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19-1919; Rev 1; 01/02
24V Internal Switch, 100% Duty Cycle, Step-Down Converters
General Description
The MAX1836/MAX1837 high-efficiency step-down converters provide a preset 3.3V or 5V output voltage from supply voltages as high as 24V. Using external feedback resistors, the output voltage may be adjusted from 1.25V to VIN. An internal current-limited switching MOSFET delivers load currents up to 125mA (MAX1836) or 250mA (MAX1837). The unique current-limited control scheme, operating with duty cycles up to 100%, minimizes the dropout voltage (120mV at 100mA). Additionally, this control scheme reduces supply current under light loads to 12A. High switching frequencies allow the use of tiny surface-mount inductors and output capacitors. The MAX1836/MAX1837 step-down converters with internal switching MOSFETs are available in a 6-pin SOT23 package, making them ideal for low-cost, lowpower, space-sensitive applications. For increased output drive capability, use the MAX1776 step-down converter that uses an internal 24V switch to deliver up to 500mA. For even higher currents, use the MAX1626/ MAX1627 step-down controllers that drive an external P-channel MOSFET to deliver up to 20W.
____________________________Features
o 4.5V to 24V Input Voltage Range o Preset 3.3V or 5V Output o Adjustable Output from 1.25V to VIN o Output Currents Up to 125mA (MAX1836) or 250mA (MAX1837) o Internal P-Channel MOSFET o Efficiency Over 90% o 12A Quiescent Current o 3A Shutdown Current o 100% Maximum Duty Cycle for Low Dropout o Current-Limiting and Overtemperature Protection o Small 6-Pin SOT23 Package
MAX1836/MAX1837
Ordering Information
PART MAX1836EUT50-T MAX1836EUT33-T MAX1837EUT50-T MAX1837EUT33-T TEMP RANGE -40C to +85C -40C to +85C -40C to +85C -40C to +85C PINPACKAGE 6 SOT23-6 6 SOT23-6 6 SOT23-6 6 SOT23-6 TOP MARK AANW AANY AANX AANZ
________________________Applications
9V Battery Systems Notebook Computers Distributed Power Systems Backup Supplies 4mA to 20mA Loop Power Supplies Industrial Control Supplies Hand-Held Devices
Note: The MAX1836/MAX1837 require special solder temperature profile described in the Absolute Maximum Ratings.
Selector Guide
PART MAX1836EUT50 MAX1836EUT33 MAX1837EUT50 MAX1837EUT33 PRESET OUTPUT VOLTAGE (V) 5 3.3 5 3.3 LOAD CURRENT (mA) 125 125 250 250
Typical Operating Circuit
INPUT 4.5V TO 24V OUTPUT 3.3V OR 5V IN SHDN OUT MAX1836 MAX1837 GND FB LX
Pin Configuration
TOP VIEW
FB 1 6 OUT
GND 2
MAX1836 MAX1837
5
SHDN
IN 3
4
LX
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
SOT23
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
24V Internal Switch, 100% Duty Cycle, Step-Down Converters MAX1836/MAX1837
ABSOLUTE MAXIMUM RATINGS
IN, SHDN to GND ...................................................-0.3V to +25V LX to GND.......................................................-2V to (VIN + 0.3V) OUT, FB to GND.......................................................-0.3V to +6V Continuous Power Dissipation (TA = +70C) (Note 1) 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
Note 1: Thermal properties are specified with product mounted on PC board with 1in2 of copper area and still air.
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
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER Input Supply Range Input Undervoltage Lockout Threshold Input Supply Current Input Supply Current in Dropout Input Shutdown Current SYMBOL VIN VUVLO IIN IIN(DROP) VIN = 5V SHDN = GND FB = GND, ILOAD = 0 to 125mA (MAX1836) or 250mA (MAX1837) (Note 2) MAX183_EUT50 MAX183_EUT33 4.80 3.168 1.25 1.200 VOUT = 5V IFB Threshold tOFF(MIN) tON(MAX) RLX ILIM VFB = 1.3V VIN = 6V MAX1836 MAX1837 250 500 -75 VFB = 0 or 1.25V, TA = +25C VFB rising or falling -25 50 0.2 7 100 0.4 10 1.1 312 625 1.25 2.5 VIN rising VIN falling CONDITIONS MIN 4.5 3.55 3.45 4.0 3.9 12 18 3 5.00 3.30 7 5.20 V 3.432 VIN 1.300 7.4 +25 150 0.6 13 2 450 850 +75 V V A nA mV s s mA mV TYP MAX 24 4.4 4.3 25 UNITS V V A A A
Output Voltage (Preset Mode)
VOUT
