View MAX608_126909.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

19-0438; Rev 0; 9/95
NUAL KIT MA ATION ET EVALU TA SHE WS DA FOLLO
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller
____________________________Features
o 1.8V to 16.5V Input Range o 85% Efficiency for 30mA to 1.5A Load Currents o Up to 10W Output Power o 110A Max Supply Current o 5A Max Shutdown Current o Preset 5V or Adjustable Output (3V to 16.5V) o Current-Limited PFM Control Scheme o Up to 300kHz Switching Frequency o Evaluation Kit Available
_______________General Description
The MAX608 low-voltage step-up controller operates from a 1.8V to 16.5V input voltage range. Pulse-frequency-modulation (PFM) control provides high efficiency at heavy loads, while using only 85A (typical) when operating with no load. In addition, a logic-controlled shutdown mode reduces supply current to 2A typical. The output voltage is factory-set at 5V or can be adjusted from 3V to 16.5V with an external resistor divider. The MAX608 is ideal for two- and three-cell batterypowered systems. An operating frequency of up to 300kHz allows use with small surface-mount components. The MAX608 operates in "bootstrapped" mode only (with the chip supply, OUT, connected to the DC-DC output). For a 12V output without external resistors, or for nonbootstrapped applications (chip supply connected to input voltage), refer to the pin-compatible MAX1771. The MAX608 is available in 8-pin DIP and SO packages.
MAX608
________________________Applications
High-Efficiency DC-DC Converters Battery-Powered Applications Positive LCD-Bias Generators Portable Communicators
______________Ordering Information
PART MAX608C/D MAX608EPA MAX608ESA TEMP. RANGE 0C to +70C -40C to +85C -40C to +85C PIN-PACKAGE Dice* 8 Plastic DIP 8 SO
* Contact factory for dice specifications.
__________Typical Operating Circuit
INPUT 1.8V TO VOUT OUTPUT 5V EXT CS N
__________________Pin Configuration
TOP VIEW
ON/OFF
MAX608
SHDN REF
EXT OUT
1 2
8
CS GND AGND REF
MAX608
7 6 5
FB 3 SHDN 4
FB AGND GND OUT
DIP/SO
________________________________________________________________ Maxim Integrated Products
1
Call toll free 1-800-998-8800 for free samples or literature.
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller MAX608
ABSOLUTE MAXIMUM RATINGS
Supply Voltage OUT to GND.............................................................-0.3V, 17V EXT, CS, REF, SHDN, FB to GND ...............-0.3V, (VOUT + 0.3V) GND to AGND.............................................................0.1V, -0.1V Continuous Power Dissipation (TA = +70C) Plastic DIP (derate 9.09mW/C above +70C) ............727mW SO (derate 5.88mW/C above +70C) .........................471mW 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
