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19-1666; Rev 0; 7/00
28V Internal Switch LCD Bias Supply in SOT23
General Description
The MAX1605 boost converter contains a 0.5A internal switch in a tiny 6-pin SOT23 package. The IC operates from a +2.4V to +5.5V supply voltage but can boost battery voltages as low as 0.8V up to 28V at the output. The MAX1605 uses a unique control scheme providing the highest efficiency over a wide range of load conditions. An internal 0.5A MOSFET reduces external component count, and a high switching frequency (up to 500kHz) allows for tiny surface-mount components. The current limit can be set to 500mA, 250mA, or 125mA, allowing the user to reduce the output ripple and component size in low-current applications. Additional features include a low quiescent supply current and a shutdown mode to save power. The MAX1605 is ideal for small LCD panels with low current requirements but can also be used in other applications. A MAX1605EVKIT evaluation kit (EV kit) is available to help speed up design time. o Adjustable Output Voltage up to 28V o 20mA at 20V from a Single Li+ Battery o 88% Efficiency o Up to 500kHz Switching Frequency o Selectable Inductor Current Limit (125mA, 250mA, or 500mA) o 18A Operating Supply Current o 0.1A Shutdown Current o Small 6-Pin SOT23 Package
Features
MAX1605
________________________Applications
LCD Bias Generators Cellular or Cordless Phones Palmtop Computers Personal Digital Assistants (PDAs) Organizers Handy Terminals
PART MAX1605EUT-T
Ordering Information
TEMP. RANGE -40C to +85C PINPACKAGE 6 SOT23-6 SOT MARK AAHP
Typical Operating Circuit
VIN = 0.8V TO VOUT L1 10H
Pin Configuration
TOP VIEW
SHDN 1 6 FB
VCC = 2.4V TO 5.5V
VOUT = VIN TO 28V VCC MAX1605 LX
VCC 2
MAX1605
5
LIM
GND 3
LIM ON OFF SHDN GND FB
4
LX
SOT23-6
________________________________________________________________ Maxim Integrated Products
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28V Internal Switch LCD Bias Supply in SOT23 MAX1605
ABSOLUTE MAXIMUM RATINGS
VCC, FB, LIM, SHDN to GND....................................-0.3V to +6V LX to GND ..............................................................-0.3V to +30V Continuous Power Dissipation (TA = +70C) 6-Pin SOT23 (derate 8.7mW/C above +70C) ...........696mW 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
(VCC = SHDN = 3.3V, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER Supply Voltage Inductor Input Voltage Range VCC Undervoltage Lockout Quiescent Supply Current Shutdown Supply Current VCC Line Regulation VIN Line Regulation Load Regulation Efficiency Feedback Set Point Feedback Input Bias Current LX LX Voltage Range LX Switch Current Limit VLX LIM = VCC ILX(MAX) LIM = floating LIM = GND LX On-Resistance LX Leakage Current Maximum LX On-Time Minimum LX Off-Time CONTROL INPUTS VIH SHDN Input Threshold VIL SHDN Input Bias Current ISHDN 2.4V VCC 5.5V VCC = 5.5V, V SHDN = 0 to 5.5V -1 2.4V VCC 5.5V 0.8 x VCC 0.2 x VCC 1 tON tOFF VFB > 1.1V VFB < 0.8V (soft-start) RLX