View MAX1795_27449.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

19-1798; Rev 0; 12/00
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown
General Description
The MAX1795/MAX1796/MAX1797 are high efficiency step-up DC-DC converters intended for small portable hand-held devices. These devices feature Maxim's True-ShutdownTM circuitry, which fully disconnects the output from the input in shutdown, improves efficiency, and eliminates costly external components. All three devices also feature Maxim's proprietary LX-damping circuitry for reduced EMI in noise-sensitive applications. For additional in-system flexibility, a battery monitoring comparator (LBI/LBO) remains active even when the DC-DC converter is in shutdown. The input voltage range is +0.7V to VOUT, where VOUT can be set from +2V to +5.5V. Startup is guaranteed from +0.85V. The MAX1795/MAX1796/MAX1797 have a preset, pin-selectable 5V or 3.3V output. The output can also be adjusted to other voltages, using two external resistors. The three devices differ only in their current limits, allowing optimization of external components for different loads: The MAX1795, MAX1796, and MAX1797 have current limits of 0.25A, 0.5A, and 1A, respectively. All devices are packaged in a compact 8pin MAX package that is only 1.09mm tall and half the size of an 8-pin SO. o >95% Efficiency o True-Shutdown Circuitry Output Disconnects from Input in Shutdown No External Schottky Diode Needed o 25A Quiescent Supply Current o Low-Noise Antiringing Feature o LBI/LBO Comparator Enabled in Shutdown o 2A Shutdown Current o 8-Pin MAX Package
Features
MAX1795/MAX1796/MAX1797
Ordering Information
PART MAX1795 EUA MAX1796 EUA MAX1797 EUA TEMP. RANGE -40C to +85C -40C to +85C -40C to +85C PIN-PACKAGE 8 MAX 8 MAX 8 MAX
________________________Applications
Portable Digital Audio Players PDAs/Palmtops Wireless Handsets Portable Terminals
True-Shutdown is a trademark of Maxim Integrated Products.
Pin Configuration
IN 0.7V TO 5.5V LBI FB LBO SHDN 1 2 3 4 8 BATT OUT LX GND OFF ON
Typical Operating Circuit
TOP VIEW
BATT MAX1795 MAX1796 MAX1797
LX
MAX1795 MAX1796 MAX1797 MAX
7 6 5
LBI LBO SHDN
OUT FB
OUT
GND
________________________________________________________________ Maxim Integrated Products
1
For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown MAX1795/MAX1796/MAX1797
ABSOLUTE MAXIMUM RATINGS
OUT, LX, SHDN, LBI, LBO, BATT to GND................-0.3V to +6V FB .............................................................-0.3V to (VOUT + 0.3V) ILX, IOUT ..............................................................................1.5A Output Short-Circuit Duration ...................................................5s Continuous Power Dissipation 8-Pin MAX (derate 4.1mW/C above +70C) .............330mW 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
(VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Minimum Input Voltage Operating Voltage Startup Voltage Startup Voltage Tempco Output Voltage Adjustable Output Voltage Range BATT = +2V, FB = OUT (VOUT = +3.3V) Steady-State Output Current IOUT BATT = +2V, FB = GND (VOUT = +5.0V) Feedback Set-Point Voltage (Adjustable Mode) Feedback Input Current Internal NFET, PFET OnResistance LX Switch Current Limit (NFET only) LX Leakage Current Synchronous Rectifier Turn-Off Current Limit Damping Switch On-Resistance Operating Current into OUT (Note 2) RDAMP VFB = +1.4V 100 VFB IFB RDS(ON) VOUT = +2V to +5.5V VFB = +1.24V VOUT = +3.3V, ILX = 100mA MAX1795 ILIM ILEAK MAX1796 MAX1797 VLX = 0 and +5.5V, VOUT = +5.5V NFET PFET 0.2 0.4 0.8 MAX1795 MAX1796 MAX1797 MAX1795 MAX1796 MAX1797 VOUT FB = OUT FB = GND 3.17 4.80 2.0 100 200 400 50 100 250 1.20 180 300 550 120 200 370 1.24 4 0.17 0.27 0.25 0.5 1.0 0.2 25 200 25 400 45 1.28 100 0.3 0.45 0.35 0.625 1.25 A mA A A V nA mA VBATT SYMBOL After startup (Note 1) TA = +25C, RL = 3k 1.0 0.85 -2.2 3.3 5.0 3.43 5.20 5.5 CONDITIONS MIN TYP 0.7 5.5 1.0 MAX UNITS V V V mV/C V V
2
_______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown
ELECTRICAL CHARACTERISTICS (continued)
(VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = 0C to +85C, unless otherwise noted. Typical values are TA = +25C.)