(VOUT = 5V, ILOAD = 0mA, TA = -40C to +85C where indicated. TA = -25C to +85C for all other limits. Typical values are at TA = +25C.) PARAMETER Input Voltage Range (Note 2) Minimum Start-Up Voltage SYMBOL CONDITIONS TA = -25C to +85C TA = -40C to +85C (Note 1) No load VOUT = 16.5V, SHDN 0.4V VOUT = 10V, SHDN 1.6V Output Voltage (Note 3) Output Voltage Line Regulation (Note 4) Output Voltage Load Regulation (Note 4) Maximum Switch On-Time tON(max) Minimum Switch Off-Time Efficiency Reference Voltage REF Load Regulation REF Line Regulation FB Trip Point Voltage (Note 5) FB Input Current SHDN Input High Voltage SHDN Input Low Voltage SHDN Input Current 2 VFB IFB VIH VIL IIN VREF tOFF(min) VIN = 4V, VOUT = 5V, ILOAD = 500mA, circuit of Figure 2a IREF = 0A 0A IREF 100A 3V VOUT 16.5V TA = -25C to +85C TA = -40C to +85C (Note 1) TA = -25C to +85C TA = -40C to +85C (Note 1) VOUT = 1.8V to 16.5V VOUT = 1.8V to 16.5V VOUT = 16.5V, SHDN = 0V or 16.5V 1.6 0.4 1 1.4625 1.4475 -4 TA = -25C to +85C TA = -40C to +85C (Note 1) 1.4625 1.4475 -4 40 1.5 VIN = 2.0V to 5.0V, over full load range, circuit of Figure 2a TA = -25C to +85C TA = -40C to +85C (Note 1) TA = -25C to +85C TA = -40C to +85C (Note 1) TA = -25C to +85C TA = -40C to +85C (Note 1) 4.825 4.800 5.0 5.0 7 60 12 1.8 16 2.3 87 1.5 1.5375 1.5525 10 100 1.5375 1.5525 20 40 20 2.8 2 MIN 1.8 1.9 1.6 85 TYP MAX 16.5 16.5 1.8 110 120 5 10 5.175 V 5.200 mV/V mV/A s s % V mV V/V V nA UNITS V V A A
Supply Current
VIN = 2.7V to 4.0V, VOUT = 5V, ILOAD = 500mA, circuit of Figure 2a VIN = 2V, VOUT = 5V, ILOAD = 0mA to 500mA, circuit of Figure 2a
V V
A
_______________________________________________________________________________________
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller
ELECTRICAL CHARACTERISTICS (continued)
(VOUT = 5V, ILOAD = 0mA, TA = -40C to +85C where indicated. TA = -25C to +85C for all other limits. Typical values are at TA = +25C.) PARAMETER Current-Limit Trip Level CS Input Current EXT Rise Time EXT Fall Time EXT On-Resistance SYMBOL VCS ICS VOUT = 5V, 1nF from EXT to GND VOUT = 5V, 1nF from EXT to GND EXT = high or low CONDITIONS VOUT = 3V to 16.5V TA = -25C to +85C TA = -40C to +85C (Note 1) MIN 85 80 0.01 50 50 15 30 TYP 100 MAX 115 120 1 UNITS mV A ns
MAX608
Note 1: Limits over this temperature range are guaranteed by design. Note 2: The MAX608 must be operated in bootstrapped mode with OUT connected to the DC-DC circuit output. The minimum output voltage set point is +3V. Note 3: Output voltage guaranteed using preset voltages. See Figures 4a-4d for output current capability versus input voltage. Note 4: Output voltage line and load regulation depend on external circuit components. Note 5: Operation in the external-feedback mode is guaranteed to be accurate to the VFB trip level, and does not include resistor tolerance.
__________________________________________Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (VOUT = 5V)
MAX608-01
EFFICIENCY vs. LOAD CURRENT (VOUT = 12V)
MAX608-02
EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V)
MAX608-03
100 VIN = 4.0V 90 EFFICIENCY (%) VIN = 3.5V
100 VIN = 9.0V 90 EFFICIENCY (%) VIN = 6.0V VIN = 5.0V
100
90 EFFICIENCY (%)
VIN = 3.0V
80 VIN = 3.0V 70 VIN = 2.0V
80 VIN = 3.0V VIN = 2.0V
80
70
70
VIN = 2.0V
60 1 10 100 1000 LOAD CURRENT (mA)
60 1 10 100 1000 LOAD CURRENT (mA)
60 1 10 100 1000 LOAD CURRENT (mA)
LOAD CURRENT vs. MINIMUM START-UP INPUT VOLTAGE
MAX608-04
LOAD CURRENT vs. MINIMUM START-UP INPUT VOLTAGE
VOUT = 12V CIRCUIT OF FIGURE 2b EXTERNAL FET THRESHOLD LIMITS FULL-LOAD START-UP BELOW 3.6V