VCC = 5V, ILX = 100mA VCC = 3.3V, ILX = 100mA VLX = 28V 10 0.8 3.9 13 1.0 5.0 0.40 0.20 0.10 0.50 0.25 0.125 0.8 1 2 2 16 1.2 6.0 28 0.56 0.285 0.15 A s s A V VFB IFB VFB = 1.3V VLNR VLNR VLDR SYMBOL VCC VIN VUVLO ICC (Note 2) (Note 2) VCC falling, 50mV typical hysteresis VFB = 1.3V SHDN = GND VOUT = 18V, ILOAD = 1mA, VIN = 5V, VCC = VLIM = 2.4V to 5.5V VOUT = 18V, ILOAD = 1mA, VCC = VLIM = 5V, VIN = 2.4V to 12V VOUT = 18V, VCC = VIN = VLIM = 5V, ILOAD = 0mA to 20mA L1 = 100H, VIN = 3.6V, ILOAD = 10mA 1.225 CONDITIONS MIN 2.4 0.8 2.0 2.2 18 0.1 0.1 0.15 0.1 88 1.25 5 1.275 100 TYP MAX 5.5 VOUT 2.37 35 1 UNITS V V V A A %/V %/V %/mA % V nA
V
A
2
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28V Internal Switch LCD Bias Supply in SOT23 MAX1605
ELECTRICAL CHARACTERISTICS (continued)
(VCC = SHDN = 3.3V, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER LIM Input Low Level LIM Input Float Level LIM Input High Level LIM Input Bias Current ILIM SYMBOL CONDITIONS 2.4V VCC 5.5V 2.4V VCC 5.5V, ILIM = 0.5A 2.4V VCC 5.5V SHDN = VCC, LIM = GND or VCC SHDN = GND (VCC / 2) 0.2V VCC - 0.4V -2 0.1 2 1 MIN TYP MAX 0.4 (VCC / 2) + 0.2V UNITS V V V A
ELECTRICAL CHARACTERISTICS
(VCC = SHDN = 3.3V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Supply Voltage Inductor Input Voltage Range VCC Undervoltage Lockout Quiescent Supply Current Shutdown Supply Current Feedback Set Point Feedback Input Bias Current LX LX Voltage Range LX Switch Current Limit LX On-Resistance LX Leakage Current Maximum LX On-Time Minimum LX Off-Time CONTROL INPUTS VIH SHDN Input Threshold VIL SHDN Input Bias Current ISHDN 2.4V VCC 5.5V VCC = 5.5V, VSHDN = 0 to 5.5V -1 2.4V VCC 5.5V 0.8 x VCC 0.2 x VCC 1 tON tOFF VFB > 1.1V VFB < 0.8V VLX LIM = VCC ILX(MAX) RLX LIM = floating LIM = GND VCC = 3.3V, ILX = 100mA VLX = 28V 9 0.75 3.8 0.35 0.18 0.08 28 0.58 0.30 0.17 2 2 17 1.25 6.0 A s s A V VFB IFB VFB = 1.3V SYMBOL VCC VIN VUVLO ICC (Note 2) (Note 2) VCC falling, 50mV typical hysteresis VFB = 1.3V SHDN = GND 1.215 CONDITIONS MIN 2.4 0.8 2.0 MAX 5.5 VOUT 2.37 35 1 1.285 100 UNITS V V V A A V nA
V
A
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28V Internal Switch LCD Bias Supply in SOT23 MAX1605
ELECTRICAL CHARACTERISTICS (continued)
(VCC = SHDN = 3.3V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER LIM Input Low Level LIM Input Float Level LIM Input High Level LIM Input Bias Current ILIM SYMBOL CONDITIONS 2.4V VCC 5.5V 2.4V VCC 5.5V, ILIM = 0.5A 2.4V VCC 5.5V SHDN = VCC, LIM = GND or VCC SHDN = GND (VCC / 2) - 0.25V VCC - 0.4V -2 MIN MAX 0.4 (VCC / 2) + 0.25V UNITS V V V 2 1 A
Note 1: All devices are 100% tested at TA = +25C. All limits over the temperature range are guaranteed by design. Note 2: The MAX1605 requires a supply voltage between +2.4V and +5.5V; however, the input voltage used to power the inductor can vary from +0.8V to VOUT.
Typical Operating Characteristics
(VCC = 3.3V, VIN = 3.6V, L1 = 10H, SHDN = LIM = VCC, VOUT(NOM) = 18V (Figure 3), TA = +25C, unless otherwise noted.)
