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19-3420; Rev 0; 9/04
ILABLE N KIT AVA EVALUATIO
High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET
General Description Features
Low-Cost, High-Current Linear Regulator 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 Power-Good (PGOOD) Open-Drain Output with 3ms Startup Delay Programmable Soft-Start Shutdown with Output Discharge
MAX8704
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. 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 MAX(R) package.
Applications
VMCH and VCCP CPU Supplies Notebook Computers Desktop Computers Servers VID Power Supplies Low-Voltage Bias Supplies
PART MAX8704EUB
Ordering Information
TEMP RANGE -40C to +85C PIN-PACKAGE 10 MAX
Pin Configuration
TOP VIEW
VIN 1 VCC PGOOD PLIM SS/EN 2 3 4 5 10 DRV 9 GND CSP CSN FB
MAX8704
8 7 6
MAX
MAX is a registered trademark of Maxim Integrated Products, Inc.
________________________________________________________________ Maxim Integrated Products
1
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.
High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET MAX8704
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 = +70C) 10-Pin MAX (derated 5.6mW/C above +70C) .........444mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
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 = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Input Voltage Range Preset Output Voltage (Fixed) Feedback Voltage Accuracy (Adjustable) Load-Regulation Error Line-Regulation Error FB Input Bias Current CSN Input Bias Current DRV Output Voltage Swing DRV Slew Rate Quiescent Supply Current (VCC) Quiescent Supply Current (VIN) Shutdown Supply Current (VCC) Shutdown Supply Current (VIN) FAULT DETECTION Thermal-Shutdown Threshold VCC Undervoltage-Lockout Threshold Current-Limit Threshold Power-Limit Threshold Power-Limit Conversion Gain Power-Limit Conversion Gain Variation VCSLIMIT VPWRLIMIT KPLIM TSHDN Rising edge, 20C hysteresis Rising edge, 15mV hysteresis PLIM = GND Rising edge VCSP - VCSN = 30mV, VCSN = 0.5V, VIN = 3.5V VCSP - VCSN = 25mV to 45mV, VCSN = 0.5V, VIN = 2V to 4.5V 45 0.96 155 +140 4.2 50 1.0 200 12 4.45 57 1.04 233 C V mV V A/V2 % ICC IIN IFB ICSN VDRV SYMBOL VIN VCC VOUT VFB FB = VCC FB = GND FB = CSN VCSP - VCSN = 45mV VIN = 1V to 5.5V VFB = 0.6V VCSN = 1.6V Output high Output low CDRV = 40nF FB forced above the regulation point, VCSN = 1.6V FB forced above the regulation point, VCSN = 1.6V SS/EN = GND SS/EN = GND VCC 1.0 -1 50 VCC 0.7 0.7 0.2 1.5 5 35 5 3 10 70 10 1.0 V/s mA A A A CONDITIONS MIN 1.0 4.5 1.462 1.170 490 -2.5 1.50 1.20 500 -2 0.01 +1 100 TYP MAX 5.5 5.5 1.538 1.230 510 UNITS V V mV % % A A V
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High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 2.5V, VCC = 5.0V, PLIM = FB = GND, CSP = CSN, SS/EN floating, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER PLIM Output Current PLIM Output Current Offset CSP Input Current SOFT-START AND SHUTDOWN Soft-Start Charge Current SS/EN Full Current Threshold SS/EN Enable Threshold SS/EN Discharge Current Discharge-Mode On-Resistance INPUTS AND OUTPUTS PGOOD Trip Threshold PGOOD Startup Delay PGOOD Output Low Voltage PGOOD Leakage Current IPGOOD ISINK = 4mA VFB = 1.0V (PGOOD high impedance), PGOOD forced to 5V -1 With respect to error-comparator threshold, 2% hysteresis -10 1 -8 3 -6 5 0.3 +1 % ms V A ISS/EN RCSN Rising edge VSS/EN = 1.5V, thermal fault, bias fault condition, or UVLO 0.4 ISS VSS/EN = 1.5V 4 5 2 0.5 10 10 0.6 20 6 A V V A SYMBOL CONDITIONS VCSP - VCSN = 30mV, VIN = 3.5V, VCSN = 0.5V CSP = CSN, VIN = 1.0V, VCSN = 0.5V VCSN = 1.50V, VCSP = 1.55V -1 MIN 14 TYP 18 0.5 MAX 21 2 +1 UNITS A A A
MAX8704
ELECTRICAL CHARACTERISTICS
(VIN = 2.5V, VCC = 5.0V, PLIM = FB = GND, CSP = CSN, SS/EN floating, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage Range Preset Output Voltage (Fixed) Feedback Voltage Accuracy (Adjustable) DRV Output Voltage Swing SYMBOL VIN VCC VOUT VFB FB = VCC FB = GND FB = CSN Output high Output low Quiescent Supply Current (VCC) Quiescent Supply Current (VIN) Shutdown Supply Current (VCC) Shutdown Supply Current (VIN) ICC IIN FB forced above the regulation point, VCSN = 1.6V FB forced above the regulation point, VCSN = 1.6V SS/EN = GND SS/EN = GND CONDITIONS MIN 1.0 4.5 1.455 1.158 485 VCC 1.1 1.1 3 10 70 10 mA A A A MAX 5.5 5.5 1.545 1.242 515 UNITS V V mV
VDRV
V
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High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET MAX8704
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 2.5V, VCC = 5.0V, PLIM = FB = GND, CSP = CSN, SS/EN floating, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER FAULT DETECTION VCC Undervoltage-Lockout Threshold Current-Limit Threshold Power-Limit Threshold PLIM Output Current SOFT-START AND SHUTDOWN Soft-Start Charge Current SS/EN Enable Threshold INPUTS AND OUTPUTS PGOOD Trip Threshold PGOOD Startup Delay PGOOD Output Low Voltage ISINK = 4mA With respect to error-comparator threshold, 2% hysteresis -11 0.5 -5 5.5 0.3 % ms V ISS VSS/EN = 0 Rising edge 4 0.4 6 0.6 A V VCSLIMIT VPWRLIMIT Rising edge, 15mV hysteresis PLIM = GND Rising edge VCSP - VCSN = 30mV, VIN = 3.5V, VCSN = 0.5V 43 0.90 13 4.45 60 1.10 22 V mV V A SYMBOL CONDITIONS MIN MAX UNITS
Note 1: Specifications to -40C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1, VOUT = 1.5V, TA = +25C, unless otherwise noted.)
OUTPUT VOLTAGE DEVIATION vs. LOAD CURRENT
MAX8704 toc01
PLIM VOLTAGE vs. LOAD CURRENT
MAX8704 toc02
OUTPUT VOLTAGE vs. INPUT VOLTAGE
1.6 1.4 1.2 VOUT (V) 10mA LOAD
MAX8704 toc03
0 -0.5 VOUT DEVIATION (%) -1.0 -1.5 -2.0 -2.5 -3.0 0 1 2 3 4 5 LOAD CURRENT (A) VIN = 3.3V VIN = 1.8V
1.5 POWER LIMIT 1.2 PLIM VOLTAGE (V) VIN = 3.3V
1.8
0.9
VIN = 2.5V VIN = 1.8V
1.0 0.8 0.6 0.4 1A LOAD
0.6
0.3 CURRENT LIMIT 0 0 1 2 3 4 5 LOAD CURRENT (A)
0.2 0 1 2 3 INPUT VOLTAGE (V) 4 5
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High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VOUT = 1.5V, TA = +25C, unless otherwise noted.)
PLIM VOLTAGE vs. INPUT VOLTAGE
MAX8704 toc04
MAX8704
5V BIAS SUPPLY CURRENT vs. INPUT VOLTAGE
MAX8704 toc05
MOSFET POWER DISSIPATION vs. DRAIN-TO-SOURCE VOLTAGE
MAX8704 toc06
1.5
5 1A LOAD BIAS SUPPLY CURRENT (mA) 4
2.5
1.2 PLIM VOLTAGE (V)
OUTPUT SHORT
2.0 1A LOAD POWER (W)
0.9 1A LOAD 0.6
3
1.5 OUTPUT SHORT 1.0
2 POWER LIMIT 1 DROPOUT 0
0.3
10mA LOAD
0.5
0 1 2 3 INPUT VOLTAGE (V) 4 5
0 1 2 3 INPUT VOLTAGE (V) 4 5 1 2 3 VDS (V) 4 5
OUTPUT CURRENT LIMIT vs. SS/EN VOLTAGE
MAX8704 toc07
SOFT-START (CSS = 10nF)
2V 1V
SHUTDOWN SEQUENCE (NO LOAD)
MAX8704 toc08 MAX8704 toc09
6 5 CURRENT LIMIT (A) 4 3 2 1 0 0 0.5 1.0 1.5 2.0 2.5
5V 0 A 5V 0 B 1.5V
A
0 1.5V 0 1A 0 3.0 C
B
C 0 1A 0 200s/div A. EN/SS, 1V/div C. FET CURRENT, 1A/div B. LDO OUTPUT, 1V/div 1.5 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
SS/EN VOLTAGE (V)
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High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET MAX8704
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VOUT = 1.5V, TA = +25C, unless otherwise noted.)
POWER LIMIT
MAX8704 toc10
3A LOAD TRANSIENT (IRF7401)
MAX8704 toc11
3A LOAD TRANSIENT (FDS6570A)
MAX8704 toc12
5V
A
3.5A 0.5A
A
3.5A 0.5A
A
0 0 1.5V 0 4A 0
B
3.5V 3.0V
B 3.0V 2.5V C 1.8V 1.7V 1.50V D 20s/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 1.45V
B
C
1.8V 1.7V 1.50V
C
D 4ms/div C. LDO OUTPUT, 1V/div A. EN/SS, 2V/div D. FET CURRENT, 5A/div B. PLIM, 0.5V/div 0.4 LOAD, CSS = 10nF, VIN = 1.8V, RPLIM = 200k, CPLIM = 0.1F
1.45V
D 20s/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
LINE TRANSIENT (1.8V TO 2.5V)
MAX8704 toc13
GAIN AND PHASE vs. FREQUENCY
60 40 20 0 -20 0.001 180 90 0 -90 -180 0.001 0.01 0.1 FREQUENCY (MHz) FB = CSN, VOUT = 0.5V, VIN = 1.0V COUT = 2 x 22F 1206 CERAMIC, IOUT = 0.5A 1 10 0.01 0.1 1 GAIN (dB)
MAX8704 toc14
2.5V 2.0V 1.5V 3.5V A
10
1.50V 1.49V
C
40s/div A. INPUT: 1.8V TO 2.5V, 0.5V/div C. OUTPUT, 10mV/div B. DRV, 200mV/div ROUT = 1.5
PHASE
3.3V 1.51V
B
GAIN AND PHASE vs. FREQUENCY
60 40 20 0 -20 0.001 180 90 0 -90 -180 0.001 0.01 0.1 FREQUENCY (MHz) FB = CSN, VOUT = 0.5V, VIN = 1.0V COUT = 100F 70m SANYO 4TPB100M, IOUT = 0.5A 1 10 0.01 0.1 1
MAX8704 toc15
PSRR
MAX8704 toc16
GAIN (dB)
40 0
10
PSRR (dB)
-40
PHASE
-80
-120 10 0.01 0.1 1 FREQUENCY (MHz) FB = VCC, VOUT = 1.5V, VIN = 2.5V COUT = 2 x 22F 1206 CERAMIC, IOUT = 0.5A 0.001
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High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET
Pin Description
PIN 1 2 NAME VIN VCC FUNCTION Input Voltage Sense. The MAX8704 senses the voltage across the external MOSFET (VIN - VCSN) to determine the MOSFET's power dissipation. Analog and Driver Supply Input. Connect to the system supply voltage (+5.0V). Bypass VCC to analog ground with a 1F or greater ceramic capacitor. 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. 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. 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). 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. 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 10 resistor to GND. 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. Ground Gate Drive for the External n-Channel MOSFET
MAX8704
3
PGOOD
4
PLIM
5
SS/EN
6
FB
7
CSN
8 9 10
CSP GND DRV
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
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High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET MAX8704
load current. Bypass VCC with a 1F 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.
INPUT 1.8V TO 5.5V CIN1 100F CIN2 10F DRV VIN 5V BIAS SUPPLY C1 1.0F VCC R3 100k CSP CSN FB R1 20k N1 IRF7401 RSENSE 10m OUTPUT (VOUT) 1.5V AT 5A (MAX) COUT 2 x 22F
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 10F 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.
MAX8704
PGOOD GND
R2 10k
POWERGOOD SS/EN ON OFF CSS 0.01F PLIM RPLIM 200k CPLIM 0.1F
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 = 1k), automatically forcing the MAX8704 into shutdown. When properly
Figure 1. Standard Application Circuit
powered (VCC above UVLO), the MAX8704 charges the soft-start capacitor with a constant 5A 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)
MOSFET FDS6574A Si4836DY RDS(ON) (m) 2.5V 7 4 1.8V 9 5 VDS (V) 20 12 CISS* (nF) 8 7 PACKAGE SO-8 (2.5W) SO-8 (2.5W) VENDOR Fairchild Siliconix (Vishay)
*CISS when VDS = 1V
Table 2. MOSFET Selection (0.5V to 1.5V Output-Voltage Applications)
MOSFET IRF7401 NDS8425 FDS6572A FDS7064N Si9426DY Si4866DY Si7882DP RDS(ON) (m) 4.5V 22 22 6 7.5 13.5 5.5 2.5V 30 28 8 -- 16 8 VDS (V) 20 20 20 30 20 12 CISS* (nF) 2.7 1.4 6.2 3.7 3.5 3.2 PACKAGE SO-8 (2.5W) SO-8 (2.5W) SO-8 (2.5W) Bottomless SO-8 (3W) SO-8 (2.5W) SO-8 (2.5W) PowerPAK (5W) VENDOR International Rectifier Fairchild Fairchild Fairchild Siliconix (Vishay) Siliconix (Vishay)
*CISS when VDS = 1V 8 _______________________________________________________________________________________
High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET MAX8704
INPUT 1.0V TO 5.5V CIN N1 RSENSE COUT OUTPUT (VOUT)
VIN
DRV
CSP
CSN
10
SHDN FB 5V BIAS SUPPLY C1 THERMAL SHDN CONTROL BLOCK VCC DUAL-MODE FEEDBACK ERROR AMP 0.5V GND
R1
R2
5A
OFF ON SS/EN CSS 1k LOGIC SUPPLY R3 POWERGOOD PGOOD DELAY LOGIC RSS/EN 0.5V
SHDN
CURRENT LIMIT (FIGURE 3)
POK 0.91 x REF
FB
PLIM MULTIPLIER PLIM CPLIM 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 10 discharge FET. Drive SS/EN with a push/pull output to bypass soft-start.
Dual Mode is a trademark of Maxim Integrated Products, Inc.
Output Voltage and Dual ModeTM 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
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High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET MAX8704
VCC
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. Thermal-Overload Protection Thermal-overload protection prevents the MAX8704 from overheating. When the junction temperature exceeds +140C, the linear regulator automatically pulls PGOOD low and enters shutdown--the MAX8704 pulls SS/EN low with an internal 1k pulldown resistor. This disables the driver and discharges the output, allowing the device to cool. Once the junction temperature cools by 20C, 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.
1V
PLIM SS/EN
CURRENT LIMIT TO CONTROL BLOCK
2V
CSP
CSN
Power-Good
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 100k resistor to minimize current consumption.
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.
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)
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
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
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High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET
that provide on-resistance specifications with a 1.8V gate-to-source voltage. 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: RDIS(MAX) = TJ(MAX) - TA JC + CA
0 1 2 3 4 5 LOAD CURRENT (A) 50 VIN > VOUT + 0.2V 40
MAX8704
OUTPUT CAPACITANCE vs. LOAD CURRENT
COUT (F)
30
20
10
where TJ(MAX) is the maximum junction temperature (+150C), TA is the ambient temperature, JC is the thermal resistance from the die junction to the package case, and CA 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.
Figure 4. Output Capacitance vs. Load Current
conversion gain (K PLIM ) equals 200A/V 2 , and the power-limit threshold (VPWRLIMIT) equals 1.0V. An external capacitor (CPLIM) adjusts the power-limit time constant (PLIM = 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 10F 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.
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.
Output Capacitor Selection (COUT)
The MAX8704 requires 10F/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.
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 x KPLIM x RSENSE
where RSENSE is the current-sense resistor, PLIMIT is the maximum MOSFET power dissipation, the power-limit
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11
High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET MAX8704
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.
INPUT
GROUND CIN
RSENSE 5V BIAS
GROUND GROUND MAX8704
COUT OUTPUT
PC Board Layout Guidelines
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.
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 = 5A 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.
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
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High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET
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.)
10LUMAX.EPS
MAX8704
e
10
4X S
10
INCHES 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 0 6
MILLIMETERS 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
H 0 0.500.1 0.60.1
1
1
0.60.1
TOP VIEW
BOTTOM VIEW
D2 GAGE PLANE A2 A b D1 A1
E2 c E1 L1
L
FRONT VIEW
SIDE VIEW
PROPRIETARY INFORMATION TITLE:
PACKAGE OUTLINE, 10L uMAX/uSOP
APPROVAL DOCUMENT CONTROL NO. REV.
21-0061
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 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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