MAX608-05
SUPPLY CURRENT vs. INPUT VOLTAGE
MAX608-06
700 600 LOAD CURRENT (mA) 500 400 300 200 100 0 1.8 2.2 2.6 3.0 3.4 3.8 VOUT = 5V CIRCUIT OF FIGURE 2a EXTERNAL FET THRESHOLD LIMITS FULL-LOAD START-UP BELOW 3.7V
500
200
SUPPLY CURRENT (A)
400 LOAD CURRENT (mA)
150
300
100
200
50
100 0 4.0 1.8 2.2 2.6 3.0 3.4 3.8 4.0 MINIMUM START-UP VOLTAGE (V) MINIMUM START-UP VOLTAGE (V) 0 0 1 2 3 4 5 INPUT VOLTAGE (V)
_______________________________________________________________________________________
3
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller MAX608
____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
EXT RISE/FALL TIME vs. SUPPLY VOLTAGE
MAX608-07
REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE
REFERENCE OUTPUT RESISTANCE ()
MAX608-08
REFERENCE vs. TEMPERATURE
MAX608-09
250 200
250
1.506 1.504 1.502 REFERENCE (V) 1.500 1.498 1.496 1.494
EXT RISE/FALL TIME (ns)
CEXT = 2200pF CEXT = 1000pF CEXT = 470pF CEXT = 100pF
200 10A 150
150
100
100
50A 100A
50
50
0 2 4 6 8 10 12 SUPPLY VOLTAGE (V)
0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
1.492 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
MAXIMUM SWITCH ON-TIME vs. TEMPERATURE
MAX608-10
SHUTDOWN CURRENT vs. TEMPERATURE
3.5 SHUTDOWN CURRENT (A) 3.0 2.5 2.0 1.5 V+ = 8V 1.0 0.5 V+ = 4V 2.20 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) V+ = 15V tOFF(min) (s)
MAX608-11
MINIMUM SWITCH OFF-TIME vs. TEMPERATURE
MAX608-12
16.5
4.0
2.30
tON(max) (s)
16.0
2.25
15.5 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
0
-60 -30
0
30
60
90
120 150
TEMPERATURE (C)
HEAVY-LOAD SWITCHING WAVEFORMS (VOUT = 5V)
A B VOUT A 0V ILIM B 0A C C
MEDIUM-LOAD SWITCHING WAVEFORMS (VOUT = 5V)
VOUT 0V ILIM
0A
2s/div VIN = 3V, IOUT = 930mA, VOUT = 5V A = EXT VOLTAGE, 5V/div B = INDUCTOR CURRENT, 1A/div C = VOUT RIPPLE, 50mV/div, AC-COUPLED
20s/div VIN = 3V, IOUT = 490mA, VOUT = 5V A = EXT VOLTAGE, 5V/div B = INDUCTOR CURRENT, 1A/div C = VOUT RIPPLE, 50mV/div, AC-COUPLED
4
_______________________________________________________________________________________
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller
____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
HEAVY-LOAD SWITCHING WAVEFORMS (VOUT = 12V)
VOUT A 0V ILIM B 0A C C B 0A A
MAX608
MEDIUM-LOAD SWITCHING WAVEFORMS (VOUT = 12V)
VOUT 0V ILIM
2s/div VIN = 4V, IOUT = 490mA, VOUT = 12V A = EXT VOLTAGE, 10V/div B = INDUCTOR CURRENT, 1A/div C = VOUT RIPPLE, 50mV/div, AC-COUPLED
10s/div VIN = 4V, IOUT = 300mA, VOUT = 12V A = EXT VOLTAGE, 10V/div B = INDUCTOR CURRENT, 1A/div C = VOUT RIPPLE, 50mV/div, AC-COUPLED
LINE-TRANSIENT RESPONSE (VOUT = 5V)
LOAD-TRANSIENT RESPONSE (VOUT = 5V)
A
4.0V 2.7V
500mA A 0A
B
B
5ms/div IOUT = 500mA, VOUT = 5V A = VIN, 2.7V TO 4.0V, 1V/div B = VOUT RIPPLE, 100mV/div, AC-COUPLED
2ms/div VIN = 2V, VOUT = 5V A = LOAD CURRENT, 0mA TO 500mA, 500mA/div B = VOUT RIPPLE, 50mV/div, AC-COUPLED
EXITING SHUTDOWN
A 0V 5V B 0V 200s/div IOUT = 500mA, VIN = 3.5V A = SHDN, 2V/div B = VOUT, 2V/div
_______________________________________________________________________________________
5
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller MAX608