PARAMETER Operating Current into BATT Shutdown Current into BATT LX Switch MaxImum On-Time LX Switch Minimum Off-Time LBI Threshold Voltage Falling LBI Hysteresis LBI Input Current ILBI VLBI = +0.8V VBATT = VLBI = +0.975V, sinking 20A (50 typ) LBO Low Output Voltage VBATT = VLBI = +1.1V, sinking 100A (25 typ) LBO Off-Leakage Current VIL SHDN Input Voltage VIH Shutdown Input Current VSHDN = 0 and +5.5V 0.8 VBATT 100 VLBO = +5.5V 1 0.1 100 0.2 VBATT nA tON tOFF VLBI SYMBOL CONDITIONS VFB = +1.4V, VLBI = +1V SHDN = BATT, VLBI = +1V VFB = +1V, if current limit not reached VFB = +1V VBATT = +2V VBATT = LBI 3 0.8 0.8 0.875 MIN TYP 2 2 4 1 0.85 0.925 25 9 100 0.1 V MAX 4 4 5 1.2 0.90 0.975 UNITS A A s s V mV nA
MAX1795/MAX1796/MAX1797
V
nA
ELECTRICAL CHARACTERISTICS
(VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = -40C to +85C, unless otherwise noted.) (Note 3)
PARAMETER Operating Voltage Output Voltage Adjustable Output Voltage Range MAX1795 FB = OUT (VOUT = +3.3V) Steady-State Output Current (Note 1) IOUT FB = GND (VOUT = +5.0V) Feedback Set-Point Voltage (Adjustable Mode) Feedback Input Current VFB IFB VOUT = +2V to +5.5V VFB = +1.25V MAX1796 MAX1797 MAX1795 MAX1796 MAX1797 SYMBOL VBATT VOUT Note 1 FB = OUT FB = GND CONDITIONS MIN 1.0 3.13 4.75 2.0 100 200 400 60 125 250 1.19 1.29 100 V nA mA MAX 5.5 3.47 5.25 5.5 UNITS V V V
_______________________________________________________________________________________
3
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown MAX1795/MAX1796/MAX1797
ELECTRICAL CHARACTERISTICS (continued)
(VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = -40C to +85C, unless otherwise noted.) (Note 3)
PARAMETER Internal NFET, PFET OnResistance LX Switch Current Limit (NFET only) LX Leakage Current Damping Switch On-Resistance Operating Current into OUT (Note 2) Operating Current into BATT Shutdown Current into BATT LX Switch Maximum On-Time LX Switch Minimum Off-Time LBI Threshold Voltage LBI Input Current tON tOFF VLBI ILBI SYMBOL RDS(ON) ILIM VOUT = +3.3V, ILX = 100mA MAX1795 MAX1796 MAX1797 ILEAK RDAMP VFB = +1.4V VFB = +1.4V, VLBI = +1V SHDN = BATT, VLBI = +1V VFB = +1V, if current limit not reached VFB = +1V VBATT = +2V VBATT = LBI VLBI = +0.8V VBATT = VLBI = +0.975V, sinking 20A (50 typ) LBO Low Output Voltage VBATT = VLBI = +1.1V, sinking 100A (25 typ) LBO Off-Leakage Current VIL SHDN Input Voltage VIH Shutdown Input Current VSHDN = 0 and +5.5V 0.8 VBATT 100 VLBO = +5.5V 0.1 100 0.2 VBATT nA 2.75 0.7 0.8 0.875 VLX = 0 and +5.5V, VOUT = +5.5V 100 400 45 4 4 5.25 1.3 0.90 0.975 100 0.1 V CONDITIONS NFET PFET 0.19 0.35 0.8 MIN MAX 0.3 0.45 0.37 0.7 1.32 A A A A s s V nA A UNITS
V
nA
Note 1: Operating Voltage: Since the regulator is bootstrapped to the output, once started it will operate down to a 0.7V input. Note 2: Device is bootstrapped (power to IC comes from OUT). This correlates directly with the actual battery supply current. Note 3: Specifications to -40C are guaranteed by design, not production tested.
4
_______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown
Typical Operating Characteristics
(L = 22H, CIN = 47F, COUT = 47F, TA = +25C, unless otherwise noted.)
MAX1795 EFFICIENCY vs. LOAD CURRENT (+5V)
MAX1795/96/97 toc01
MAX1795/MAX1796/MAX1797
MAX1795 EFFICIENCY vs. LOAD CURRENT (+3.3V)
MAX1795/96/97 toc02
MAX1796 EFFICIENCY vs. LOAD CURRENT (+5V)
VBATT = +3.6V 90 80 EFFICIENCY (%) 70 60 50 40 30 20 VBATT = +2.4V VBATT = +1.2V
MAX1795/96/97 toc03
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0
VBATT = +3.6V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0
VBATT = +2.4V
100
VBATT = +2.4V
VBATT = +1.2V
VBATT = +1.2V
L = 10H 0.1 1 10 100 1000
10 0 0.1 1 10 100 1000
0.1
1
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MAX1796 EFFICIENCY vs. LOAD CURRENT (+3.3V)
MAX1795/96/97 toc04
MAX1797 EFFICIENCY vs. LOAD CURRENT (+5V)
MAX1795/96/97 toc05
MAX1797 EFFICIENCY vs. LOAD CURRENT (+3.3V)
90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 VBATT = +1.2V VBATT = +2.4V
MAX1795/96/97 toc06
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0
VBATT = +2.4V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 VBATT = +2.4V VBATT = +1.2V VBATT = +3.6V
100
VBATT = +1.2V
0.1
1
10
100
1000
0.1
1
10
100
1000
0.1
1
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
NO-LOAD BATTERY CURRENT vs. INPUT BATT VOLTAGE
MAX1795/96/97 toc07
STARTUP VOLTAGE vs. LOAD CURRENT
3.5 3.0 EFFICIENCY (%) 2.5 VOUT = +3.3V 2.0 1.5 1.0
MAX1795/96/97 toc08
SHUTDOWN THRESHOLD vs. INPUT BATT VOLTAGE
MAX1795/96/97 toc09
300 250 BATTERY CURRENT (A) 200 150 100 50 VOUT = +3.3V 0 VOUT = +5V
4.0
3.0 2.5 2.0 1.5 1.0 0.5 0
0.5 0 0.1 1 10 100 1000
SHUTDOWN THRESHOLD (V)
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 BATT VOLTAGE (V)
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 BATT VOLTAGE (V)
LOAD CURRENT (mA)
_______________________________________________________________________________________
5
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown MAX1795/MAX1796/MAX1797
Typical Operating Characteristics (continued)
(L = 22H, CIN = 47F, COUT = 47F, TA = +25C, unless otherwise noted.)
LOW-BATTERY INPUT THRESHOLD vs. INPUT BATT VOLTAGE
MAX1795/96/97 toc10
LOW-BATTERY INPUT THRESHOLD vs. TEMPERATURE
MAX1795/96/97 toc12
MAX1795 MAXIMUM OUTPUT CURRENT vs. BATT INPUT VOLTAGE
MAX1795/96/97 toc12
0.950 LOW-BATTERY INPUT THRESHOLD (V) 0.925 0.900 INCREASING VLBI 0.875 0.850 0.825 0.800
0.950 LOW-BATTERY INPUT THRESHOLD (V) VBATT = +3.6V 0.925 0.900 0.875 0.850 0.825 0.800 DECREASING VLBI INCREASING VLBI
250
200 LOAD CURRENT (mA) VOUT = +3.3V 150 VOUT = +5.0V 100
DECREASING VLBI
50
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 BATT VOLTAGE (V)
-40
-15
10
35
60
85
0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 BATT VOLTAGE (V)
TEMPERATURE (C)
MAX1796 MAXIMUM OUTPUT CURRENT vs. BATT INPUT VOLTAGE
MAX1795/96/97 toc13
MAX1797 MAXIMUM OUTPUT CURRENT vs. BATT INPUT VOLTAGE
MAX1795/96/97 toc14
500
1000
400 LOAD CURRENT (mA)
800 LOAD CURRENT (mA) VOUT = +3.3V 600
300 VOUT = +3.3V 200 VOUT = +5.0V 100
400
VOUT = +5.0V
200
0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 BATT VOLTAGE (V)
0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 BATT VOLTAGE (V)
OUT LEAKAGE CURRENT vs. OUTPUT VOLTAGE
SHDN = BATT VOUT = +5V VBATT = +2.4V
MAX1795/96/97 toc15
HEAVY-LOAD SWITCHING WAVEFORMS
MAX1795/96/97 toc16
2.0
OUT LEAKAGE CURRENT (A)
1.6
VLX 5V/div
1.2 IINDUCTOR 500mA/div 0.8 VOUT (AC-COUPLED) 100mV/div
0.4
0
OUT BIASED WITH EXTERNAL VOLTAGE SOURCE 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 OUTPUT VOLTAGE (V) VIN = +3.6V VOUT = +5.0V ILOAD = 400mA 4.00s/div
6
_______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown
Typical Operating Characteristics (continued)
(L = 22H, CIN = 47F, COUT = 47F, TA = +25C, unless otherwise noted.)
LIGHT-LOAD SWITCHING WAVEFORMS
MAX1795/96/97 toc17
MAX1795/MAX1796/MAX1797
LINE-TRANSIENT RESPONSE
MAX1795/96/97 toc18
VLX 5V/div
VBATT +2.7V TO +3V
IINDUCTOR 500mA/div VOUT (AC-COUPLED) 20mV/div
VOUT (AC-COUPLED) 100mV/div
20s/div VBATT = +3.6V VOUT = +5.0V ILOAD = 40mA
10s/div VBATT = +2.7V TO +3V VOUT = +5.0V NO LOAD
LOAD-TRANSIENT RESPONSE
MAX1795/96/97 toc19
STARTUP-SHUTDOWN WAVEFORMS
MAX1795/96/97 toc20
IOUT 100mA/div
VSHDN 5V/div
VOUT 2V/div VOUT 100mV/div IINDUCTOR 500mA/div 40s/div VBATT = +2.4V VOUT = +3.3V ILOAD = 0 TO 325mA VBATT = +2.4V VOUT = +5.0V ILOAD = 200mA
2ms/div
_______________________________________________________________________________________
7
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown MAX1795/MAX1796/MAX1797
Pin Description
PIN 1 NAME LBI FUNCTION Low-Battery Comparator Input. Internally set to trip at +0.85V. This function remains operational in shutdown. Dual-ModeTM Feedback Input. Connect to GND for preset 5.0V output. Connect to OUT for preset 3.3V output. Connect a resistive voltage-divider from OUT to GND to adjust the output voltage from 2V to 5.5V. Low-Battery Comparator Output, Open-Drain Output. LBO is low when VLBI < 0.85V. This function remains operational in shutdown. Shutdown Input. If SHDN is high, the device is in shutdown mode, OUT is high impedance, and LBI/LBO are still operational. Connect shutdown to GND for normal operation. Ground Inductor Connection Power Output. OUT provides bootstrap power to the IC. Battery Input and Damping Switch Connection
2
FB
3 4 5 6 7 8
LBO SHDN GND LX OUT BATT
Detailed Description
The MAX1795/MAX1796/MAX1797 compact step-up DC-DC converters start up with voltages as low as 0.85V and operate with an input voltage down to +0.7V. Consuming only 25A of quiescent current, these devices have an internal synchronous rectifier that reduces cost by eliminating the need for an external diode and improves overall efficiency by minimizing losses in the circuit (see Synchronous Rectification section for details). The internal N-channel MOSFET power switch resistance is typically 0.17, which minimizes losses. The LX switch current limits of the MAX1795/ MAX1796/MAX1797 are 0.25A, 0.5A, and 1A, respectively. All three devices offer Maxim's proprietary TrueShutdown circuitry, which disconnects the output from the input in shutdown and puts the output in a high impedance state. These devices also feature Maxim's proprietary LX-damping circuitry, which reduces EMI in noise-sensitive applications. For additional in-system flexibility, the LBI/LBO comparator remains active in shutdown. (Figure 1 is a typical application circuit).
VIN 22H 1M BATT SHDN LBO MAX1795 MAX1796 MAX1797 LX VOUT = 3.3V VIN OUT COUT* 47F
GND LBI FB
*SEE TABLE 1 FOR COMPONENT VALUES.
Figure 1. Typical Application Circuit
Control Scheme
A unique minimum-off-time, current-limited control scheme is the key to the MAX1795/MAX1796/ MAX1797s' low operating current and high efficiency over a wide load range. The architecture combines the high output power and efficiency of a pulse-width-modulation (PWM) device with the ultra-low quiescent curDual Mode is a trademark of Maxim Integrated Products. 8
rent of a traditional pulse-skipping controller (Figure 2). Switching frequency depends upon the load current and input voltage, and can range up to 500kHz. Unlike conventional pulse-skipping DC-DC converters (where ripple amplitude varies with input voltage), ripple in these devices does not exceed the product of the switch current limit and the filter-capacitor equivalent series resistance (ESR).
_______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown MAX1795/MAX1796/MAX1797
BATT
R1 1M SHDN LBO LBI
MAX1795 MAX1796 MAX1797
ZEROCROSSING AMPLIFIER S Q R
+ _ + _
0.85V
+
OUT BODY DIODE CONTROL
OUT
47F
S Q R
BATT BATT 22H
TIMER BLOCK S Q START OUT TON MAX TOFF MAX R LX
47F
R2 FB FB SELECT REFERENCE
+ _
+ _
ERROR AMPLIFIER CURRENT-LIMIT AMPLIFIER
R3
+ _
GND
Figure 2. Functional Diagram
Synchronous Rectification
The internal synchronous rectifier eliminates the need for an external Schottky diode, reducing cost and board space. During the cycle off-time, the P-channel MOSFET turns on and shunts the MOSFET body diode. As a result, the synchronous rectifier significantly improves efficiency without the addition of an external component. Conversion efficiency can be as high as 95%, as shown in the Typical Operating Characteristics.
ance node. The LBI/LBO comparator remains active in shutdown. As shown in Figure 1, the MAX1795/MAX1796/MAX1797 may be automatically shut down when the input voltage drops below a preset threshold by connecting LBO to SHDN (see Low-Battery Detection section).
BATT/Damping Switch
The MAX1795/MAX1796/MAX1797 each contain an internal damping switch to minimize ringing at LX. The damping switch connects a resistor across the inductor when the inductor's energy is depleted (Figure 3). Normally, when the energy in the inductor is insufficient to supply current to the output, the capacitance and inductance at LX form a resonant circuit that causes ringing. The ringing continues until the energy is dissipated through the series resistance of the inductor. The damping switch supplies a path to quickly dissi9
Shutdown
The device enters shutdown when V SHDN is high, reducing supply current to less than 2A. During shutdown, the synchronous rectifier disconnects the output from the input, eliminating the DC conduction path that normally exists with traditional boost converters in shutdown mode. In shutdown, OUT becomes a high-imped-
_______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown MAX1795/MAX1796/MAX1797
VIN
pate this energy, minimizing the ringing at LX. Damping LX ringing does not reduce VOUT ripple, but does reduce EMI (Figures 3, 4, and 5).
Setting the Output Voltage
BATT MAX1795 MAX1796 MAX1797 R1 200 DAMPING SWITCH LX OUT 47F
47F
22H
VOUT can be set to 3.3V or 5.0V by connecting the FB pin to GND (5V) or OUT (3.3V). To adjust the output voltage, connect a resistive voltage-divider from OUT to FB to GND (Figure 6). Choose a value less than 250k for R2.
VIN
VOUT
BATT LX
Figure 3. Simplified Diagram of Inductor Damping Switch
R3 LBI R4 MAX1795 MAX1796 MAX1797
OUT SHDN 1M R1 LBO FB R2
OUTPUT 2V TO 5.5V 47F
LOW-BATTERY OUTPUT
GND
VLX 1V/div
Figure 6. Setting an Adjustable Output
Use the following equation to calculate R1: R1 = R2 [(VOUT / VFB) - 1]
2s/div
where VFB = +1.245V, and VOUT may range from +2V to +5.5V.
Figure 4. LX Ringing for Conventional Step-Up Converter (without Damping Switch)
Low-Battery Detection
The MAX1795/MAX1796/MAX1797 each contain an onchip comparator for low-battery detection. If the voltage at LBI is above 0.85V, LBO (an open-drain output) sinks current to GND. If the voltage at LBI is below 0.85V, LBO goes high impedance. The LBI/LBO function remains active even when the part is in shutdown. Connect a resistive voltage-divider to LBI from BATT to GND. The low-battery monitor threshold is set by two resistors, R3 and R4 (Figure 6). Since the LBI bias current is typically 2nA, large resistor values (R4 up to 250k) can be used to minimize loading of the input supply. Calculate R3 using the following equation: R3 = R4[(VTRIP / 0.85V) - 1]
VLX 1V/div
2s/div
Figure 5. LX Waveform with Damping Switch 10 ______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown MAX1795/MAX1796/MAX1797
VTRIP is the input voltage where the low-battery detector output goes high impedance. For single-cell applications, LBI may be connected to the battery. When V BATT <1.0V>, the LBI threshold increases to 0.925V (see Typical Operating Characteristics). Connect a pullup resistor of 100k or greater from LBO to OUT for a logic output. LBO is an open-drain output and can be pulled as high as 6V regardless of the voltage at OUT. When LBI is below the threshold, the LBO output is high impedance. If the low-battery comparator is not used, ground LBI and LBO. I IOUT(MAX) = ILIM - RIP (1 - D) 2 where: IRIP = Inductor ripple current (A) VOUT = Output voltage (V) ILIM = Device current limit (0.25A, 0.5A, or 1A) RPFET = On-resistance of P-channel MOSFET () (typ 0.27) LESR = ESR of Inductor () (typ 0.095) VBATT = Input voltage (V) L = Inductor value in H tOFF = LX switch's off-time (s) (typ 1s) D = Duty cycle RNFET = On-resistance of N-channel MOSFET () (typ 0.17) IOUT(MAX) = Maximum output current (A)
Applications Information
Inductor Selection
An inductor value of 22H performs well in most applications. The MAX1795/MAX1796/MAX1797 will also work with inductors in the 10H to 47H range. Smaller inductance values typically offer a smaller physical size for a given series resistance, allowing the smallest overall circuit dimensions, but have lower output current capability. Circuits using larger inductance values exhibit higher output current capability, but are physically larger for the same series resistance and current rating. The inductor's incremental saturation current rating should be greater than the peak switch-current limit, which is 0.25A for the MAX1795, 0.5A for the MAX1796, and 1A for the MAX1797. However, it is generally acceptable to bias the inductor into saturation by as much as 20% although this will slightly reduce efficiency. Table 1 lists some suggested components for typical applications. The inductor's DC resistance significantly affects efficiency. Calculate the maximum output current (IOUT(MAX)) as follows, using inductor ripple current (IRIP) and duty cycle (D):
IRIP = VOUT + ILIM L t OFF VOUT + ILIM - x (RPFET + LESR ) - VBATT (RPFET + LESR ) + 2 IRIP (RPFET + LESR ) - VBATT 2
Capacitor Selection
Table 1 lists suggested tantalum or polymer capacitor values for typical applications. The ESR of both input bypass and output filter capacitors affects efficiency and output ripple. Output voltage ripple is the product of the peak inductor current and the output capacitor ESR. High-frequency output noise can be reduced by connecting a 0.1F ceramic capacitor in parallel with the output filter capacitor. (See Table 2 for a list of suggested component suppliers.)
PC Board Layout and Grounding
Careful printed circuit layout is important for minimizing ground bounce and noise. Keep the IC's GND pin and the ground leads of the input and output filter capacitors less than 0.2in (5mm) apart. In addition, keep all connections to the FB and LX pins as short as possible. In particular, when using external feedback resistors, locate them as close to FB as possible. To maximize output power and efficiency and minimize output ripple voltage, use a ground plane and solder the IC's GND pin directly to the ground plane.
D=
I VOUT + ILIM - RIP (RPFET - RNFET + LESR ) 2
and
______________________________________________________________________________________
11
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown MAX1795/MAX1796/MAX1797
Table 1. Suggested Components for Typical Applications
COMPONENT COMPONENT VALUE (MAX1797, 1A CURRENT LIMIT) COMPONENT VALUE (MAX1796, 0.5A CURRENT LIMIT) COMPONENT VALUE (MAX1795, 0.25A CURRENT LIMIT) Sumida CR32-220, 22H Sumida CDRH6D28-220, 22H Inductor Coilcraft DS3316P-223, 22H Sanyo POSCAP 6TPA47M, 47F AVX TPSD476M016R0150, 47F Output Capacitor Taiyo Yuden UMK316BI150KH, 0.1F Taiyo Yuden UMK316BI150KH, 0.1F Taiyo Yuden UMK316BI150KH, 0.1F Coilcraft DS1608C-223, 22H Sanyo POSCAP 6TPA47M, 47F AVX TPSD226M016R0150, 22F Sumida CDRH4D28-220, 22H Sumida CR32-100, 10H Murata CQH3C100K34, 10H Murata CQH4N100K(J)04, 10H Coilcraft DS1608C-223, 22H Coilcraft DS1608C-103, 10H Input Capacitor Sanyo POSCAP 6TPA47M, 47F AVX TPSD106M016R0150, 10F
Table 2. Component Suppliers
COMPANY AVX Coilcraft Coiltronics Murata PHONE USA 803-946-0690 USA 847-639-6400 USA 561-241-7876 USA 814-237-1431 1-800-831-9172 USA 805-867-2555 Japan 81-3-34947411 US A 619-661-6835 Ja pan 81- 7- 20706306 USA 603-224-1961 USA 647-956-0666 Japan 81-3-36075111 USA 408-573-4150 FAX USA 803-626-3123 USA 847-639-1238469 USA 561-241-9339 USA 814-238-0490 USA 805-867-2556 Japan 81-3-34947414 US A 619-661-1055 Ja pan 81- 7- 20701174 USA 603-224-1430 US A 647-956-0702 Ja pan 81- 3- 36075144 USA 408-573-4159
Chip Information
TRANSISTOR COUNT: 1100 PROCESS: BiCMOS
Nihon
Sanyo Sprague Sumida Taiyo Yuden
12
______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters with True-Shutdown
Package Information
8LUMAXD.EPS
MAX1795/MAX1796/MAX1797
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________13 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

▲Up To Search▲

Price & Availability of MAX1795

	To Download MAX1795 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .