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19-1307; Rev 0a; 11/97
KIT ATION EVALU BLE AVAILA
Step-Up/Down DC-DC Converter in QSOP Package
____________________________Features
o Step-Up/Down Voltage Conversion o 1.8V to 11V Input Range o 3.3V/5V or Adjustable Output Voltage Range o Output Current: 300mA at 5V (VIN 2.5V) 150mA at 5V (VIN 1.8V) o Smaller Inductor than SEPIC and Flybacks o Load Disconnects from Input in Shutdown o Supply Current from Battery: 85A (No-Load) 0.1A (Shutdown) o PGI/PGO Low-Battery Comparator o 16-Pin QSOP Package (same footprint as 8-pin SO) o No External FETs Required o Thermal and Short-Circuit Protection
________________General Description
The MAX1672 integrates a step-up DC-DC converter with a linear regulator to provide step-up/down voltage conversion. This device provides a constant output voltage for inputs that vary above and below the output voltage. It has a 1.8V to 11V input range and a preset 3.3V or 5V output. The output can also be set from 1.25V to 5.5V using two resistors. Typical efficiency is 85%. The MAX1672's step-up/linear-regulator configuration permits the use of a single, physically smaller inductor than can be used with competing SEPIC and flyback configurations. Switch current is also selectable, permitting the use of smaller inductors in low-current applications. The linear regulator also acts as a filter to reduce output ripple voltage. The MAX1672 has a low 85A quiescent supply current, which is further reduced to 0.1A in logic-controlled shutdown. The output voltage is disconnected from the input in shutdown. The MAX1672 also has a PGI/PGO low-battery detector. The MAX1672 comes in a 16-pin QSOP package (same size as a standard 8-pin SO). For a larger device that delivers more output current, refer to the MAX710/MAX711. The preassembled MAX1672 evaluation kit is available to speed designs.
MAX1672
________________________Applications
Single-Cell, Lithium-Powered Portable Devices 3.3V and Other Low-Voltage Systems Digital Cameras 2-Cell to 4-Cell AA Alkaline Hand-Held Equipment Battery-Powered Devices with AC Input Adapters
______________Ordering Information
PART MAX1672C/D MAX1672EEE TEMP. RANGE 0C to +70C -40C to +85C PIN-PACKAGE Dice* 16 QSOP
*Dice are tested at TA = +25C.
__________Typical Operating Circuit
INPUT 1.8V TO 11V IN PGI LX PS
__________________Pin Configuration
TOP VIEW
LX 1 PGND 2
16 LX 15 PGND 14 GND
MAX1672
ON ON 3.3V 0.5A OFF OFF 5V 0.8A ONA ONB 3/5 ILIM REF PGND GND OUT 3.3V/5V OUTPUT
ONB 3 ONA 4 3/5 5 PGI 6 PG0 FB LOW-BATTERY DETECTOR OUTPUT PGO 7 ILIM 8
MAX1672
13 REF 12 IN 11 PS 10 FB 9 OUT
QSOP
________________________________________________________________ Maxim Integrated Products
1
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Step-Up/Down DC-DC Converter in QSOP Package MAX1672
ABSOLUTE MAXIMUM RATINGS
IN, PS, LX, OUT, PGO to GND ......................... -0.3V to +11.5V ILIM, ONA, ONB, FB, 3/5, REF, PGI to GND......................................-0.3V to (VPS + 0.3V) PGND to GND .......................................................-0.3V to +0.3V OUT Short Circuit to GND ..........................................Continuous Output Current ..................................................................350mA Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derate above +70C by 8.3mW/C).......667mW Operating Temperature Range ......................... -40C to +85C Junction Temperature .................................................... +150C Storage Temperature Range ........................... -65C to +160C Lead Temperature (soldering, 10sec) ............................ +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