______________________________________________________________Pin Description
PIN 1 2 3 NAME EXT OUT FB FUNCTION Gate Drive for External N-Channel Power Transistor Power-Supply and Voltage-Sense Input. Always connect OUT to circuit output. Feedback Input for Adjustable-Output Operation. Connect to ground for fixed-output operation. Use a resistor divider network to adjust the output voltage. See Setting the Output Voltage section. Active-High TTL/CMOS Logic-Level Shutdown Input. In shutdown mode, VOUT is a diode drop below the input voltage (due to the DC path from the input voltage to the output). Connect to ground for normal operation. 1.5V Reference Output that can source 100A for external loads. Bypass to GND with 0.1F. The reference is disabled in shutdown. Analog Ground High-Current Ground Return for the Output Driver Positive Input to the Current-Sense Amplifier. Connect the current-sense resistor between CS and AGND.
4
SHDN
5 6 7 8
REF AGND GND CS
REF
FB DUAL-MODE COMPARATOR SHDN BIAS CIRCUITRY
MAX608
50mV 1.5V REFERENCE MIN OFF-TIME ONE-SHOT Q TRIG 2.3s F/F S R MAX ON-TIME ONE-SHOT TRIG Q 16s LOW-VOLTAGE OSCILLATOR 2.5V Q N ERROR COMPARATOR LOW-VOLTAGE START-UP COMPARATOR
OUT
EXT CURRENT-SENSE AMPLIFIER 0.1V CS
Figure 1. Functional Diagram
6
_______________________________________________________________________________________
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller MAX608
VIN = 2V
C2 0.1F 5 REF C3 0.1F 4 SHDN 3 FB 6 AGND GND 7 C2 0.1F 2 OUT L1 22H C1 150F D1 1N5817 N MMFT3055EL CS 8 RSENSE 50m C4 200F R2 = (R1) GND 7 FB 3 5 REF C3 0.1F 2 OUT L1 22H
VIN = 2V
C1 150F D1 1N5817 N MMFT3055EL RSENSE 50m R1 58k
VOUT = 12V @ 0.3A
C4 200F
MAX608
VOUT = 5V @ 0.5A
4 SHDN MAX608 6
EXT
1
EXT
1
AGND
CS
8 R2 402k
( VOUT -1) V
REF
VREF = 1.5V
Figure 2a. 5V Preset Output
Figure 2b. 12V Output
_______________Detailed Description
The MAX608 is a BiCMOS, step-up, switch-mode power-supply controller that provides a preset 5V output, in addition to adjustable-output operation. Its unique control scheme combines the advantages of pulse-frequency modulation (low supply current) and pulse-width modulation (high efficiency with heavy loads), providing high efficiency over a wide output current range, as well as increased output current capability over previous PFM devices. In addition, the external sense resistor and power transistor allow the user to tailor the output current capability for each application. Figure 1 shows the MAX608 functional diagram. The device has a shutdown mode that reduces the supply current to 5A max. Figure 2 shows the standard application circuits. The IC is powered from the output, and the input voltage range is 1.8V to VOUT (this configuration is commonly known as bootstrap operation). The voltage applied to the gate of the external power transistor is switched from VOUT to ground. The MAX608's output voltage can be set to 5V by connecting FB to ground; it can also be adjusted from 3V to 16.5V using external resistors. Use 1% external feedback resistors when operating in adjustable-output mode (Figures 2b, 2c) to achieve an overall output voltage accuracy of 5%.
C2 0.1F 2 5 REF C3 0.1F OUT
VIN = 2V
L1 22H
C1 150F D1 1N5817 N SI6426 RSENSE 50m R1 50k
VOUT = 3.3V @ 0.6A
C4 200F
4 SHDN MAX608 6
EXT
1
AGND
CS
8 R2 60k
FB GND 7 R2 = (R1)
3
( VOUT -1) V
REF
C5 47pF
VREF = 1.5V
Figure 2c. 3.3V Output
PFM Control Scheme
The MAX608 uses a proprietary current-limited PFM control scheme to provide high efficiency over a wide range of load currents. This control scheme combines the ultralow supply current of PFM converters (or pulse skippers) with the high full-load efficiency of PWM converters.
_______________________________________________________________________________________
7
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller
Unlike traditional PFM converters, the MAX608 uses a sense resistor to control the peak inductor current. The device also operates with high switching frequencies (up to 300kHz), allowing the use of miniature external components. As with traditional PFM converters, the power transistor is not turned on until the voltage comparator senses the output is out of regulation. However, unlike traditional PFM converters, the MAX608 switch uses the combination of a peak current limit and a pair of one-shots that set the maximum on-time (16s) and minimum offtime (2.3s); there is no oscillator. Once off, the minimum off-time one-shot holds the switch off for 2.3s. After this minimum time, the switch either 1) stays off if the output is in regulation, or 2) turns on again if the output is out of regulation. The control circuitry allows the IC to operate in continuous-conduction mode (CCM) while maintaining high efficiency with heavy loads. When the power switch is turned on, it stays on until either 1) the maximum ontime one-shot turns it off (typically 16s later), or 2) the switch current reaches the peak current limit set by the current-sense resistor. The MAX608 switching frequency is variable (depending on load current and input voltage), causing variable switching noise. However, the subharmonic noise generated does not exceed the peak current limit times the filter capacitor equivalent series resistance (ESR). For example, when generating a 5V output at 500mA from a 2V input, only 75mV of output ripple occurs, using the circuit of Figure 2a.
MAX608
R2 FB VOUT R1
MAX608
C5*
R1 = 10k TO 500k GND R2 = R1
( VOUT -1) REF
V
VREF = 1.5V * OPTIONAL, SEE TEXT FOR VALUE
Figure 3. Adjustable Output Circuit
__________________Design Procedure
Setting the Output Voltage
The MAX608's output voltage is preset to 5V (FB = 0V), or it can be adjusted from 16.5V down to 3V using external resistors R1 and R2, configured as shown in Figure 3. For adjustable-output operation, select feedback resistor R1 in the 10k to 500k range. R2 is given by: VOUT R2 = (R1) ----- -1 VREF
(
)
Low-Voltage Start-Up Oscillator
The MAX608 features a low input voltage start-up oscillator that guarantees start-up with no load for input voltages down to 1.8V. At these low voltages, the output voltage is not large enough for proper error-comparator operation and internal biasing. The start-up oscillator has a fixed 50% duty cycle and the MAX608 disregards the error-comparator output when the output voltage is less than 2.5V. Above 2.5V, the error-comparator and normal one-shot timing circuitry are used.
where VREF equals 1.5V. OUT must always be connected to the circuit output. Figure 2 shows various circuit configurations for preset/ adjustable operation.
Determining RSENSE
Use the theoretical output current curves shown in Figures 4a-4d to select R SENSE . They are derived using the minimum (worst-case) current-limit comparator threshold value over the extended temperature range (-40C to +85C). No tolerance was included for RSENSE. The voltage drop across the diode is assumed to be 0.5V, and the drop across the power switch rDS(ON) and coil resistance is assumed to be 0.3V.