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1605 toc02
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
VIN = 3.6V LIM = VCC (500mA) IOUT = 5mA
MAX1605 toc01
OUTPUT VOLTAGE vs. INPUT VOLTAGE
21.5 21.3 21.1 OUTPUT VOLTAGE (V) 20.9 20.7 20.5 20.3 20.1 19.9 19.7 VCC = 3.3V LIM = VCC (500mA) 0 3 6 INPUT VOLTAGE (V) 9 12 IOUT = 1mA IOUT = 5mA 18.4 18.3 18.2 OUTPUT VOLTAGE (V) 18.1 18.0 17.9 17.8 17.7 17.6 17.5 17.4 0 5 LIM = GND (125mA)
18.1 OUTPUT VOLTAGE (V) 18.0 17.9 17.8 17.7 17.6 2.0
LIM = VCC (500mA)
IOUT = 1mA
LIM = OPEN (250mA)
19.5 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V)
10
15
20
25
LOAD CURRENT (mA)
EFFICIENCY vs. SUPPLY VOLTAGE (L1 = 10H)
MAX1605 toc04
EFFICIENCY vs. INPUT VOLTAGE (L1 = 10H)
IOUT = 5mA 80 EFFICIENCY (%) 70 60 50 40 30 VCC = 3.3V ILIM = 500mA 0 3 6 INPUT VOLTAGE (V) 9 12 IOUT = 1mA
MAX1605 toc05
EFFICIENCY vs. LOAD CURRENT (L1 = 10H)
88 86 EFFICIENCY (%) 84 82 80 78 76 74 0 5 10 15 20 25 LOAD CURRENT (mA) LIM = VCC (500mA) LIM = OPEN (250mA) LIM = GND (125mA)
MAX1605 toc06
80 IOUT = 5mA IOUT = 10mA
90
90
78 EFFICIENCY (%)
76
74
IOUT = 1mA 80 VIN = 3.6V ILIM = 500mA
72
70 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V)
4
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MAX1605 toc03
18.2
28V Internal Switch LCD Bias Supply in SOT23
Typical Operating Characteristics (continued)
(VCC = 3.3V, VIN = 3.6V, L1 = 10H, SHDN = LIM = VCC, VOUT(NOM) = 18V (Figure 3), TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (L1 = 47H)
MAX1605 toc07
MAX1605
EFFICIENCY vs. LOAD CURRENT (L1 = 100H)
MAX1605 toc08
CURRENT LIMIT vs. SUPPLY VOLTAGE
LIM = VCC 500 CURRENT LIMIT (mA)
MAX1605 toc09
90 88 86 EFFICIENCY (%) 84 82 80 78 76 74 0 5 10 15 20 LIM = VCC (500mA) LIM = OPEN (250mA) LIM = GND (125mA)
90 88 86 EFFICIENCY (%) 84 82 80 78 76 74 LIM = VCC (500mA) LIM = GND (125mA) LIM = OPEN (250mA)
600
400
300
LIM = OPEN
200 LIM = GND 100
25
0
5
10
15
20
25
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
LOAD CURRENT (mA)
LOAD CURRENT (mA)
SUPPLY VOLTAGE (V)
CURRENT LIMIT vs. INPUT VOLTAGE
MAX1605 toc10
SUPPLY CURRENT vs. SUPPLY VOLTAGE (NO LOAD)
MAX1605 toc11
SUPPLY CURRENT vs. LOAD CURRENT
LIM = GND (125mA) LIM = OPEN (250mA)
MAX1605 toc12
600 LIM = VCC
25
3.0 2.5 SUPPLY CURRENT (mA) 2.0 1.5 1.0 0.5 0
500 CURRENT LIMIT (mA)
20 SUPPLY CURRENT (A)
400 LIM = OPEN 300
15
10
200
LIM = GND
5
LIM = VCC (500mA)
100 0 3 6 INPUT VOLTAGE (V) 9 12
0 0 1 2 3 4 5 SUPPLY VOLTAGE (V)
0
5
10
15
20
25
LOAD CURRENT (mA)
LINE TRANSIENT
MAX1605 toc13
LOAD TRANSIENT
MAX1605 toc14
SHUTDOWN WAVEFORM
MAX1605 toc15
4V IOUT 10mA/div 10mA A 2V/div 0 18.1V 18V B 100mV/div 2V 0 IL1 250mA/div 200s/div VOUT = 18V, ROUT = 1.8k VCC = 3.3V, VIN = 3.6V VOUT 10V/div 500mA 250mA 0 IL1 500mA/div 20V 10V 0 VSHDN 2V/div
6V 4V 2V
18.1V 18 17.9V
17.9V 500mA 0
200s/div A: VIN = VCC = 2.4V TO 5.5V B: VOUT = 18V, ROUT = 3.6k
40s/div VOUT = 18V, IOUT = 1mA TO 10mA VCC = 3.3V, VIN = 3.6V
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VOUT 100mV/div
5
28V Internal Switch LCD Bias Supply in SOT23 MAX1605
Pin Description
PIN 1 2 3 4 5 6 NAME SHDN VCC GND LX LIM FB FUNCTION Active-Low Shutdown Input. A logic low shuts down the device and reduces the supply current to 0.1A. Connect SHDN to VCC for normal operation. IC Supply Voltage (+2.4V to +5.5V). Bypass VCC to GND with a 0.1F or greater capacitor. Ground Inductor Connection. The drain of an internal 28V N-channel MOSFET. LX is high impedance in shutdown. Inductor Current Limit Selection. Connect LIM to VCC for 500mA, leave LIM floating for 250mA, or connect LIM to GND for 125mA. Feedback Input. Connect to a resistive-divider network between the output (VOUT) and FB to set the output voltage between VIN and 28V. The feedback threshold is 1.25V.
L1 10H
VIN = 0.8V TO VOUT
LX CFF VCC = 2.4V TO 5.5V CONTROL LOGIC VCC LIM CURRENT LIMIT N
VOUT = VIN TO 28V
COUT
SHUTDOWN LOGIC
R1 FB
ON OFF
SHDN
ERROR AMPLIFIER 1.25V GND MAX1605
R2
Figure 1. Functional Diagram
Detailed Description
The MAX1605 compact, step-up DC-DC converter operates from a +2.4V to +5.5V supply. Consuming only 18A of supply current, the device includes an internal switching MOSFET with 1 on-resistance and selectable current limit (Figure 1). During startup, the MAX1605 extends the minimum off-time, limiting initial
6
surge current. The MAX1605 also features a shutdown mode.
Control Scheme
The MAX1605 features a minimum off-time, current-limited control scheme. The duty cycle is governed by a pair of one-shots that set a minimum off-time and a maximum on-time. The switching frequency can be up
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28V Internal Switch LCD Bias Supply in SOT23 MAX1605
VCC (2.4V TO 5.5V) VCC MAX1605 LIM NO CONNECTION LIM VCC (2.4V TO 5.5V) VCC MAX1605 LIM VCC (2.4V TO 5.5V) VCC MAX1605
GND
GND
GND
IPEAK = 500mA
IPEAK = 250mA
IPEAK = 125mA
Figure 2. Setting the Peak Inductor Current Limit
to 500kHz and depends upon the load and input voltage. The peak current limit of the internal N-channel MOSFET is pin selectable and may be set at 125mA, 250mA, or 500mA (Figure 2).
below VIN when the MAX1605 is shut down. The capacitance and load at OUT determine the rate at which V OUT decays. SHDN can be pulled as high as 6V, regardless of the input and output voltages.
Setting the Output Voltage (FB)
Adjust the output voltage by connecting a voltagedivider from the output (VOUT) to FB (Figure 3). Select R2 between 10k to 200k. Calculate R1 with the following equation: R1 = R2 [(VOUT / VFB) - 1] where VFB = 1.25V and VOUT may range from VIN to 28V. The input bias current of FB has a maximum value of 100nA, which allows large-value resistors to be used. For less than 1% error, the current through R2 should be greater than 100 times the feedback input bias current (IFB).
Separate/Same Power for L1 and VCC
Separate voltage sources can supply the inductor (VIN) and the IC (VCC). This allows operation from low-voltage batteries as well as high-voltage sources (0.8V to 28V) because chip bias is provided by a logic supply (2.4V to 5.5V) while the output power is sourced directly from the battery to L1. Conversely, VIN and VCC can also be supplied from one supply if it remains within VCC's operating limits (+2.4V to +5.5V).
VIN = 0.8V TO VOUT CIN 10F VCC = 2.4V TO 5.5V C1 0.1F LIM ON OFF SHDN
L1 10H
Current Limit Select Pin (LIM)
The MAX1605 allows a selectable inductor current limit of 125mA, 250mA, or 500mA (Figure 2). This allows flexibility in designing for higher current applications or for smaller, compact designs. The lower current limit allows the use of a physically smaller inductor in spacesensitive, low-power applications. Connect LIM to VCC for 500mA, leave floating for 250mA, or connect to GND for 125mA.
D1 VCC MAX1605 FB R2 165k GND LX R1 2.2M
VOUT = 18V CFF 10pF COUT 1F
Shutdown (SHDN)
Pull SHDN low to enter shutdown. During shutdown, the supply current drops to 0.1A and LX enters a highimpedance state. However, the output remains connected to the input through the inductor and output rectifier, holding the output voltage to one diode drop
Figure 3. Typical Application Circuit 7
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28V Internal Switch LCD Bias Supply in SOT23 MAX1605
Design Procedure
Inductor Selection
Smaller inductance values typically offer smaller physical size for a given series resistance or saturation current. Circuits using larger inductance values may start up at lower input voltages and exhibit less ripple, but also provide reduced output power. This occurs when the inductance is sufficiently large to prevent the maximum current limit from being reached before the maximum on-time expires. The inductor's saturation current rating should be greater than the peak switching current. However, it is generally acceptable to bias the inductor into saturation by as much as 20%, although this will slightly reduce efficiency.
Input Bypass Capacitor
Two inputs, VCC and VIN, require bypass capacitors. Bypass VCC with a 0.1F ceramic capacitor as close to the IC as possible. The input supplies high currents to the inductor and requires local bulk bypassing close to the inductor. A 10F low-ESR surface-mount capacitor is sufficient for most applications.
PC Board Layout and Grounding
Careful printed circuit layout is important for minimizing ground bounce and noise. Keep the MAX1605's ground pin and the ground leads of the input and output capacitors less than 0.2in (5mm) apart. In addition, keep all connections to FB and LX as short as possible. In particular, when using external feedback resistors, locate them as close to FB as possible. To minimize output voltage ripple, and to maximize output power and efficiency, use a ground plane and solder GND directly to the ground plane. Refer to the MAX1605EVKIT evaluation kit for a layout example.
Picking the Current Limit
The peak LX current limit (ILX(MAX)) required for the application may be calculated from the following equation:
ILX(MAX) VOUT - VIN(MIN) x t OFF(MIN) VOUT x IOUT(MAX) + VIN(MIN) 2xL
(
)
Applications Information
Negative Voltage for LCD Bias
The MAX1605 can also generate a negative output by adding a diode-capacitor charge-pump circuit (D1, D2, and C3) to the LX pin as shown in Figure 4. Feedback is still connected to the positive output, which is not loaded, allowing a very small capacitor value at C4. For best stability and lowest ripple, the time constant of the R1-R2 series combination and C4 should be near or less than that of C2 and the effective load resistance. Output load regulation of the negative output is somewhat looser than with the standard positive output circuit, and may rise at very light loads due to coupling through the capacitance of D2. If this is objectionable, reduce the resistance of R1 and R2, while maintaining their ratio, to effectively preload the output with a few hundred microamps. This is why the R1-R2 values shown in Figure 3 are about 10-times lower than typical values used for a positive-output design. When loaded, the negative output voltage will be slightly lower (closer to ground by approximately a diode forward voltage) than the inverse of the voltage on C4.
where tOFF(MIN) = 0.8s, and VIN(MIN) is the minimum voltage used to supply the inductor. The set current limit must be greater than this calculated value. Select the appropriate current limit by connecting LIM to VCC, GND, or leaving it unconnected (see Current Limit Select Pin and Figure 2).
Diode Selection
The high maximum switching frequency of 500kHz requires a high-speed rectifier. Schottky diodes, such as the Motorola MBRS0530 or the Nihon EP05Q03L, are recommended. To maintain high efficiency, the average current rating of the Schottky diode should be greater than the peak switching current. Choose a reverse breakdown voltage greater than the output voltage.
Output Filter Capacitor
For most applications, use a small ceramic surfacemount output capacitor, 1F or greater. For small ceramic capacitors, the output ripple voltage is dominated by the capacitance value. If tantalum or electrolytic capacitors are used, the higher ESR increases the output ripple voltage. Decreasing the ESR reduces the output ripple voltage and the peak-to-peak transient voltage. Surface-mount capacitors are generally preferred because they lack the inductance and resistance of their through-hole equivalents.
Output Disconnected in Shutdown
When the MAX1605 is shut down, the output remains connected to the input (Figure 3), so the output voltage falls to approximately V IN - 0.6V (the input voltage minus a diode drop). For applications that require output isolation during shutdown, add an external PNP transistor as shown in Figure 4. When the MAX1605 is active, the voltage set at the transistor's emitter exceeds the input voltage, forcing the transistor into the
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28V Internal Switch LCD Bias Supply in SOT23 MAX1605
VIN = 0.8V TO VOUT L1 10H R3 1 C3 0.1F D1* D2* VNEG -19V C2 1F
C5 1F D3** VCC C6 0.1F LIM ON OFF SHDN GND MAX1605 FB LX R1 240k
VCC = 2.4V TO 5.5V
C1 1000pF
C4 0.01F
R2 16.5k *D1, D2 = CENTRAL SEMICONDUCTOR CMPD7000 DUAL **D3 = CENTRAL SEMICONDUCTOR CMSD4448 (1N4148)
Figure 4. Negative Voltage for LCD Bias
saturation region. When shut down, the input voltage exceeds the emitter voltage so the inactive transistor provides high-impedance isolation between the input and output. Efficiency will be slightly degraded due to the PNP transistor saturation voltage and base current.
Chip Information
TRANSISTOR COUNT: 2329
VIN = 0.8V TO VOUT
L1 10H
R3 = 180k
VCC = 2.4V TO 5.5V VCC MAX1605 LIM FB R2 ON OFF SHDN GND LX R1
VSET = 18.3V (VOUT +0.3V) 2N2907A
VOUT = 18V
Figure 5. Output Disconnected in Shutdown
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9
28V Internal Switch LCD Bias Supply in SOT23 MAX1605
Package Information
6LSOT.EPS
10
______________________________________________________________________________________
28V Internal Switch LCD Bias Supply in SOT23 MAX1605
NOTES
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28V Internal Switch LCD Bias Supply in SOT23 MAX1605
NOTES
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.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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