IO − Output Current − A Figure 6 1 10 Figure 7 TPS64202 TPS64203 OUTPUT VOLTAGE vs OUTPUT CURRENT OUTPUT VOLTAGE vs OUTPUT CURRENT 3.40 3.38 TA = 25°C, VO = 3.3 V 1.29 VI = 6 V VI = 5 V 1.27 VO − Output Voltage − V VO − Output Voltage − V 3.36 TA = 25°C, VO = 1.2 V 3.34 3.32 3.30 VI = 3.6 V 3.28 VI = 4.2 V 3.26 1.25 VI = 3.6 V VI = 5 V VI = 6 V 1.23 1.21 VI = 1.8 V 1.19 VI = 2.5 V 3.24 1.17 3.22 3.20 0.0001 0.001 0.01 0.1 IO − Output Current − A Figure 8 1 10 1.15 0.0001 0.001 0.01 0.1 IO − Output Current − A 1 10 Figure 9 7 www.ti.com SLVS485 − AUGUST 2003 TPS64200 TPS64201 SWITCHING FREQUENCY vs OUTPUT CURRENT SWITCHING FREQUENCY vs OUTPUT CURRENT 500 400 VO = 3.3 V VO = 3.3 V VI = 5 V 450 350 400 250 f − Frequency − kHz f − Frequency − kHz 300 VO = 1.2 V 200 150 VI = 5 V 350 300 250 VO = 1.2 V 200 150 100 100 50 50 0 0.001 0.01 0.1 1 0 0.001 10 IO − Output Current − A 0.01 Figure 10 1 TPS64202 TPS64203 SWITCHING FREQUENCY vs OUTPUT CURRENT SWITCHING FREQUENCY vs OUTPUT CURRENT 900 VI = 4.2 V VI = 3.8 V 10 Figure 11 600 550 0.1 IO − Output Current − A VI = 5 V 800 500 700 400 f − Frequency − kHz f − Frequency − kHz 450 350 300 250 200 150 50 500 VO = 1.2 V 400 300 100 VO = 3.3 V 0 0.001 0.01 0.1 IO − Output Current − A Figure 12 8 VO = 3.3 V, Cff = 165 pF 200 VI = 3.8 V, Cff = 165 pF 100 600 1 10 0 0.001 0.01 0.1 1 IO − Output Current − A Figure 13 10 www.ti.com SLVS485 − AUGUST 2003 TPS642000 OPERATING QUIESCENT CURRENT vs INPUT VOLTAGE TPS64200 OUTPUT VOLTAGE RIPPLE VI = 3.8 V, VO = 1.2 V, RL = 1.2 Ω, TA = 25°C 35 TA = 25°C TA = 85°C 20 mV/Div 30 TA = −40°C 25 VO 20 I(coil) 15 10 5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 2 ms/Div VI − Input Voltage − V Figure 14 Figure 15 TPS64200 TPS64203 LINE TRANSIENT RESPONSE LOAD TRANSIENT RESPONSE VI = 3.8 V to 5 V, VO = 1.2 V, RL = 1.2 Ω, TA = 25°C VI 1 A/Div IO VI = 5 V, VO = 3.3 V, IL = 200 mA to 1800 mA, TA = 25°C VO 50 mV/Div 20 mV/Div 1 V/Div IO = 1000 mA 200 mA/Div IQ − Operating Quiescent Current − µ A 40 40 ms/Div Figure 16 VO 50 ms/Div Figure 17 9 www.ti.com SLVS485 − AUGUST 2003 TPS64200 STARTUP TIMING EN VO II I(Inductor) VI = 3.8 V, VO = 3.3 V, RL = 1.66 Ω, TA = 25°C 100 ms/Div Figure 18 DETAILED DESCRIPTION Operation The TPS6420x is a nonsynchronous step-down controller which is operating with a minimum on-time/minimum off-time control. An external PMOS is turned on until the output voltage reaches its nominal value or the current limit is exceeded. If the current limit is exceeded, the PMOS is switched off and stays off for the minimum off-time. After that the PMOS is switched on again. When the nominal output voltage is reached, the PMOS is switched off and stays off until the output voltage dropped below its nominal value. Operating Modes When delivering low or medium output current, the TPS6420x operate in discontinuous mode. With every switching cycle, the current in the inductor starts at zero, rises to a maximum value and ramps down to zero again. As soon as the current in the inductor drops to zero, ringing occurs at the resonant frequency of the inductor and stray capacitance, due to residual energy in the inductor when the diode turns off. Ringing in discontinuous mode is normal and does not have any influence on efficiency. The ringing does not contain much energy and can easily be damped by an RC snubber. See the application section for further details. With high output current, the TPS6420x operate in continuous current mode. In this mode, the inductor current does not drop to zero within one switching cycle. The output voltage in continuous mode is directly dependant on the duty cycle of the switch. Variable Minimum On-Time (TPS64201 to TPS64202 Only) The minimum on-time of the device is 1.6 µs. At light loads, this would cause a low switching frequency in the audible range because the energy transferred to the output during the on-time would cause a higher rise in the output voltage than needed and therefore lead to a long off−time until the output voltage dropped again. To avoid a switching frequency in the audible range the TPS64201 and TPS64202 can internally reduce the minimum on time in three steps from 1.6 µs to 800 ns, 400 ns and 200 ns. The on-time is reduced by one step if the switching frequency dropped to a lower value than 50 kHz. This keeps the frequency above the audio frequency over a wide load range and also keeps the output voltage ripple low. 10 www.ti.com SLVS485 − AUGUST 2003 Soft Start The TPS6420x has an internal soft start circuit that limits the inrush current during start up. This prevents possible voltage drops of the input voltage in case a battery or a high impedance power source is connected to the input of the TPS6420x. During soft start the current limit is increased from 25% of its maximum to the maximum within about 250 µs. 100% Duty Cycle Low Dropout Operation The TPS6420x offers the lowest possible input to output voltage difference while still maintaining regulation with the use of the 100% duty cycle mode. In this mode the P-channel switch is constantly turned on. This is particularly useful in battery powered applications to achieve longest operation time by taking full advantage of the whole battery voltage range. Enable A voltage higher than the EN trip point of 1.3 V up to the input voltage forces the TPS64200 into shutdown. In shutdown, the power switch, drivers, voltage reference, oscillator, and all other functions are turned off. The supply current is reduced to less than 1 µA in shutdown. Pulling enable low starts up the TPS64200 with the softstart as described under the chapter softstart. Undervoltage Lockout The undervoltage lockout circuit prevents the device from misoperation at low input voltages. Basically, it prevents the converter from turning on the external PMOS under undefined conditions. Current Limit The ISENSE input is used to set the current limit for the external PMOS. The sense resistor must be connected between VI and source of the external PMOS. The ISENSE pin is connected to the source of the external PMOS. The maximum current is calculated by: V I (cur lim) + (ISENSE) R S (1) For low cost solutions the rDS(on) of the external PMOS can also be used to set the current limit. In this case the ISENSE pin is connected to the drain of the PMOS. The current in the PMOS is automatically sampled by the TPS6420x some 10 ns after the PMOS is turned on. The ISENSE pin should always be connected to either the source of the PMOS or the drain if an additional sense resistor is used. Otherwise there is no working overcurrent protection and no soft start in the system. The maximum drain current if the rDS(on) is used as a sense resistor is calculated by: V I (cur lim) + (ISENSE) r DS(on) (2) Short-Circuit Protection With a controller only limited short circuit protection is possible because the temperature of the external components is not supervised. In an overload condition, the current in the external diode may exceed the maximum rating. To protect the diode against overcurrent, the off-time of the TPS6420x is increased when the voltage at the feedback pin is lower than its nominal value. The off-time when the output is shorted (feedback voltage is zero) is about 4 µs. This allows the current in the external diode to drop until the PMOS is turned on again and the overcurrent protection switches off the PMOS again. The off-time is directly proportional to the voltage at feedback. 11 www.ti.com SLVS485 − AUGUST 2003 THEORY OF OPERATION The basic application circuit for the TPS64200 is shown in Figure 1. External component selection is driven by the load requirement. It begins with the selection of the current sense resistor R(ISENSE) followed by the output diode, the inductor L, and the output and input capacitors. The inductor is chosen based on the desired amount of ripple current and switching frequency. The output capacitor is chosen large enough to meet the required output ripple and transient requirements. The ESR of the output capacitor is needed for stability of the converter. Therefore, an output capacitor with a certain amount of ESR is needed for the standard application circuit. See the application information for more details. The input capacitor must be capable of handling the required RMS input current. Setting the Inductor Current Limit The ISENSE pin is connected to an internal current comparator with a threshold of 120 mV/R(ISENSE). The current comparator sets the peak inductor current. As the current limit is intended to protect the external PMOS the limit must not be reached in normal operation. Set the current limit to about 1.3 times the maximum output current or higher if desired. This takes into account a certain amount of inductor current ripple. The current limit may also influence the start-up time when the current limit is exceeded during start up. V R (ISENSE) v min IO — maximum output current in continuous conduction mode (ISENSE) V(ISENSE), min = 90 mV 1.3 I (3) O The current sense resistor’s power rating should be: P (ISENSE) ǒV(ISENSE) maxǓ w R 2 V(ISENSE), max = 120 mV (4) (ISENSE) Setting the Output Voltage The output voltage of the TPS64200 to TPS64202 can be set using an external resistor divider. The sum of R1 and R2 should not exceed 1 MΩ to keep the influence of leakage current into the feedback pin low. V O +V FB R1 ) R2 R2 ǒ Ǔ V R1 + R2 V O * R2 with VFB = 1.2 V (5) FB In some applications, depending on the layout, the capacitance may be too high from FB to GND. In this case, the internal comparator may not switch fast enough to operate with the minimum on-time or the minimum off-time given in this data sheet. For such applications a feedforward capacitor (Cff) in the range of 4.7 pF to 47 pF (typical) is added in parallel with R1 to speed up the comparator. Choose a capacitor value that is high enough that the device turns on the PMOS for its minimum on-time with no load at the output. Selecting the Input Capacitor The input capacitor is used to reduce peak currents drawn from the power source and reduces noise and voltage ripple on the input of the converter, caused by its switching action. Use low ESR tantalum capacitors or preferably X5R or X7R ceramic capacitors with a voltage rating higher than the maximum supply voltage in the application. In continuous conduction mode, the input capacitor must handle an rms-current which is given by: I Cin(rms) [I O Ǹ V O V , min I (6) Select the input capacitor according to the calculated rms-input current requirements and according to the maximum voltage ripple. Use a minimum value of 10 µF: ǒ Ǔ 2 ǒ 1 L 1 L 0.3 DI 2 L 2 C , min + [ I V V V (ripple) I (ripple) 12 I Ǔ O V I 2 with: V(ripple) − voltage ripple at CI ∆IL − inductor current ripple (7) www.ti.com SLVS485 − AUGUST 2003 For a first approximation use: L = 10 µH V(ripple) = 150 mV (verify in the application) Selecting the Inductor Value The main parameters when choosing the inductor are current rating and inductance. The inductance mainly determines the inductor current ripple. The TPS6420x operates with a wide range of inductor values. Values between 4.7 µH and 47 µH work in most applications. Select an inductor with a current rating exceeding the limit set by R(ISENSE) or rDS(on). The first step in inductor design is to determine the operating mode of the TPS64200. The device can either work with minimum-on-time or minimum-off-time, depending on input voltage and output voltage. The device works with minimum-on-time if: t V *V *I I O O r DS(on) *R RL xI w O off , min ǒVO ) VSCHOTTKY ) RRL I O Ǔ t on, min (8) with RRL − inductor resistance L+V with DI Dt For minimum-on-time: ǒVI–VO–IO L+ r –R DS(on) RL DI I O Ǔ t on, min with: ∆I ≤ 0.3 × IO (9) For minimum-off-time: L+ ǒVO ) VSCHOTTKY ) RRL I O Ǔ t off , min (10) DI Table 1. List of Inductors Tested With the TPS6420x MANUFACTURER TYPE INDUCTANCE DC RESISTANCE SATURATION CURRENT TDK SLF7032T−100M1R4 10 µH ±20% 53 mΩ ±20% 1.4 A TDK SLF6025−150MR88 15 µH ±20% 85 mΩ ±20% 0.88 A Sumida CDRH6D28−5R0 5 µH 23 mΩ 2.4 A Sumida CDRH103R−100 10 µH 45 mΩ 2.4 A Sumida CDRH4D28−100 10 µH 95 mΩ 1.0 A Sumida CDRH5D18−6R2 6.2 µH 71 mΩ 1.4 A Coilcraft DO3316P−472 4.7 µH 18 mΩ 5.4 A Coilcraft DT3316P−153 15 µH 60 mΩ 1.8 A Coilcraft DT3316P−223 22 µH 84 mΩ 1.5 A Wurth 744 052 006 6.2 µH 80 mΩ 1.45 A Wurth 74451115 15 µH 90 mΩ 0.8 A 13 www.ti.com SLVS485 − AUGUST 2003 Selecting the External PMOS An external PMOS must be used for a step-down converter with the TPS64200. The selection criteria for the PMOS are threshold voltage, rDS(on), gate charge and current and voltage rating. Since the TPS64200 can operate down to 1.8 V, the external PMOS must have a VGS(th) much lower than that if it is operated with such a low voltage. As the gate of the PMOS finds the full supply voltage applied to the TPS64200, the PMOS must be able to handle that voltage at the gate. The drain to source breakdown voltage rating should be at least a few volts higher than the supply voltage in the application. The rms-current in the PMOS assuming low inductor current ripple and continuous conduction mode, is: I PMOS(rms) [ I ǸD + I O O Ǹ V O V I (11) The power dissipated in the PMOS is comprised of conduction losses and switching losses. The conduction losses are a function of the rms−current in the PMOS and the rDS(on) at a given temperature. They are calculated using: P ǒ Ǔ + I ǸD (cond) O ǒ1 ) TC ƪTJ–25°CƫǓ [ ǒIO ǸDǓ 2 r DS(on) 2 r DS(on) (12) with TC = 0.005/°C Table 2. PMOS Transistors Used in the Application Section TYPE MANUFACTURER ID PACKAGE Vishay Siliconix rDS(on) 0.11 Ω at VGS = −2.5 V VDS Si5447DC −20 V −3.5 A at 25°C 1206 Si5475DC Vishay Siliconix 0.041 Ω at VGS = −2.5 V −12 V −6.6 A at 25°C 1206 Si2301ADS Vishay Siliconix 0.19 Ω at VGS = −2.5 V −20 V −1.4 A at 25°C SOT23 Si2323DS Vishay Siliconix 0.41 Ω at VGS = −2.5 V −20 V −4.1 A at 25°C SOT23 FDG326P Fairchild 0.17 Ω at VGS = −2.5 V −20 V −1.5 A SC70 Selecting the Output Diode The output diode conducts in the off phase of the PMOS and carries the full output current. The high switching frequency demands a high-speed rectifier. Schottky diodes are recommended for best performance. Make sure that the peak current rating of the diode exceeds the peak current limit set by the sense resistor R(ISENSE) or rDS(on). Select a Schottky diode with a low reverse leakage current to avoid an increased supply current. The average current in the diode in continuous conduction mode, assuming low inductor current ripple, is: I (diode)(Avg) ǒ Ǔ V [ I (1–D) + I 1– O O O V I (13) Table 3. Tested Diodes 14 TYPE MANUFACTURER VR IF PACKAGE MBRM120LT3 On Semiconductor MBR0530T1 On Semiconductor 20 V 1A DO216AA 30 V 0.5 A ZHCS2000TA SOD123 Zetex 40 V 2A SOT23−6 B320 Diodes Inc. 20 V 3A SMA www.ti.com SLVS485 − AUGUST 2003 Selecting the Output Capacitor The value of the output capacitor depends on the output voltage ripple requirements as well as the maximum voltage deviation during a load transient. The TPS6420x require a certain ESR value for proper operation. Low ESR tantalum capacitors or PosCap work best in the application. A ceramic capacitor with up 1 µF may be used in parallel for filtering short spikes. The output voltage ripple is a function of both the output capacitance and the ESR value of the capacitor. For a switching frequency which is used with the TPS6420x, the voltage ripple is typically between 90% and 95% due to the ESR value. ƪ DV pp + DI ESR ) ǒ 1 C O 8 ƒ Ǔƫ [ 1.1 DI ESR (14) DV pp (15) 1.1 DI The output capacitance typically increases with load transient requirements. For a load step from zero output current to its maximum, the following equation can be used to calculate the output capacitance: ESR, max [ DI L 2 O C + O (V * V ) x DV I O (16) Table 4. Capacitors Used in the Application TYPE MANUFACTURER CAPACITANCE ESR VOLTAGE RATING 6TPB47M (PosCap) Sanyo 47 µF 0.1 Ω 6.3 V T491D476M010AS Kemet 47 µF 0.8 Ω 10 V B45197A Epcos 47 µF 0.175 Ω 16 V B45294−R1107−M40 Epcos 100 µF 0.045 Ω 6.3 V 594D476X0016C2 Vishay 47 µF 0.11 Ω 16 V Output Voltage Ripple Output voltage ripple causes the output voltage to be higher or lower than set by the resistor divider at the feedback pin. If the application runs with minimum on-time, the ripple (half of the peak-to-peak value) adds to the output voltage. In an application which runs with minimum off-time, the output voltage is lower by the amount of ripple (half of the peak-to-peak value) at the output. Snubber Design For low output current, the TPS6420x work in discontinuous current mode. When the current in the inductor drops to zero, the inductor and parasitic capacitance form a resonant circuit, which causes oscillations when both, diode and PMOS do not conduct at the end of each switching cycle. The oscillation can easily be damped by a RC-snubber. The first step in the snubber design is to measure the oscillation frequency of the sine wave. Then, a capacitor has to be connected in parallel to the Schottky diode which causes the frequency to drop to half of its original value. The resistor is selected for optimum transient response (aperiodic). R + 2pfL f − measured resonant frequency L − inductance used (17) Selecting the Right Device for the Application The TPS6420x step-down controllers either operate with a fixed on-time or a fixed off-time control. It mainly depends on the input voltage to output voltage ratio if the switching frequency is determined by the minimum-on-time or the minimum-off-time. To select the right device for an application see the table below: INPUT TO OUTPUT VOLTAGE RATIO VI >> VO (e.g. VI = 5 V VO = 1.5 V) VI ≈ VO (e.g. VI = 3.8 V VO = 3.3 V) SWITCHING FREQUENCY DETERMINED BY PROPOSED DEVICE FOR HIGH SWITCHING FREQUENCY PROPOSED DEVICE FOR LOW SWITCHING FREQUENCY Minimum on−time TPS64203 TPS64200, TPS64201 Minimum off−time TPS64202 TPS64200, TPS64201 15 www.ti.com SLVS485 − AUGUST 2003 APPLICATION INFORMATION Li-lon 3.3 V to 4.2 V R(ISENSE) = 33 mΩ CIN 10 µF TPS64202 1 2 3 EN SW GND VIN FB ISENSE 6 Si5475DC 5 CDRH6D28-5R0 4 3.3 V / 2 A 5 µH MBRM120LT3 R1 620 kΩ Cff 4.7 pF Co 47 mF PosCap 6TPA47M R2 360 kΩ Figure 19. Application For a Li-Ion to 3.3-V / 2-A Conversion The TPS64202 was used for this application because for a low input to output voltage difference, the switching frequency is determined by the minimum off-time. The TPS64202 with its minimum off-time of 300 ns provides a higher switching frequency compared to the other members of the TPS6420x family. Li-lon 3.3 V to 4.2 V CIN 10 µF TPS64202 1 2 3 EN SW GND VIN FB ISENSE 6 5 4 MBRM120LT3 Si5475DC CDRH6D28-5R0 5 µH R1 620 kΩ R3 150 Ω C3 470 pF 3.3 V / 2 A Cff 4.7 pF R2 360 kΩ Co 47 mF PosCap 6TPA47M Figure 20. Application For a Li-Ion to 3.3-V / 2-A Conversion Using rDS(on) Sense and RC Snubber Network For the Schottky Diode 16 www.ti.com SLVS485 − AUGUST 2003 DESIGN EXAMPLE FOR AN APPLICATION USING A LI-ION CELL (3.3 V TO 4.2 V) TO GENERATE 3.3 V/500 mA 1. Calculate the sense resistor for the current limit: V R (ISENSE) v min (ISENSE) + 90 mV + 138 mW 1.3 I 1.3 0.5 A O (18) Choose the next lower standard value : R(ISENSE) = 120 mΩ. Verify the inductor current ripple after the inductor has been determined in step 5. If the rDS(on) of the PMOS is used to sense the inductor current, a PMOS with less than 138 mΩ must be used for the application. 2. Calculate the resistors for the output voltage divider using VO = 3.3 V and VFB = 1.21 V R1 + R2 ǒ Ǔ V V O –R2 + 1.72 (19) R2 FB Choose R2 = 360 kΩ, and then get R1 = 619 kΩ. Select the next standard value: R1 = 620 kΩ 3. Select the external PMOS For a Li-Ion to 3.3-V conversion, the minimum input voltage is 3.3 V. Therefore, the converter runs in 100% mode (duty cycle=1) and the maximum PMOS current is equal to the output current. I (PMOS) +I O + 0.5 A (20) The Si2301ADS is selected for this application because it meets the requirements when an external sense resistor is used. Otherwise a PMOS with less rDS(on) must be selected. Verify the maximum power dissipation of the PMOS using: P (cond) ǒ OǓ 2 + I r DS(on) + (0.5 A) 2 0.19 W + 48 mW (21) 4. Select the external diode For the Schottky diode, the worst case current is at high input voltage (4.2 V for a Li-Ion cell). I (diode)(Avg) [I ǒ Ǔ V 1– O O V I +I O VǓ + 0.11 A ǒ1– 3.3 4.2 V (22) The MBR0530T1 is selected because it meets the voltage and current requirements. The forward voltage is about 0.3 V. Do not use a Schottky diode which is much larger than required as it also typically has more leakage current and capacitance which reduces efficiency. 5. Calculate the inductor value. If the output voltage is close to the input voltage, the switching frequency is determined by the minimum off-time. Therefore, the TPS64202 is used for the maximum switching frequency possible. Allow an inductor ripple current of 0.3 × IO for the application. For the inductor, a series resistance of 100 mΩ is assumed. For minimum-off-time, the inductor value is: L+ ǒVO ) V(SCHOTTKY) ) RRL DI I Ǔ O (23) t off , min + (3.3 V ) 0.3 V ) 0.05 V) 0.3 0.5 A 0.3 ms + 7.3 mH For a low inductor current ripple, select the next available larger inductor with L = 10 µH. This provides an inductor ripple current of 110 mA (peak-to-peak). 17 www.ti.com SLVS485 − AUGUST 2003 DI + ǒVO ) V(SCHOTTKY) ) RRL I O Ǔ t off , min L + 110 mA (24) The current rating for the inductor must be: I, inductor u I O ) DI + 555 mA 2 (25) 6. Select the input and output capacitor The output capacitor is selected for an output voltage ripple of less than 20 mVpp. With ESR, max [ DV pp 1.1 DI + 1.1 0.02 V + 165 mW 0.11 A (26) A 47-µF PosCap with an ESR of 100 mΩ was selected to meet the ripple requirements. The input capacitor was selected to its minimum value of 10 µF. 1 Li-lon Cell R(ISENSE) = 120 mΩ 10 µF TPS64202 1 2 3 EN SW GND VIN FB ISENSE 6 Si2301DS 5 CDRH4D18-100 4 10 µH MBR0530T1 3.3 V / 0.5 A R1 620 kΩ 47 µF PosCap 6TPA47M R2 360 kΩ Figure 21. Application Circuit 18 www.ti.com SLVS485 − AUGUST 2003 5V R(ISENSE) = 33 mΩ 10 µF TPS64200 1 2 3 EN SW GND VIN FB ISENSE 6 Si5447DC 5 CDRH103R-100 4 3.3 V / 2 A 10 µH ZHCS200 R1 620 kΩ 47 mF PosCap 6TPA47M R2 360 kΩ Figure 22. Application For a 5-V to 3.3-V / 2-A Conversion Inverter Using TPS64200 VI 2.7 V to 4.2 V R(ISENSE) = 33 mΩ 10 µF TPS64200 1 EN 6 SW 2 GND 3 FB ISENSE Si2301DS 5 VIN MBR0530T1 4 −5 V / 0.1 A SW R2 24 kΩ R1 VI _ OPA363 100 kΩ CDRH4D28-100 10 µH 47 µF X7R + Figure 23. Application For an Inverter Using TPS64200 19 www.ti.com SLVS485 − AUGUST 2003 The TPS6420x can be used for an inverter. Only one additional operational amplifier is required for this application. When the PMOS is switched on, the current in the inductor ramps up to its maximum, set by Rs. Then the PMOS is switched off, the energy stored in the inductor is transferred to the output. The output voltage and the maximum output current can be calculated using: V + R1 O R2 V FB I O V max [ 0.8 –V V (ISENSE) 2R (ISENSE) I O (27) OLED Power Supply The TPS6420x can be combined with a TPS61045 boost converter for a OLED power supply. 4.7 µH 7 V / 50 mA 4.7 µF 8 1 L SW 2 VIN DO 5 FB CTRL 6 GND PGND 56 kΩ 22 pF 1 µF X7R 3 4 7 12 kΩ TPS61045 VI 1.8 V to 5.5 V R(ISENSE) = 150 mΩ 10 µF TPS64200 1 2 3 EN GND 6 SW Si2301DS 5 VIN MBR0530T1 4 FB ISENSE −7 V / 50 mA SW R2 130 kΩ R1 VI _ OPA363 750 kΩ CDRH4D28-100 10 µH + Figure 24. Application For a OLED Power Supply 20 47 µF X7R www.ti.com SLVS485 − AUGUST 2003 5V 10 µF X5R TPS64203 1 2 3 EN SW GND VIN FB ISENSE 6 Si2323DS 5 Wurth 744052006 4 1.2 V / 1.2 A 6.2 µH MBRM120LT3 100 µF/6.3 V B45294-R1107-M40 Figure 25. Application For a 5-V to 1.2-V / 1.2-A Conversion 5V R(ISENSE) = 20 mΩ CIN 22 µF TPS64202 1 2 3 EN SW GND VIN FB ISENSE 6 Si5475DC 5 4 DO3316P−472 3.3 V / 3 A 4.7 µH B320 R1 620 kΩ Cff 4.7 pF R2 360 kΩ Co 100 mF PosCap 6TPC100M Figure 26. Application For a 5-V to 3.3-V / 3-A Conversion 21 www.ti.com SLVS485 − AUGUST 2003 Ceramic Output Capacitor 5V R(ISENSE) = 33 mΩ CI 10 µF TPS64203 1 EN SW 6 2 GND VIN 5 3 4 FB ISENSE R(GATE) 10 Ω Si5475DC CDRH6D28-100 MBRM120LT3 10 µH R1a 680 kΩ R1b 2.2 MΩ 3.3 V / 2 A Cff 82 pF R2 300 kΩ Co 22 µF X5R 6.3 V Figure 27. Application Using a Ceramic Output Capacitor The control scheme of the TPS6420x usually requires an output capacitor with some tens of milliohms of ESR for stability, which is usually the case for tantalum capacitors. This application circuit above also works with ceramic capacitors. Resistor R1b is used to add an additional control signal to the feedback loop, which is coupled into the FB pin. The circuit works best with R1b = 2 …4 x R1a. If the resistance of R1b is too low compared to R1a, the more load regulation the output voltage shows, but stability is best. The advantage of this circuit is a very low output voltage ripple and small size. The gate resistor shown can be used in every application. It minimizes switching noise of the converter and, therefore, increases stability and provides lower output voltage ripple. However, it decreases efficiency slightly because the rise and fall time, and the associated losses are larger. R1 + 1 1 ) 1 R1a R1b R1b + 1 1 – 1 R1 R1a (28) Use the following equation to calculate R1a if R1b = 4R1a R1a + 5 R1 4 22 (29) IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. 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