Shutdown Mode
When SHDN is high, the MAX608 enters shutdown mode. In this mode, the internal biasing circuitry is turned off (including the reference), and V OUT falls to a diode drop below V IN (due to the DC path from the input to the output). In shutdown mode, the supply current drops to less than 5A. SHDN is a TTL/CMOS logic-level input. Connect SHDN to GND for normal operation.
Determining the Inductor (L)
Practical inductor values range from 10H to 300H. 22H is a good choice for most applications. In applications with large input/output differentials, the IC's output-current capability will be much less when the inductance value is too low, because the IC will always operate in discontinuous mode. If the inductor value is too low, the
8
_______________________________________________________________________________________
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller MAX608
2.0 VOUT = 3.3V L = 22H MAXIMUM OUTPUT CURRENT (A) MAXIMUM OUTPUT CURRENT (A) 1.5 RSENSE = 25m RSENSE = 35m 1.0 RSENSE = 50m 3.5 3.0 2.5 2.0 1.5 1.0 0.5 RSENSE = 100m 0 2.0 2.5 3.0 INPUT VOLTAGE (V) 3.5 0 2 3 4 INPUT VOLTAGE (V) 5 RSENSE = 50m VOUT = 5V L = 22H RSENSE = 20m RSENSE = 25m RSENSE = 35m
0.5
RSENSE = 100m
Figure 4a. Maximum Output Current vs. Input Voltage (VOUT = 3.3V)
3.5 MAXIMUM OUTPUT CURRENT (A) 3.0 2.5 2.0 1.5 1.0 0.5 0 2 4 RSENSE = 100m 6 8 INPUT VOLTAGE (V) 10 12 RSENSE = 50m
Figure 4b. Maximum Output Current vs. Input Voltage (VOUT = 5V)
3.5 MAXIMUM OUTPUT CURRENT (A) 3.0 2.5 2.0 1.5 1.0 0.5 0 2 4 6 RSENSE = 100m 8 10 12 INPUT VOLTAGE (V) 14 16 RSENSE = 50m
VOUT = 12V L = 22H RSENSE = 20m RSENSE = 25m RSENSE = 35m
VOUT = 15V L = 22H RSENSE = 20m RSENSE = 25m RSENSE = 35m
Figure 4c. Maximum Output Current vs. Input Voltage (VOUT = 12V)
Figure 4d. Maximum Output Current vs. Input Voltage (VOUT = 15V)
current will ramp up to a high level before the current-limit comparator can turn off the switch. The minimum on-time for the switch (tON(min)) is approximately 2s; select an inductor that allows the current to ramp up to ILIM. The standard operating circuits use a 22H inductor. If a different inductance value is desired, select L such that: VIN(max) x 2s L -----------------ILIM Larger inductance values tend to increase the start-up time slightly, while smaller inductance values allow the coil current to ramp up to higher levels before the switch turns off, increasing the ripple at light loads.
Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for use with high switching frequencies. Make sure the inductor's saturation current rating (the current at which the core begins to saturate and the inductance starts to fall) exceeds the peak current rating set by R SENSE. However, it is generally acceptable to bias the inductor into saturation by approximately 20% (the point where the inductance is 20% below the nominal value). For highest efficiency, use a coil with low DC resistance, preferably under 20m. To minimize radiated noise, use a toroid, a pot core, or a shielded coil. Table 1 lists inductor suppliers and specific recommended inductors.
_______________________________________________________________________________________
9
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller MAX608
Power Transistor Selection
Use an N-channel MOSFET power transistor with the MAX608. Use logic-level or low-threshold N-FETs to ensure the external N-channel MOSFET (N-FET) is turned on completely and that start-up occurs. N-FETs provide the highest efficiency because they do not draw any DC gate-drive current. When selecting an N-FET, some important parameters to consider are the total gate charge (Qg), on-resistance (rDS(ON)), reverse transfer capacitance (CRSS), maximum drain to source voltage (VDS max), maximum gate to source voltage (VGS max), and minimum threshold voltage (VTH min). Qg takes into account all capacitances associated with charging the gate. Use the typical Qg value for best results; the maximum value is usually grossly overspecified since it is a guaranteed limit and not the measured value. The typical total gate charge should be 50nC or less. With larger numbers, the EXT pins may not be able to adequately drive the gate. The EXT rise/fall time varies with different capacitive loads as shown in the Typical Operating Characteristics. The two most significant losses contributing to the N-FET's power dissipation are I2R losses and switching losses. Select a transistor with low r DS(ON) and low CRSS to minimize these losses. Determine the maximum required gate-drive current from the Qg specification in the N-FET data sheet. Select an N-FET with a BVDSS > VOUT, BVGSS > VOUT, and a minimum VTH of 0.5V below the minimum input voltage. When using a power supply that decays with time (such as a battery), the N-FET transistor will operate in its linear region when the voltage at EXT approaches the threshold voltage of the FET, dissipating excessive power. Prolonged operation in this mode may damage the FET. To avoid this condition, make sure VEXT is above the VTH of the FET, or use a voltage detector (such as the MAX8211) to put the IC in shutdown mode once the input supply voltage falls below a predetermined minimum value. Excessive loads with low input voltages can also cause this condition. The MAX608's maximum allowed switching frequency during normal operation is 300kHz. However, at startup, the maximum frequency can be 500kHz, so the maximum current required to charge the N-FET's gate is f(max) x Qg(typ). Use the typical Qg number from the transistor data sheet. For example, the MMFT3055EL has a Qg(typ) of 7nC (at VGS = 5V), therefore the current required to charge the gate is:
10
IGATE (max) = (500kHz) (7nC) = 3.5mA. Figure 2a's application circuit uses a 4-pin MMFT3055EL surface-mount N-FET that has 150m on-resistance with 4.5V VGS, and a guaranteed VTH of less than 2V. Figure 2c's application circuit uses an Si6426DQ logic-level NFET with a threshold voltage (VTH) of 1V.
Diode Selection
The MAX608's high switching frequency demands a high-speed rectifier. Schottky diodes such as the 1N5817-1N5822 are recommended. Make sure the Schottky diode's average current rating exceeds the peak current limit set by RSENSE, and that its breakdown voltage exceeds V OUT . For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents; high-speed silicon diodes such as the MUR105 or EC11FS1 can be used instead. At heavy loads and high temperatures, the benefits of a Schottky diode's low forward voltage may outweigh the disadvantage of high leakage current.
Capacitor Selection
Output Filter Capacitor The primary criterion for selecting the output filter capacitor (C4) is low effective series resistance (ESR). The product of the peak inductor current and the output filter capacitor's ESR determines the amplitude of the ripple seen on the output voltage. Two OS-CON 100F, 16V output filter capacitors in parallel with 35m of ESR each typically provide 75mV ripple when stepping up from 2V to 5V at 500mA (Figure 2a). Smaller-value and/or higherESR capacitors are acceptable for light loads or in applications that can tolerate higher output ripple. Since the output filter capacitor's ESR affects efficiency, use low-ESR capacitors for best performance. See Table 1 for component selection. Input Bypass Capacitors The input bypass capacitor (C1) reduces peak currents drawn from the voltage source and also reduces noise caused by the switching action of the MAX608 at the voltage source. The input voltage source impedance determines the size of the capacitor required at the OUT input. As with the output filter capacitor, a low-ESR capacitor is recommended. For output currents up to 1A, 150F (C1) is adequate, although smaller bypass capacitors may also be acceptable. Bypass the IC with a 0.1F ceramic capacitor (C2) placed as close as possible to the OUT and GND pins. Reference Capacitor Bypass REF with a 0.1F capacitor (C3). REF can source up to 100A of current for external loads.
______________________________________________________________________________________
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller MAX608
PRODUCTION INDUCTORS Sumida CD54 series CDR125 series Coiltronics CTX20 series Coilcraft DO3316 series DO3340 series CAPACITORS Matsuo 267 series Sprague 595D series AVX TPS series Sanyo OS-CON series TRANSISTORS Siliconix Si9410DY Si4410DY Si6426DQ Si6946DQ Motorola MTP3055EL MTD20N03HDL MMFT3055ELT1 DIODES Central Semiconductor CMPSH-3 CMPZ5240 Nihon EC11 FS1 series (highspeed silicon) Motorola MBRS1100T3 MMBZ5240BL Motorola 1N5817-1N5822 MUR105 (high-speed silicon) SUPPLIER AVX Central Semiconductor Coilcraft Coiltronics Matsuo Motorola Nichicon Nihon Sanyo Siliconix Sprague Sumida PHONE USA: (803) 448-9411 USA: (516) 435-1110 USA: (708) 639-6400 USA: (407) 241-7876 USA: (714) 969-2491 Japan: 81-6-337-6450 USA: (800) 521-6274 USA: (708) 843-7500 USA: (805) 867-2555 USA: (619) 661-6835 Japan: 81-7-2070-1005 USA: (800) 554-5565 USA: (603) 224-1961 USA: (708) 956-0666 Japan: 81-3-3607-5111 FAX (803) 448-1943 (516) 435-1824 (708) 639-1469 (407) 241-9339 (714) 960-6492 81-6-337-6456 (602) 952-4190 (708) 843-2798 (805) 867-2556 (619) 661-1055 81-7-2070-1174 (408) 970-3950 (603) 224-1430 (708) 956-0702 81-3-3607-5144
Surface Mount
Through Hole
Sumida RCH855 series RCH110 series
Sanyo OS-CON series Nichicon PL series
Feed-Forward Capacitor When adjusting the output voltage, it may be necessary to parallel a 47pF to 220pF capacitor across R2, as shown in Figures 2 and 3. Choose the lowest capacitor value that insures stability; high capacitance values may degrade line regulation.
__________Applications Information
Starting Up Under Load
The Typical Operating Characteristics show the Start-Up Voltage vs. Load Current graphs for 5V and 12V output voltages. These graphs depend on the type of power switch used. The MAX608 is not designed to start up under full load with low input voltages.
Layout Considerations
Due to high current levels and fast switching waveforms, which radiate noise, proper PC board layout is essential. Protect sensitive analog grounds by using a star ground configuration. Minimize ground noise by connecting GND, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point (star ground configuration). Also, minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise. Place input bypass capacitor C2 as close as possible to OUT and GND. If an external resistor divider is used (Figures 2 and 3), the trace from FB to the resistors must be extremely short.
______________________________________________________________________________________
11
5V or Adjustable, Low-Voltage, Step-Up DC-DC Controller MAX608
___________________Chip Topography
EXT
OUT
CS
0.126" (3.200mm)
GND FB AGND
SHDN
0.080" (2.032mm)
REF
TRANSISTOR COUNT: 501 SUBSTRATE CONNECTED TO OUT
________________________________________________________Package Information
DIM INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.157 0.150 0.050 0.244 0.228 0.050 0.016 MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 3.80 4.00 1.27 5.80 6.20 0.40 1.27
D A e B
0.101mm 0.004in.
0-8
A1
C
L
A A1 B C E e H L
E
H
Narrow SO SMALL-OUTLINE PACKAGE (0.150 in.)
DIM PINS D D D 8 14 16
INCHES MILLIMETERS MIN MAX MIN MAX 0.189 0.197 4.80 5.00 0.337 0.344 8.55 8.75 0.386 0.394 9.80 10.00
21-0041A
12
______________________________________________________________________________________

▲Up To Search▲

Price & Availability of MAX608

	To Download MAX608 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .