19-3420; Rev 0; 9/04 ILABLE N KIT AVA EVALUATIO High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET Features ♦ Low-Cost, High-Current Linear Regulator The MAX8704 includes a fixed current limit and an adjustable power limit to protect the external MOSFET from overheating. Additionally, the MAX8704 includes an internal thermal limit to prevent damage to the controller and provide remote thermal protection for the external MOSFET. The MAX8704 features an adjustable soft-start function and generates a delayed power-good (PGOOD) signal that signals when the linear regulator is in regulation. The MAX8704 is available in a 10-pin µMAX® package. ♦ Power-Good (PGOOD) Open-Drain Output with 3ms Startup Delay ♦ External MOSFET Protection MOSFET Power Limit 50mV (typ) Current Limit Thermal Limit ♦ 1.0V to 5.5V Input Supply Voltage ♦ 1.2V or 1.5V Preset, or Adjustable Output Voltage ♦ Programmable Soft-Start ♦ Shutdown with Output Discharge Ordering Information Applications VMCH and VCCP CPU Supplies Notebook Computers PART MAX8704EUB TEMP RANGE PIN-PACKAGE -40°C to +85°C 10 µMAX Desktop Computers Servers VID Power Supplies Low-Voltage Bias Supplies Pin Configuration TOP VIEW VIN 1 VCC 10 DRV 2 MAX8704 9 GND PGOOD 3 8 CSP PLIM 4 7 CSN SS/EN 5 6 FB µMAX µMAX is a registered trademark of Maxim Integrated Products, Inc. ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX8704 General Description The MAX8704 high-current linear regulator uses an external n-channel MOSFET to generate low-voltage supplies for notebook computers. This linear regulator delivers an output voltage as low as 0.5V from an input voltage as low as 1.0V. Normally, this low input requirement would make the design of such a regulator very difficult. In this application, the 5V bias supply that is always available in the system powers the MAX8704 driver and control circuitry. MAX8704 High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET ABSOLUTE MAXIMUM RATINGS VCC, VIN to GND.......................................................-0.3V to +6V CSP, CSN, DRV to GND ...........................................-0.3V to +6V FB, PLIM, SS/EN, PGOOD to GND.............-0.3V to (VCC + 0.3V) Continuous Power Dissipation (TA = +70°C) 10-Pin µMAX (derated 5.6mW/°C above +70°C) .........444mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C 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 in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VIN = 2.5V, VCC = 5.0V, PLIM = FB = GND, CSP = CSN, SS/EN floating, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Input Voltage Range SYMBOL CONDITIONS MIN TYP MAX VIN 1.0 5.5 VCC 4.5 5.5 FB = VCC 1.462 1.50 1.538 FB = GND 1.170 1.20 1.230 FB = CSN 490 500 510 Load-Regulation Error VCSP - VCSN = 45mV -2.5 Line-Regulation Error VIN = 1V to 5.5V Preset Output Voltage (Fixed) Feedback Voltage Accuracy (Adjustable) FB Input Bias Current CSN Input Bias Current DRV Output Voltage Swing VOUT VFB IFB ICSN VDRV DRV Slew Rate VFB = 0.6V -1 50 VCC 1.0 Output low 0.7 0.2 V mV % +1 µA 100 µA VCC 0.7 CDRV = 40nF V % 0.01 VCSN = 1.6V Output high -2 UNITS V 1.0 V/µs Quiescent Supply Current (VCC) ICC FB forced above the regulation point, VCSN = 1.6V 1.5 3 mA Quiescent Supply Current (VIN) IIN FB forced above the regulation point, VCSN = 1.6V 5 10 µA Shutdown Supply Current (VCC) SS/EN = GND 35 70 µA Shutdown Supply Current (VIN) SS/EN = GND 5 10 µA FAULT DETECTION Thermal-Shutdown Threshold TSHDN VCC Undervoltage-Lockout Threshold Rising edge, 20°C hysteresis +140 Rising edge, 15mV hysteresis 4.2 °C 4.45 V Current-Limit Threshold VCSLIMIT PLIM = GND 45 50 57 mV Power-Limit Threshold VPWRLIMIT Rising edge 0.96 1.0 1.04 V VCSP - VCSN = 30mV, VCSN = 0.5V, VIN = 3.5V 155 200 233 µA/V2 Power-Limit Conversion Gain Power-Limit Conversion Gain Variation 2 KPLIM VCSP - VCSN = 25mV to 45mV, VCSN = 0.5V, VIN = 2V to 4.5V ±12 _______________________________________________________________________________________ % High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET (VIN = 2.5V, VCC = 5.0V, PLIM = FB = GND, CSP = CSN, SS/EN floating, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 14 18 21 µA 0.5 2 µA +1 µA 6 µA PLIM Output Current VCSP - VCSN = 30mV, VIN = 3.5V, VCSN = 0.5V PLIM Output Current Offset CSP = CSN, VIN = 1.0V, VCSN = 0.5V CSP Input Current VCSN = 1.50V, VCSP = 1.55V -1 VSS/EN = 1.5V 4 5 0.4 0.5 0.6 V 10 20 µA SOFT-START AND SHUTDOWN Soft-Start Charge Current ISS SS/EN Full Current Threshold 2 SS/EN Enable Threshold Rising edge SS/EN Discharge Current ISS/EN Discharge-Mode On-Resistance RCSN VSS/EN = 1.5V, thermal fault, bias fault condition, or UVLO V Ω 10 INPUTS AND OUTPUTS With respect to error-comparator threshold, 2% hysteresis PGOOD Trip Threshold PGOOD Startup Delay PGOOD Output Low Voltage PGOOD Leakage Current -10 -8 1 3 ISINK = 4mA IPGOOD VFB = 1.0V (PGOOD high impedance), PGOOD forced to 5V -1 -6 % 5 ms 0.3 V +1 µA ELECTRICAL CHARACTERISTICS (VIN = 2.5V, VCC = 5.0V, PLIM = FB = GND, CSP = CSN, SS/EN floating, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER Input Voltage Range Preset Output Voltage (Fixed) Feedback Voltage Accuracy (Adjustable) DRV Output Voltage Swing SYMBOL MIN MAX VIN 1.0 5.5 VCC 4.5 5.5 VOUT VFB VDRV CONDITIONS FB = VCC 1.455 1.545 FB = GND 1.158 1.242 FB = CSN 485 515 Output high Output low VCC 1.1 UNITS V V mV V 1.1 Quiescent Supply Current (VCC) ICC FB forced above the regulation point, VCSN = 1.6V 3 mA Quiescent Supply Current (VIN) IIN FB forced above the regulation point, VCSN = 1.6V 10 µA Shutdown Supply Current (VCC) SS/EN = GND 70 µA Shutdown Supply Current (VIN) SS/EN = GND 10 µA _______________________________________________________________________________________ 3 MAX8704 ELECTRICAL CHARACTERISTICS (continued) ELECTRICAL CHARACTERISTICS (continued) (VIN = 2.5V, VCC = 5.0V, PLIM = FB = GND, CSP = CSN, SS/EN floating, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN MAX UNITS 4.45 V FAULT DETECTION VCC Undervoltage-Lockout Threshold Rising edge, 15mV hysteresis Current-Limit Threshold VCSLIMIT PLIM = GND 43 60 mV Power-Limit Threshold VPWRLIMIT Rising edge 0.90 1.10 V 13 22 µA VCSP - VCSN = 30mV, VIN = 3.5V, VCSN = 0.5V PLIM Output Current SOFT-START AND SHUTDOWN Soft-Start Charge Current ISS SS/EN Enable Threshold VSS/EN = 0 4 6 µA Rising edge 0.4 0.6 V With respect to error-comparator threshold, 2% hysteresis -11 -5 % 0.5 5.5 ms 0.3 V INPUTS AND OUTPUTS PGOOD Trip Threshold PGOOD Startup Delay PGOOD Output Low Voltage ISINK = 4mA Note 1: Specifications to -40°C are guaranteed by design, not production tested. Typical Operating Characteristics (Circuit of Figure 1, VOUT = 1.5V, TA = +25°C, unless otherwise noted.) VIN = 1.8V -1.5 -2.0 VIN = 3.3V 1.2 VIN = 2.5V 0.6 VIN = 1.8V 1 2 3 LOAD CURRENT (A) 4 4 5 0.8 1A LOAD 0.4 0.2 0 0 0 1.0 0.6 CURRENT LIMIT -3.0 1.4 1.2 0.9 0.3 -2.5 10mA LOAD 1.6 VOUT (V) VIN = 3.3V POWER LIMIT PLIM VOLTAGE (V) -0.5 1.8 MAX8704 toc02 1.5 MAX8704 toc01 0 -1.0 OUTPUT VOLTAGE vs. INPUT VOLTAGE PLIM VOLTAGE vs. LOAD CURRENT MAX8704 toc03 OUTPUT VOLTAGE DEVIATION vs. LOAD CURRENT VOUT DEVIATION (%) MAX8704 High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET 0 1 2 3 LOAD CURRENT (A) 4 5 1 2 3 INPUT VOLTAGE (V) _______________________________________________________________________________________ 4 5 High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET 5V BIAS SUPPLY CURRENT vs. INPUT VOLTAGE 1A LOAD 0.6 0.3 4 2.0 1A LOAD 3 2 1.5 OUTPUT SHORT 1.0 POWER LIMIT 1 10mA LOAD 0.5 DROPOUT 0 0 1 2 3 4 5 0 1 INPUT VOLTAGE (V) 2 3 4 5 1 INPUT VOLTAGE (V) OUTPUT CURRENT LIMIT vs. SS/EN VOLTAGE SOFT-START (CSS = 10nF) MAX8704 toc07 6 5 4 MAX8704 toc08 5V 2V 5V 0.5 1.0 1.5 2.0 SS/EN VOLTAGE (V) 2.5 3.0 B 0 1.5V C C 0 1A 0 0 0 A A 1A 0 5 0 B 1 4 MAX8704 toc09 0 2 3 SHUTDOWN SEQUENCE (NO LOAD) 0 1.5V 3 2 VDS (V) 1V CURRENT LIMIT (A) 2.5 MAX8704 toc05 1A LOAD POWER (W) 0.9 5 BIAS SUPPLY CURRENT (mA) OUTPUT SHORT 1.2 PLIM VOLTAGE (V) MAX8704 toc04 1.5 MOSFET POWER DISSIPATION vs. DRAIN-TO-SOURCE VOLTAGE MAX8704 toc06 PLIM VOLTAGE vs. INPUT VOLTAGE 200µs/div A. EN/SS, 1V/div C. FET CURRENT, 1A/div B. LDO OUTPUT, 1V/div 1.5Ω LOAD, VIN = 1.8V D 1ms/div C. LDO OUTPUT, 1V/div A. PGOOD, 5V/div D. FET CURRENT, 2A/div B. EN/SS, 5V/div NO LOAD, CSS = 1nF, VIN = 1.8V _______________________________________________________________________________________ 5 MAX8704 Typical Operating Characteristics (continued) (Circuit of Figure 1, VOUT = 1.5V, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (Circuit of Figure 1, VOUT = 1.5V, TA = +25°C, unless otherwise noted.) 3A LOAD TRANSIENT (IRF7401) POWER LIMIT 3A LOAD TRANSIENT (FDS6570A) MAX8704 toc11 MAX8704 toc10 5V A 3.5A 0 B 3.5V MAX8704 toc12 A 0.5A B 2.5V 1.8V C C 1.8V 0 1.7V 1.7V 4A 1.50V 1.50V D 0 D 1.45V LINE TRANSIENT (1.8V TO 2.5V) A 2.0V 60 40 20 0 -20 1.5V 0.001 3.3V 1.51V B 1.50V C PHASE 3.5V 0.001 40µs/div A. INPUT: 1.8V TO 2.5V, 0.5V/div C. OUTPUT, 10mV/div B. DRV, 200mV/div ROUT = 1.5Ω 0.1 1 10 0.01 0.1 1 10 FREQUENCY (MHz) FB = CSN, VOUT = 0.5V, VIN = 1.0V COUT = 2 x 22µF 1206 CERAMIC, IOUT = 0.5A PSRR GAIN AND PHASE vs. FREQUENCY MAX8704 toc16 MAX8704 toc15 40 0 0.001 0.01 0.1 1 10 PSRR (dB) GAIN (dB) 0.01 180° 90° 0° -90° -180° 1.49V -40 -80 -120 0.001 0.01 0.1 1 10 FREQUENCY (MHz) FB = CSN, VOUT = 0.5V, VIN = 1.0V COUT = 100µF 70mΩ SANYO 4TPB100M, IOUT = 0.5A 6 20µs/div A. LOAD: 0.5A - 3.5A, 3A/div C. INPUT, 100mV/div D. OUTPUT, 50mV/div B. DRV, 0.5V/div VIN = 1.8V MAX8704 toc14 GAIN (dB) 2.5V 180° 90° 0° -90° -180° D GAIN AND PHASE vs. FREQUENCY MAX8704 toc13 60 40 20 0 -20 C 1.45V 20µs/div A. LOAD: 0.5A - 3.5A, 3A/div C. INPUT, 100mV/div D. OUTPUT, 50mV/div B. DRV, 0.5V/div VIN = 1.8V 4ms/div C. LDO OUTPUT, 1V/div A. EN/SS, 2V/div D. FET CURRENT, 5A/div B. PLIM, 0.5V/div 0.4Ω LOAD, CSS = 10nF, VIN = 1.8V, RPLIM = 200kΩ, CPLIM = 0.1µF B 3.0V 3.0V 0 1.5V A 3.5A 0.5A PHASE MAX8704 High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET 10 0.01 0.1 1 FREQUENCY (MHz) FB = VCC, VOUT = 1.5V, VIN = 2.5V COUT = 2 x 22µF 1206 CERAMIC, IOUT = 0.5A 0.001 _______________________________________________________________________________________ High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET PIN NAME FUNCTION 1 VIN Input Voltage Sense. The MAX8704 senses the voltage across the external MOSFET (VIN - VCSN) to determine the MOSFET’s power dissipation. 2 VCC Analog and Driver Supply Input. Connect to the system supply voltage (+5.0V). Bypass VCC to analog ground with a 1µF or greater ceramic capacitor. 3 PGOOD Open-Drain Power-Good Output. PGOOD is low when the output voltage is more than 8% (typ) below the nominal regulation voltage. PGOOD is also pulled low during soft-start and in shutdown. Approximately 3ms (typ) after the LDO reaches the regulation voltage, PGOOD becomes high impedance as long as the output remains in regulation. 4 PLIM Power-Limit Adjustment. The PLIM output sources a current directly proportional to the MOSFET’s power dissipation. If the PLIM voltage exceeds the 1.0V power-limit threshold, the regulator reduces the power dissipation by folding back the current limit. An external resistor between PLIM and GND sets the maximum MOSFET’s power dissipation. Additionally, an external capacitor filters the PLIM voltage, allowing short high-power transients to occur periodically. 5 SS/EN Soft-Start and Enable Input. Connect SS/EN to an open-drain output. When SS/EN is pulled low, the linear regulator shuts down and pulls the output to ground. Connect a soft-start capacitor from SS/EN to GND to slowly ramp up the current limit during startup (see the Soft-Start and Enable section). 6 FB Feedback Input. Connect FB to VCC for a fixed 1.5V output, or connect FB to GND for a fixed 1.2V output. For an adjustable output, connect FB to a resistive divider from the output voltage. The FB regulation level is 0.5V. 7 CSN Negative Current-Sense Input and Output Sense Input. Connect to the negative terminal of the current-sense element as shown in Figure 1. CSN serves as the feedback input in fixed-voltage mode (FB = GND or VCC). When the MAX8704 is disabled, the output is discharged through a 10Ω resistor to GND. 8 CSP Positive Current-Sense Input. Connect to the positive terminal of the current-sense element as shown in Figure 1. The MAX8704 driver reduces the gate voltage when the current-limit threshold is exceeded. 9 GND Ground 10 DRV Gate Drive for the External n-Channel MOSFET Detailed Description The MAX8704 is a low-dropout, external n-channel MOSFET linear regulator for low-voltage notebook power supplies. The regulator uses two separate supplies—the notebook’s 5V bias supply (VCC) for driving the external n-channel MOSFET, and the lowest system supply available for the power input (VIN). By using separate bias and power inputs, the MAX8704 maximizes the gate drive while minimizing the power loss. The regulator provides an accurate (-2% typ load regulation) output that delivers up to 5A for powering the low-voltage (1.0V, 1.2V, 1.5V, and 1.8V) supplies required by notebook chipsets. Figure 1 shows the standard application circuit, and Figure 2 shows the functional diagram. The MAX8704 standard application circuit delivers up to 5A and operates with input voltages up to 5.5V, but not simultaneously. Continuous high output currents can only be achieved when the input-to-output differential voltage is low (Figure 1). 5.0V Bias Supply (VCC) The VCC input powers the control circuitry and provides the gate drive to the external n-channel MOSFET. This improves efficiency by allowing VIN to be powered from a low-voltage system supply. Power VCC from a wellregulated 5V supply. Current drawn from the VCC supply remains relatively constant with variations in VIN and _______________________________________________________________________________________ 7 MAX8704 Pin Description MAX8704 High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET load current. Bypass VCC with a 1µF or greater ceramic capacitor as close to the MAX8704 as possible. Undervoltage Lockout (UVLO) The VCC input undervoltage-lockout (UVLO) circuitry ensures that the regulator starts up with a gate-drive voltage that can adequately bias the external n-channel MOSFET. The UVLO threshold is 4.2V (typ), and VCC must remain above this level for proper operation. Power-Supply Input (VIN) The power input supply (V IN ) sources the current required by the linear regulator’s output (VOUT). VIN connects to the drain of the external n-channel power MOSFET. VIN may be as low as 1.0V, minimizing the power dissipation across the n-channel MOSFET. Bypass VIN with a 10µF or greater capacitor as close to the external MOSFET as possible. To avoid input voltage sag during a load transient, the input supply should provide a low source impedance. If a highimpedance source is used, additional input bulk capacitance is required near the MAX8704. Soft-Start and Enable (SS/EN) As shown in Figure 2, a capacitor on SS/EN allows a gradual buildup of the MAX8704 current limit, reducing the initial inrush current peaks at startup. The input supply UVLO and thermal-overload fault trigger the internal SS/EN pulldown resistor (RSS/EN = 1kΩ), automatically forcing the MAX8704 into shutdown. When properly INPUT 1.8V TO 5.5V CIN1 100µF RSENSE 10mΩ N1 IRF7401 OUTPUT (VOUT) 1.5V AT 5A (MAX) CIN2 10µF COUT 2 x 22µF DRV VIN CSP 5V BIAS SUPPLY C1 1.0µF R1 20kΩ CSN VCC R3 100kΩ FB R2 10kΩ MAX8704 PGOOD GND SS/EN PLIM POWERGOOD ON OFF RPLIM 200kΩ CSS 0.01µF Figure 1. Standard Application Circuit powered (VCC above UVLO), the MAX8704 charges the soft-start capacitor with a constant 5µA current source (see the Soft-Start Capacitor Selection section). Once the SS/EN voltage rises above 0.5V, the linear regulator Table 1. MOSFET Selection (>1.5V Output-Voltage Applications) RDS(ON) (mΩ) 2.5V 1.8V VDS (V) CISS* (nF) PACKAGE FDS6574A 7 9 20 8 SO-8 (2.5W) Fairchild Si4836DY 4 5 12 7 SO-8 (2.5W) Siliconix (Vishay) MOSFET VENDOR *CISS when VDS = 1V Table 2. MOSFET Selection (0.5V to 1.5V Output-Voltage Applications) RDS(ON) (mΩ) 4.5V 2.5V VDS (V) CISS* (nF) PACKAGE VENDOR IRF7401 22 30 20 2.7 SO-8 (2.5W) International Rectifier NDS8425 22 28 20 1.4 SO-8 (2.5W) Fairchild MOSFET FDS6572A 6 8 20 6.2 SO-8 (2.5W) Fairchild FDS7064N 7.5 — 30 3.7 Bottomless SO-8 (3W) Fairchild Si9426DY 13.5 16 20 3.5 SO-8 (2.5W) Siliconix (Vishay) Si4866DY Si7882DP 5.5 8 12 3.2 SO-8 (2.5W) PowerPAK (5W) Siliconix (Vishay) *CISS when VDS = 1V 8 CPLIM 0.1µF _______________________________________________________________________________________ High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET CIN OUTPUT (VOUT) COUT VIN DRV CSP CSN MAX8704 RSENSE N1 INPUT 1.0V TO 5.5V 10Ω R1 SHDN FB VCC 5V BIAS SUPPLY R2 DUAL-MODE FEEDBACK C1 THERMAL SHDN 5µA ERROR AMP CONTROL BLOCK 0.5V GND SHDN OFF ON SS/EN 0.5V CURRENT LIMIT (FIGURE 3) CSS 1kΩ RSS/EN LOGIC SUPPLY POK 0.91 x REF R3 FB PLIM MULTIPLIER PLIM PGOOD POWERGOOD CPLIM DELAY LOGIC RPLIM MAX8704 Figure 2. Functional Diagram is enabled. As the voltage on SS/EN continues to increase, the current-limit threshold slowly ramps up, effectively limiting the input inrush current during power-up (Figure 3). The MAX8704 reaches the full current limit when the SS/EN voltage exceeds 2V. When SS/EN is pulled low—either by an external opendrain output or by the internal power-OK (POK) lockout signal—the MAX8704 pulls the driver (DRV) low and discharges the output through a 10Ω discharge FET. Drive SS/EN with a push/pull output to bypass soft-start. Output Voltage and Dual Mode™ Feedback The MAX8704’s Dual-Mode operation allows the selection of two common preset voltages without requiring external components. Connect FB to VCC for a fixed 1.5V output, or connect FB to GND for a fixed 1.2V output. Alternatively, the output voltage can be adjusted using a resistive voltage-divider (Figure 2). The adjusted output voltage is: R1 VOUT = VFB 1 + R2 Dual Mode is a trademark of Maxim Integrated Products, Inc. _______________________________________________________________________________________ 9 MAX8704 High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET 1.0V, the MAX8704 folds back the current limit to reduce the power dissipation across the external components (Figure 3). The power limit allows an output short for an indefinite period of time without damaging the MAX8704 or its external components. VCC 1V PLIM SS/EN CURRENT LIMIT TO CONTROL BLOCK 2V CSP Thermal-Overload Protection Thermal-overload protection prevents the MAX8704 from overheating. When the junction temperature exceeds +140°C, the linear regulator automatically pulls PGOOD low and enters shutdown—the MAX8704 pulls SS/EN low with an internal 1kΩ pulldown resistor. This disables the driver and discharges the output, allowing the device to cool. Once the junction temperature cools by 20°C, the thermal protection circuit releases the SS/EN input, allowing the MAX8704 to automatically power up using the soft-start sequence. A continuous thermal-overload condition results in a pulsed output. CSN Power-Good Figure 3. Current-Limit Functional Diagram where the feedback threshold (VFB) equals 0.5V, as specified in the Electrical Characteristics table. The minimum adjustable output voltage is 0.5V (FB = CSN). The maximum adjustable output voltage is limited by the gate driver’s output-voltage swing range (see the Electrical Characteristics table) and the gate threshold of the selected n-channel MOSFET. Fault Protection Current Limit The MAX8704 features a current limit (Figure 3) that monitors the voltage across the current-sense resistor, typically limiting the CSP to CSN voltage to 50mV. When the CSP to CSN voltage reaches the current-limit threshold, the MAX8704 regulates the output current rather than the output voltage. During startup, the softstart circuit ramps the current limit to reduce the input surge current (see the Soft-Start Capacitor Selection section). MOSFET Power-Limit Protection The MAX8704 includes a proprietary power-limit circuit to protect the external n-channel MOSFET, especially under short-circuit conditions. The MAX8704 uses an internal multiplier circuit to generate an output current (I PLIM ) that is directly proportional to the MOSFET power dissipation. When the PLIM voltage exceeds 10 The MAX8704 provides an open-drain PGOOD output that goes high 3ms (typ) after the output initially reaches regulation. PGOOD transitions low immediately after the output voltage drops below 92% (typ) of the nominal regulation voltage, or when the MAX8704 enters shutdown. Connect a pullup resistor from PGOOD to VCC for a logic-level output. Use a 100kΩ resistor to minimize current consumption. Design Procedure External MOSFET Selection The external MOSFET selection depends on the gate threshold voltage, input-to-output voltage, and package power dissipation. The MAX8704 uses an external nchannel MOSFET controlled by a 5V driver, so the maximum gate-to-source voltage across the MOSFET (VGS(MAX)) is equivalent to: VGS(MAX) = VDRV(MAX) - VCSP where the maximum drive voltage is approximately VCC - 1V. The selected MOSFET’s on-resistance must be low enough to support the minimum input-to-output differential voltage (dropout voltage) and maximum load required by the application: RDS(ON)(MIN) = VIN(MIN) − VCSLIMIT − VOUT IOUT(MAX) For output voltages less than 1.5V, standard MOSFETs that provide on-resistance specifications with 2.5V gate-to-source voltages are sufficient. For output voltages greater than 1.5V, use low-threshold MOSFETs ______________________________________________________________________________________ High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET OUTPUT CAPACITANCE vs. LOAD CURRENT 50 VIN > VOUT + 0.2V 40 COUT (µF) MOSFET Power Dissipation The maximum power dissipation of the MAX8704 depends on the thermal resistance of the external nchannel MOSFET package, the board layout, the temperature difference between the die and ambient air, and the rate of airflow. The power dissipated in the MOSFET is: PDIS = IOUT x (VIN - VCSP) The maximum power dissipation allowed is determined by the following formula: MAX8704 that provide on-resistance specifications with a 1.8V gate-to-source voltage. 30 20 10 RDIS(MAX) = TJ(MAX) − TA θJC + θCA where TJ(MAX) is the maximum junction temperature (+150°C), TA is the ambient temperature, θJC is the thermal resistance from the die junction to the package case, and θCA is the thermal resistance from the case through the PC board, copper traces, and other materials to the surrounding air. Standard SO-8 MOSFETs are typically rated for 2W, while new power packages (PowerPAK, DirectFET, etc.) can achieve power dissipation ratings as high as 5W. For optimum power dissipation, use a large ground plane with good thermal contact to ground and use wide input and output traces. Extra copper on the PC board increases thermal mass and reduces the thermal resistance of the board. Setting the Current Limit The current-sense voltage threshold is preset to 50mV (typ), so the achievable peak source current (IPEAK) is determined by the current-sense resistor. The currentsense resistor can be determined by: RSENSE = VCSLIMIT / IPEAK For the best current-sense accuracy, use a 1% currentsense resistor between the source of the MOSFET and the output. Setting the Power Limit The MAX8704 includes a unique power-limit protection circuit that limits the maximum power dissipation in the external MOSFET. An external resistor (RPLIM) adjusts the actual power limit as defined by the following equation: RPLIM = VPWRLIMIT PLIMIT × KPLIM × RSENSE where RSENSE is the current-sense resistor, PLIMIT is the maximum MOSFET power dissipation, the power-limit 0 1 2 3 4 5 LOAD CURRENT (A) Figure 4. Output Capacitance vs. Load Current conversion gain (K PLIM ) equals 200µA/V 2 , and the power-limit threshold (VPWRLIMIT) equals 1.0V. An external capacitor (CPLIM) adjusts the power-limit time constant (τPLIM = RPLIM x CPLIM), allowing short high-power transients while protecting against thermal stress. Short PLIM to ground to disable the power-limit protection. Input Capacitor Selection (CIN) Typically, the linear regulator is powered from the output of a step-down regulator, effectively providing a low-impedance source for the MAX8704. Under these conditions, a local 10µF or greater ceramic capacitor is sufficient for most applications. If the linear regulator is connected to a high-impedance input, low-ESR polymer capacitors are recommended on the input. Output Capacitor Selection (COUT) The MAX8704 requires 10µF/A or greater ceramic capacitor for stable operation and optimized load-transient response. For higher capacitance values, the regulator remains stable with low-ESR, polymer output capacitors as shown in the Output Capacitance vs. Load Current graph (see Figure 4). When selecting the output capacitor to provide good transient response, the capacitor’s ESR should be minimized: ∆VOUT = ∆IOUT x ESR where ∆IOUT is the maximum peak-to-peak load current step, and ∆VOUT is the transient output-voltage tolerance. ______________________________________________________________________________________ 11 INPUT MAX8704 High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET GROUND CIN COUT The MAX8704 load-transient response graphs (see the Typical Operating Characteristics) show two components of the output response: a DC load regulation and the transient response. A typical transient response for a step change in the load current from 0.5A to 3.5A is 25mV. Lowering the output impedance—increasing the output capacitor’s value and/or decreasing the ESR— attenuates the output undershoot and overshoot. PC Board Layout Guidelines RSENSE OUTPUT 5V BIAS GROUND GROUND MAX8704 Figure 5. Recommended MAX8704 Layout Using larger output capacitance can improve efficiency in applications where the load current changes rapidly. The output capacitor acts as a reservoir for the rapid transient currents, reducing the peak current supplied by the input supply and effectively lowering the I2R power loss. Soft-Start Capacitor Selection (CSS) A capacitor (CSS) connected from SS/EN to GND causes the MAX8704 output current to slowly rise during startup, reducing stress on the input supply. The rise time to full current limit (tSS) is determined by: tSS = CSS x 1.5V / ISS where ISS = 5µA is the soft-start current. Typical capacitor values between 1nF to 100nF are sufficient. Since the regulator ramps the current-limit threshold, the actual output-voltage slew rate depends on the load current and output capacitance. The MAX8704 requires proper layout to achieve the intended output power level and regulation characteristics. Proper layout involves the use of a ground plane, appropriate component placement, and correct routing of traces using appropriate trace widths (Figure 5). • Minimize high-current ground loops: connect the ground of the MAX8704, the input capacitor, and the output capacitor together at one point. • Minimize parasitic inductance: keep the input capacitor, external MOSFET, and output capacitor close together. Route the ground plane directly under the input and output power traces/planes. • To optimize performance and power dissipation, a ground plane is essential. Dedicated ground plane layers reduce trace inductance, ground impedance, and noise coupling (ground shield) between layers, and improve thermal conductivity throughout the board. • Connect the input filter capacitor less than 10mm from the MOSFET. The connecting copper trace carries large currents and must be at least 5mm wide. Use as much copper as necessary to decrease the thermal resistance of the MOSFET. In general, more copper provides better heatsinking capabilities. Chip Information TRANSISTOR COUNT: 786 PROCESS: BiCMOS Noise, PSRR, and Transient Response The MAX8704 operates with low dropout voltage and low quiescent current in notebook computers while maintaining good noise, transient response, and AC rejection. See the Typical Operating Characteristics for a graph of PSRR vs. Frequency. Improved supply-noise rejection and transient response can be achieved by increasing the values of the input and output capacitors. Use passive filtering techniques when operating from noisy sources. 12 ______________________________________________________________________________________ High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET 10LUMAX.EPS e 4X S 10 INCHES 10 H 0 0.50±0.1 0.6±0.1 1 1 0.6±0.1 BOTTOM VIEW TOP VIEW D2 MILLIMETERS MAX DIM MIN 0.043 A 0.006 A1 0.002 A2 0.030 0.037 D1 0.116 0.120 0.114 0.118 D2 0.116 E1 0.120 E2 0.114 0.118 H 0.187 0.199 L 0.0157 0.0275 L1 0.037 REF b 0.007 0.0106 e 0.0197 BSC c 0.0035 0.0078 0.0196 REF S α 0° 6° MAX MIN 1.10 0.15 0.05 0.75 0.95 3.05 2.95 3.00 2.89 3.05 2.95 2.89 3.00 4.75 5.05 0.40 0.70 0.940 REF 0.177 0.270 0.500 BSC 0.090 0.200 0.498 REF 0° 6° E2 GAGE PLANE A2 c A b D1 A1 α E1 L L1 FRONT VIEW SIDE VIEW PROPRIETARY INFORMATION TITLE: PACKAGE OUTLINE, 10L uMAX/uSOP APPROVAL DOCUMENT CONTROL NO. 21-0061 REV. I 1 1 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 13 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX8704 Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)