(VPS = 6V, CREF = 0.1F, COUT = 4.7F, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1) PARAMETER Input Voltage Startup Voltage FB = GND, IOUT = 0mA to 150mA 3/5 = GND 3/5 = PS TA = 0C to +85C TA = -40C to +85C TA = 0C to +85C TA = -40C to +85C 4.8 4.75 3.17 3.13 1.25 VIN = 2V, 3/5 = GND, FB = GND, IOUT = 10mA to 150mA VIN = 3V to 5V, 3/5 = GND, IOUT = 100mA ONA = PS or ONB = GND, current measured into PS pin, IOUT = 0mA ONA = GND, ONB = PS, current measured into PS pin IREF = 0mA OUT = FB Hysteresis = 15mV typical VFB = 1.3V VIN = GND to 11V VPS = 5.5V, ILX = 50mA VPS = 2.7V, ILX = 50mA VLX = 11V, ONA = GND, ONB = PS ILIM = GND LX Current Limit ILIM = PS Output PFET Resistance VPS = 5.5V, IOUT = 50mA VPS = 2.7V, IOUT = 50mA VPS = 5.5V 0.35 TA = 0C to +85C TA = -40C to +85C TA = 0C to +85C TA = -40C to +85C 0.35 0.3 0.6 0.5 TA = 0C to +85C TA = -40C to +85C 1.21 1.21 1.20 70 1 3 0.6 0.9 0.1 0.5 0.5 0.8 0.8 1.2 2.3 0.1 0.7 50 6 1.3 2.0 1 0.65 0.7 1.0 1.1 2.4 4.6 1 1.4 A A A 0.003 0.15 85 0.1 1.25 1.25 125 1 1.29 1.29 1.30 3.30 5.00 CONDITIONS MIN 1.8 0.9 5.2 5.25 3.43 3.47 5.5 V %/mA %/V A A V V mV nA A A V TYP MAX 11.0 UNITS V V
Output Voltage
Output Voltage Adjustment Range Output Load Regulation Output Line Regulation Quiescent Current Shutdown Quiescent Current Reference Voltage FB Voltage FB Dual-Mode Trip Threshold FB Input Current IN Input Current LX On-Resistance LX Leakage Current
Output PFET Leakage Current VOUT = 0V, ONA = GND, ONB = PS Output PFET Current Limit
2
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Step-Up/Down DC-DC Converter in QSOP Package
ELECTRICAL CHARACTERISTICS (continued)
(VPS = 6V, CREF = 0.1F, COUT = 4.7F, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1) PARAMETER Thermal Shutdown Threshold Thermal Shutdown Hysteresis PGI/PGO COMPARATOR PGI Input Bias Current Hysteresis PGI Threshold Voltage PGO Output Leakage PGO Output Low Voltage LOGIC AND CONTROL INPUTS Input Low Voltage Input High Voltage Input Bias Current ONA, ONB, 3/5; ILIM ONA, ONB, 3/5; ILIM ONA, ONB, 3/5, ILIM 1.6 1 100 0.4 V V nA TA = 0C to +85C TA = -40C to +85C VPGO = 11V IPGO = 2mA, VPGI = 1.2V 1.21 1.19 1.25 0.1 0.1 VPGI = 1.3V CONDITIONS MIN TYP 150 20 1 30 1.29 1.31 1 0.4 50 MAX UNITS C C nA mV V A V
MAX1672
Note 1: Specifications to -40C are guaranteed by design.
__________________________________________Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT (VOUT = 5V)
MAX1672-01
EFFICIENCY vs. OUTPUT CURRENT (VOUT = 3.3V)
MAX1672-02
EFFICIENCY vs. INPUT VOLTAGE (IOUT = 10mA)
MAX1672-03
100 VIN = 5V VIN = 2.7V 80 VIN = 3.6V
100 VIN = 3.3V VIN = 2.7V
100
90 EFFICIENCY (%)
90 EFFICIENCY (%)
90 EFFICIENCY (%) VOUT = 5V
80
80
70 VIN = 1.8V 60 VIN = 0.9V
70 VIN = 1.8V 60 VIN = 0.9V
70
VOUT = 3.3V
60
50 0.1 1 10 100 1000 OUTPUT CURRENT (mA)
50 0.1 1 10 100 1000 OUTPUT CURRENT (mA)
50 0 1 2 3 4 5 6 7 8 9 10 INPUT VOLTAGE (V)
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Step-Up/Down DC-DC Converter in QSOP Package MAX1672
_____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE (POWER DISSIPATION LIMIT)
MAX1672-05
MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE (VOUT = 5V)
MAX1672-04
MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE (VOUT = 3.3V)
400 MAXIMUM OUTPUT CURRENT (mA) MAXIMUM RECOMMENDED OUTPUT CURRENT 300 ILIM = PS (0.8A) 200 ILIM = GND (0.5A) 400 MAXIMUM OUTPUT CURRENT (mA)
MAXIMUM OUTPUT CURRENT (mA)
MAXIMUM RECOMMENDED OUTPUT CURRENT 300
300
MAXIMUM RECOMMENDED OUTPUT CURRENT VOUT = 5V
200
ILIM = PS (0.8A) ILIM = GND (0.5A)
200 VOUT = 3.3V 100 VOUT = 5V TA = +25C TA = +85C VOUT = 3.3V 7 8 9 10 11
100
100
0 0 1 2 INPUT VOLTAGE (V) 3 4
0 0 1 2 INPUT VOLTAGE (V) 3 4
0 4 5 6 INPUT VOLTAGE (V)
MAXIMUM RECOMMENDED INPUT VOLTAGE
MAX1672-12
12
NO-LOAD BATTERY CURRENT vs. INPUT VOLTAGE
MAX1672-07
SHUTDOWN CURRENT vs. INPUT VOLTAGE
MAX1672-08
LINEAR REGULATOR POWER-SUPPLY REJECTION RATIO vs. FREQUENCY
MAX1672-09
1000
1
70 60 50 PSRR (dB)
SUPPLY CURRENT (A)
VOUT = 5V
100 VOUT = 3.3V
SUPPLY CURRENT (A)
0.1
40 30 20
10 0 1 2 3 4 5 6 7 8 9 10 11 INPUT VOLTAGE (V)
0.01 0 1 2 3 4 5 6 7 8 9 10 11 INPUT VOLTAGE (V)
10 0.01 0.1 1 10 100 1000 FREQUENCY (kHz)
LINE-TRANSIENT RESPONSE
MAX1672-10
LOAD-TRANSIENT RESPONSE
MAX1672-11
OUTPUT RIPPLE (MEDIUM LOAD)
A
A
B B B
2ms/div A: VOUT = 5V (100mV/div, AC COUPLED) B: VIN = 2V TO 4V (IOUT = 100mA)
2ms/div A: VOUT = 5V (50mV/div, AC COUPLED) B: IOUT = 10mA TO 100mA (VIN = 2V)
10s/div A: VOUT = 5V (20mV/div, AC COUPLED) B: IL1 (500mA /div) (VIN = 2.7V, IOUT = 80mA)
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MAX1672-06
400
A
Step-Up/Down DC-DC Converter in QSOP Package
_____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
MAX1672
OUTPUT RIPPLE (HEAVY LOAD)
MAX1672-13
START-UP DELAY
MAX1672-14
TURN-OFF DELAY
MAX1672-15
A A
A B B B 10s/div A: VOUT = 5V (20mV/div, AC COUPLED) B: IL1 (500mA /div) (VIN = 2.7V, IOUT = 250mA)
50s/div A: VOUT = (2V/div) B: ONB (2V/div) (VIN = 2.7V, RLOAD = 50)
200s/div A: VOUT = (2V/div) B: ONB (2V/div) (VIN = 2.7V, RLOAD = 50)
______________________________________________________________Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 NAME LX PGND ONB ONA 3/5 PGI PGO ILIM OUT FB PS IN REF GND PGND LX FUNCTION Inductor Connection to the Drain of the Internal N-Channel Power MOSFET Power Ground On Control Input. When ONB = low or ONA = high, the IC is on. Connect ONB to GND for normal operation (Table 1). On Control Input. When ONA = low and ONB = high, the IC is off. Connect ONA to PS for normal operation (Table 1). Output Voltage Selection Input. Connect to PS for 3.3V output and to GND for 5V output. With VFB > 80mV, the state of the 3/5 pin is ignored. (Table 2). Low-Battery Detector Input (1.25V threshold) Low-Battery Detector Output (open drain). PGO pulls low when VPGI is greater than 1.25V. Inductor-Current-Limit Selection Input. Connect to PS for 0.8A current limit and to GND for 0.5A current limit. Regulator Output. Drain of internal PFET linear regulator. Bypass with a 4.7F capacitor to GND. Feedback Input. For 3.3V or 5V output, connect to GND. For adjustable output, connect to feedback resistordivider network. With VFB > 70mV, the state of the 3/5 pin is ignored. Bootstrapped Power Supply. Output of step-up switch-mode regulator and source of internal PFET linear regulator. The IC is powered from this pin. Input Voltage Sense Input. Connect to input supply. Reference Voltage Output. Bypass with a 0.1F capacitor to GND. Analog Ground Power Ground Inductor Connection to the Drain of the Internal N-Channel Power MOSFET
_______________________________________________________________________________________
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Step-Up/Down DC-DC Converter in QSOP Package MAX1672
LX REF ERROR AMP 2 FB OFF TON FIXED TOFF GENERATOR
DRV
N
IN INPUT MONITOR OUT
100mV CURRENT-LIMIT COMPARATOR
ILIM PGND
MAX1672
PS ONA ONB REF FB2 FB FB FB1 OUT REFERENCE GENERATOR REF REF + OFFSET
PS ERROR AMP 1
P
70mV
3/5 PGO REF N PGI N GND
Figure 1. Functional Diagram
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Step-Up/Down DC-DC Converter in QSOP Package
_______________Detailed Description
The MAX1672 integrates a step-up, switch-mode DCDC converter with a linear regulator to provide stepup/down voltage conversion. The step-up converter contains an N-channel power MOSFET switch, while the linear regulator contains a P-channel MOSFET pass element (Figure 1). The step-up converter and the linear regulator share the same precision voltage reference. The MAX1672's input range is from +1.8V to +11V, and the regulated output is internally preset to +3.3V or +5V, or can be adjusted with two external resistors. Boost efficiency typically exceeds 80% over a 2mA to 200mA load range. The device is bootstrapped with chip power derived from the stepped-up voltage output at PS. The MAX1672 typically starts up with a 0.9V input. The MAX1672's step-up/linear-regulator configuration permits the use of a physically smaller inductor than competing SEPIC and flyback configurations because the 1/2LI2 requirements of a step-up converter are half those of SEPIC and flyback converters. Also, high-frequency switching and selectable peak inductor current limit allow for low inductor value (10H) and low current saturation rating, respectively, further reducing the inductor's physical dimensions. The MAX1672 maximizes efficiency in both step-up and step-down operation. In step-up mode, when VIN < VOUT, only the step-up regulator is active, while the linear regulator behaves as a 1.2 (at 5V output) PFET switch. This provides optimum efficiency (typically 85%). In low-dropout, step-down operation, when VIN is slightly greater than VOUT, both the step-up regulator and linear regulator are active. The step-up regulator is automatically enabled to maintain headroom across the linear regulator (typically 1V above the 5V output). In this case, boost ripple is rejected by the linear regulator, and OUT remains in regulation with no dropout. In normal step-down operation, when VIN is significantly greater than VOUT, only the linear regulator is active. The mode of operation is automatically controlled onchip through the IN pin, which compares V IN and VOUT. Transitions between step-up, low-dropout stepdown, and normal step-down operation are stable, but can be seen as small variations in the output DC level and output ripple.
Step-Up Switch-Mode Converter
A pulse-frequency-modulation (PFM) control scheme, with a constant 1s off-time and variable on-time, controls the N-channel MOSFET switch. A pulse is initiated whenever OUT falls out of regulation. The N-channel switch then turns off when the inductor current reaches the peak current limit or after the 4s maximum on-time, whichever occurs first. This control architecture provides high-efficiency, discontinuous inductor current under light loads as well as continuous inductor current under heavy loads. The switching frequency and output ripple are a function of load current and input voltage.
MAX1672
Linear Regulator
The low-dropout linear regulator consists of a reference, an error amplifier, and a P-channel MOSFET. The reference is connected to the error amplifier input. The error amplifier compares this reference with the selected feedback voltage and amplifies the difference. The difference is conditioned and applied to the P-channel pass transistor's gate.
ILIM
The current-limit-select input, ILIM, selects between the two peak inductor current limits: 0.8A (ILIM = PS) and 0.5A (ILIM = GND). If the application requires low output current (see Typical Operating Characteristics), select 0.5A. The lower peak current limit allows for a smaller, lower-cost inductor, and reduced output ripple.
On/Off Control
The MAX1672 is turned on or off by logic inputs ONA and ONB (Table 1). When ONA = 1 or ONB = 0, the device is on. When ONA = 0 and ONB = 1, the device shuts down (see the Applications Information section). For normal (on) operation, connect ONA to PS and ONB to GND. Shutdown mode turns off the MAX1672 completely, disconnecting the input from the output and actively pulling OUT to GND.
Table 1. On/Off Logic Control
ONA 0 0 1 1 ONB 0 1 0 1 MAX1672 On Off On On
_______________________________________________________________________________________
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Step-Up/Down DC-DC Converter in QSOP Package MAX1672
__________________Design Procedure
Output Voltage Selection
For fixed output voltages of 3.3V or 5V, connect 3/5 to PS or GND and connect FB to GND (Table 2). Alternatively, adjust the output voltage from 1.25V to 5.5V by connecting two resistors, R1 and R2 (Figure 2), which form a voltage divider between OUT and FB. Choose resistor values as follows: R1 = R2[(VOUT / VREF) -1] where VREF = 1.25V. Since the input bias current at FB has a maximum value of 50nA, R1 and R2 can be large with no significant accuracy loss. Choose R2 in the 100k to 270k range and calculate R1 using the above formula. For 1% error, the current through R1 should be at least 100 times FB's bias current. Whenever the voltage at FB exceeds 70mV above GND, the state of the 3/5 pin is ignored. Connect 3/5 to GND when adjusting VOUT with a resistor divider. Never leave 3/5 unconnected.
INPUT 1.8V TO 11V C1 100F R3 L1 10H
IN PGI
LX PS C2 100F
R4
MAX1672
ON ON OFF OFF ONA ONB 3/5 OUT R1 R5 1M
1.25V TO 5.5V OUTPUT C4 4.7F
0.5A
0.8A C3 0.1F
ILIM REF PGND
PG0 FB GND R2
LOW-BATTERYDETECTOR OUTPUT
Low-Battery Detection
The MAX1672 contains a comparator for low-battery detection. If the voltage at PGI falls below VREF (typically 1.25V), the open-drain comparator output (PGO) goes high. Hysteresis is typically 30mV. Set the lowbattery detector's threshold with resistors R3 and R4 (Figure 2) using the following equation: R3 = R4[(VPGT / VREF) -1] where VPGT is the desired threshold of the low-battery detector and VREF = 1.25V. Since the input bias current at PGI has a maximum value of 50nA, R3 and R4 can be large to minimize input loading with no significant accuracy loss. Choose R4 in the 100k to 270k range and calculate R3 using the above formula. For 1% error, the current through R3 should be at least 100 times PGI's bias current. The PGO output is open-drain and should be pulled high with external resistor R5 for normal operation. If the low-battery comparator is not used, connect PGI and PGO to GND.
Figure 2. Adjustable Output Voltage Configuration
Inductor Selection
A 10H inductor performs well in most MAX1672 applications. Smaller inductor values typically offer a smaller physical size for a given series resistance, but may increase switching losses. Larger inductor values exhibit higher output current capability and larger physical dimensions for a given series resistance. For optimum performance, choose an inductor value from Table 3 or by using the following equation:
(VOUT + VDIODE ) t
ILIM < ILIM
OFF < L
(VIN(min) + VSWITCH ) 2t
ON(max)
Table 2. Output Voltage Control
5 3/5 0 1 X FB GND GND >70mV VOUT (V) +5 +3.3 +1.25 to +5.5
where ILIM is the peak switch-current limit, which is 0.8A for ILIM = PS and 0.5A for ILIM = GND. The inductor's incremental saturation current rating should also be greater than the peak switch-current limit. However, it is generally acceptable to bias most inductors into saturation by as much as 20% with slightly reduced efficiency. The inductor's DC resistance significantly affects efficiency. See Tables 4 and 5 for a list of suggested inductors and suppliers.
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_______________________________________________________________________________________
Step-Up/Down DC-DC Converter in QSOP Package MAX1672
Table 3. Suggested Inductor Values
5 3/5 0 (5V) 0 (5V) 1 (3.3V) 1 (3.3V) ILIM 0 (0.5A) 1 (0.8A) 0 (0.5A) 1 (0.8A) INDUCTOR VALUE (H) 10 to 22 10 10 4.7 to 10 CAPACITORS L1 10H
Table 4. Suggested Components
INDUCTORS Sumida CD43-100 (1.04A, 0.182) CD54-100 (1.44A, 0.100) CDRH73-100 (1.68A, 0.072) Coilcraft DT1608C-103 (0.7A, 0.095) AVX TPSE Series Sprague 593D or 595D Series Motorola MBRS130LT3 (1.0A, 30V) MBR0520LT3 (0.5A, 20V) Schottky International Rectifier 10BQ40 (1.0A, 40V) 1N5817 Equivalent
Capacitor Selection
The equivalent series resistance (ESR) of both bypass and filter capacitors affects efficiency and output ripple. Output voltage ripple is the product of peak inductor current and filter capacitor ESR. Use low-ESR capacitors for best performance, or connect two or more filter capacitors in parallel. A 100F, 16V, input bypass capacitor (C1) with low ESR reduces peak battery currents and reflected noise due to inductor current ripple. Smaller ceramic capacitors may also be used for light loads or in applications that can tolerate higher input ripple. A 100F, 16V, surface-mount (SMT) tantalum PS filter capacitor (C2) with 0.1 ESR typically exhibits 20mV output ripple (at OUT) when stepping up from 2V to 5V at 100mA load. Smaller capacitors (down to 10F with higher ESR) are acceptable for light loads or in applications that can tolerate higher output ripple. Only 4.7F is needed at OUT (C4) to maintain linear regulator stability. During boost operation, this capacitor reduces output voltage spikes from the step-up converter by forming an R-C lowpass filter along with the P-channel MOSFET on-resistance. Output ripple can be further reduced by increasing C4. See Tables 4 and 5 for a list of suggested capacitors and suppliers.
Tantalum
DIODES
Table 5. Component Suppliers
SUPPLIER AVX Coilcraft International Rectifier Motorola Sanyo Sprague Sumida PHONE (803) 946-0690 (847) 639-6400 (310) 322-3331 (602) 303-5454 (619) 661-6835 (603) 224-1961 (847) 956-0666 FAX (803) 626-3123 (847) 639-1469 (310) 322-3332 (602) 994-6430 (619) 661-1055 (603) 224-1430 (847) 956-0702
Diode Selection
The MAX1672's high switching frequency demands a high-speed rectifier. Schottky diodes, such as the 1N5817 or MBRS130T3, are recommended. Make sure the diode's current rating exceeds the maximum load current. See Tables 4 and 5 for a list of suggested diodes and suppliers.
_______________________________________________________________________________________
9
Step-Up/Down DC-DC Converter in QSOP Package MAX1672
__________Applications Information
Using a Single, Pushbutton On/Off Switch
A single pushbutton switch can be used to turn the MAX1672 on and off. As shown in Figure 3, ONA is pulled low and ONB is pulled high when the part is off. When the momentary switch is pressed, ONB is pulled low and the regulator turns on. The switch should be on long enough for the C to exit reset. The controller issues a logic high to ONA, which guarantees the part will stay on regardless of the switch state. To turn off the regulator, press the switch again. The controller reads the switch status and pulls ONA low. When the switch is released, ONB goes high, turning off the MAX1672.
1M VDD ONB OUT
MAX1672
I/O
C
ONA
I/O
Thermal Overload Protection
Thermal overload protection limits total power dissipation in the MAX1672. When the junction temperature exceeds TJ = +150C, the pass transistor turns off, allowing the MAX1672 to cool. The pass transistor turns on again after the IC's junction temperature cools by 20C, resulting in a pulsed output during thermal overload conditions. Thermal overload protection is designed to protect the MAX1672 if fault conditions occur. It is not intended to be used as an operating mode. Prolonged operation in thermal shutdown mode may reduce the IC's reliability. For continual operation, do not exceed the absolute maximum junction temperature rating TJ = +150C.
1M
Figure 3. Momentary Pushbutton On/Off Control
Layout Considerations
Proper PC board layout is essential to minimize noise due to high inductor current levels and fast switching waveforms. To maximize output power and efficiency and minimize output ripple voltage and ground noise, use the following guidelines when designing your board: * Use a ground plane. * Keep the IC's GND pin and the ground leads of C1 and C2 (Figure 2) less than 0.2in. (5mm) apart. * Make all connections to the FB and LX pins as short as possible. * Solder the IC's GND pin directly to the ground plane. Refer to the MAX1672 EV kit for a suggested PC board layout.
Power Dissipation and Operating Region
The MAX1672's maximum power dissipation in stepdown mode depends on the thermal resistance of the case and circuit board, the temperature difference between the die junction and ambient air, and the air flow rate. The power dissipated in the device is P = IOUT (VIN - VOUT) during step-down operation. The maximum power dissipation is as follows: PMAX = (TJ - TA)/(JB + BA) where (TJ - TA) is the temperature difference between the MAX1672 die junction and the surrounding air, JB (or JC) is the thermal resistance of the package, and BA is the thermal resistance throughout the printed circuit board, copper traces, and other materials to the surrounding air. The MAX1672's thermal resistance is 120C/W. See the Typical Operating Characteristics for Maximum Output Current vs. Input Voltage.
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Step-Up/Down DC-DC Converter in QSOP Package
________________________________________________________Package Information
QSOP.EPS
MAX1672
______________________________________________________________________________________
11
Step-Up/Down DC-DC Converter in QSOP Package MAX1672
NOTES
12
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