Output Voltage Range (Adjustable Mode) Feedback Set Voltage (Adjustable Mode) OUT Bias Current FB Bias Current FB Dual Mode
TM
VOUT VFB
LX Switch Minimum Off-Time LX Switch Maximum On-Time LX Switch On-Resistance LX Current Limit LX Zero-Crossing Threshold
Dual Mode is a trademark of Maxim Integrated Products, Inc. 2 _______________________________________________________________________________________
24V Internal Switch, 100% Duty Cycle, Step-Down Converters
ELECTRICAL CHARACTERISTICS (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER Zero-Crossing Timeout LX Switch Leakage Current Dropout Voltage Line Regulation Load Regulation Shutdown Input Threshold Shutdown Leakage Current Thermal Shutdown V SHDN ISHDN VDROPOUT SYMBOL CONDITIONS LX does not rise above the threshold VIN = 18V, LX = GND, TA = +25C IOUT = 100mA, VIN = 5V VIN = 5V to 24V IOUT = 0 to 125mA (MAX1836) or 250mA (MAX1837) VIN = 4.5V to 24V (Note 3) V SHDN = 0 or 24V 10C hysteresis (typ) 0.8 -1 160 120 0.05 0.3 2.4 +1 MIN TYP 30 1 MAX UNITS s A mV % % V A C
MAX1836/MAX1837
ELECTRICAL CHARACTERISTICS
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = -40C to +85C, unless otherwise noted.) (Note 4)
PARAMETER Input Supply Range Input Undervoltage Lockout Threshold Input Supply Current Input Shutdown Current SYMBOL VIN VUVLO IIN SHDN = GND FB = GND, ILOAD = 0 to 125mA (MAX1836) or 250mA (MAX1837) (Note 2) MAX183_EUT50 MAX183_EUT33 4.80 3.168 1.25 1.200 VOUT = 5V VFB rising or falling tOFF(MIN) tON(MAX) RLX ILIM VFB = 1.3V VIN = 6V MAX1836 MAX1837 250 500 50 0.2 7 VIN rising VIN falling CONDITIONS MIN 4.5 3.55 3.45 TYP MAX 24 4.4 4.3 25 7 5.20 V 3.432 VIN 1.300 7.4 150 0.6 13 2 450 900 V V A mV s s mA UNITS V V A A
Output Voltage (Preset Mode)
VOUT
Output Voltage Range (Adjustable Mode) Feedback Set Voltage (Adjustable Mode) OUT Bias Current FB Dual Mode Threshold LX Switch Minimum Off-Time LX Switch Maximum On-Time LX Switch On-Resistance LX Current Limit
VOUT VFB
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24V Internal Switch, 100% Duty Cycle, Step-Down Converters MAX1836/MAX1837
ELECTRICAL CHARACTERISTICS (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = -40C to +85C, unless otherwise noted.) (Note 4)
PARAMETER LX Zero-Crossing Threshold Shutdown Input Threshold Shutdown Leakage Current V SHDN ISHDN VIN = 4.5V to 24V (Note 3) V SHDN = 0 or 24V SYMBOL CONDITIONS MIN -75 0.8 -1 TYP MAX +75 2.4 +1 UNITS mV V A
Note 2: When using the shutdown input, the maximum output voltage allowed with external feedback is 5.5V. If the output voltage is set above 5.5V, connect shutdown to the input. Note 3: Shutdown input minimum slew rate (rising or falling) is 10V/ms. Note 4: Specifications to -40C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25C.)
MAX1836EUT33 OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1836/7 toc01
MAX1836EUT33 EFFICIENCY vs. LOAD CURRENT
MAX1836/7 toc02
MAX1837EUT33 OUTPUT VOLTAGE vs. LOAD CURRENT
FIGURE 2 3.32 OUTPUT VOLTAGE (V) 3.31 VIN = 9V 3.30 3.29 3.28 3.27 VIN = 5V VIN = 12V
MAX1836/7 toc03
3.33 FIGURE 1 3.32 OUTPUT VOLTAGE (V) 3.31 3.30 3.29 3.28 3.27 0 50 100 150 VIN = 9V to 12V VIN = 5V
100 95 EFFICIENCY (%) 90 85 80 VIN = 12V 75 70 FIGURE 1 VOUT = 3.3V VIN = 5V VIN = 9V
3.33
200
0.1
1
10
100
1000
0
50
100
150
200
250
300
350
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MAX1837EUT33 EFFICIENCY vs. LOAD CURRENT
MAX1836/7 toc04
MAX1837EUT33 SWITCHING FREQUENCY vs. LOAD CURRENT
MAX1836/7 toc05
MAX1837EUT33 OUTPUT VOLTAGE vs. INPUT VOLTAGE
IOUT = 10mA 3.32 OUTPUT VOLTAGE (V) 3.31 3.30 3.29 3.28 3.27 FIGURE 2 VOUT = 3.3V L1 = 47H 0 4 8 12 16 20 24
MAX1836/7 toc06
100 95 EFFICIENCY (%) 90 85 80 75 70 0.1 1 10 VIN = 12V 100 FIGURE 2 VOUT = 3.3V
180 160 140 FREQUENCY (kHz) 120 100 80 60 40 20 0 VIN = 5V 0 50 100 150 200 250 300 VIN = 12V FIGURE 2 VOUT = 3.3V VIN = 9V
3.33
VIN = 5V VIN = 9V
IOUT = 200mA
1000
350
LOAD CURRENT (mA)
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
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24V Internal Switch, 100% Duty Cycle, Step-Down Converters
Typical Operating Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25C.)
MAX1837EUT33 EFFICIENCY vs. INPUT VOLTAGE
MAX1836/7 toc07
MAX1836/MAX1837
MAX1837EUT33 SWITCHING FREQUENCY vs. INPUT VOLTAGE
MAX1836/7 toc08
MAX1837EUT33 PEAK INDUCTOR CURRENT vs. INPUT VOLTAGE
FIGURE 2 VOUT = 3.3V L1 = 47H
MAX1836/7 toc09
100 95 EFFICIENCY (%) 90 85 80 75 70 0 4 8 12 16 20 IOUT = 10mA IOUT = 200mA FIGURE 2 VOUT = 3.3V L1 = 47H
100 IOUT = 200mA
1000 PEAK INDUCTOR CURRENT (mA) IOUT = 200mA IOUT = 10mA 600
800
FREQUENCY (kHz)
10
FIGURE 2 VOUT = 3.3V L1 = 47H
400
IOUT = 10mA 1 24 0 4 8 12 16 20 24
200 0 0
LIMITED BY tON(MIN) LIMITED BY ILIM
4
8
12
16
20
24
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
MAX1837EUT50 OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1836/7 toc10
MAX1837EUT50 EFFICIENCY vs. LOAD CURRENT
FIGURE 6 VOUT = 5V VIN = 9V EFFICIENCY (%) 90 85 80 75 VIN = 18V VIN = 24V VIN = 12V
MAX1836/7 toc11
5.04 VIN = 12V TO 24V OUTPUT VOLTAGE (V) 5.02 VIN = 9V
100 95
VIN = 7V
5.00
4.98
VIN = 7V
4.96 0
FIGURE 6 70 50 100 150 200 250 300 0.1 1 10 100 1000 LOAD CURRENT (mA) LOAD CURRENT (mA)
MAX1837EUT50 DROPOUT VOLTAGE vs. LOAD CURRENT
MAX1836/7 toc12
NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE
MAX1836/7 toc13
400 350 DROPOUT VOLTAGE (mV) 300 250 200 150 100 50 0 0
FIGURE 6 VOUT = 5V
15
14 SUPPLY CURRENT (A)
13
12
11
10 100 200 300 0 4 8 12 16 20 24 LOAD CURRENT (mA) INPUT VOLTAGE (V)
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24V Internal Switch, 100% Duty Cycle, Step-Down Converters MAX1836/MAX1837
Typical Operating Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25C.)
MAX1837EUT50 LOAD TRANSIENT
MAX1836/7 toc14
MAX1837EUT50 LINE TRANSIENT
MAX1836/7 toc15
400mA 200mA 0 5.02V 5.00V 4.98V 750mA 250mA 0 100s/div A: IOUT = 10mA to 250mA, 200mA/div B: VOUT = 5V, 20mV/div C: IL, 500mA/div VIN = 12V, FIGURE 6 C B A
20V 10V 0 5.1V 5.0V 4.9V 500mA 0 400s/div A: VIN = 9V to 18V, 10V/div B: VOUT = 5V, ROUT = 100, 100mV/div C: IL, 500mA/div FIGURE 6 C B A
MAX1837EUT50 LINE TRANSIENT NEAR DROPOUT
MAX1836/7 toc16
MAX1837EUT50 STARTUP WAVEFORM
MAX1836/7 toc17
15V 10V 5V 5.1V 5.0V 4.9V 500mA C 0 400s/div A: VIN = 5V to 12V, 5V/div B: VOUT = 5V, ROUT = 100, 100mV/div C: IL, 500mA/div FIGURE 6 0 200s/div A: VSHDN = 0 to 2V, 2V/div B: VOUT = 5V, ROUT = 100, 2V/div C: IL, 500mA/div VIN = 12V, FIGURE 6 B A 2V 0 4V 2V 0 500mA C B A
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24V Internal Switch, 100% Duty Cycle, Step-Down Converters
Pin Description
PIN NAME FUNCTION Dual-Mode Feedback Input. Connect to GND for the preset 3.3V (MAX183_EUT33) or 5.0V (MAX183_EUT50) output. Connect to a resistive divider between the output and FB to adjust the output voltage between 1.25V and VIN, and connect the OUT pin to GND. When setting output voltages above 5.5V, permanently connect SHDN to IN. Ground Input Voltage. 4.5V to 24V input range. Connected to the internal P-channel power MOSFET's source. Inductor Connection. Connected to the internal P-channel power MOSFET's drain. Shutdown Input. A logic low shuts down the MAX1836/MAX1837 and reduces supply current to 3A. LX is high impedance in shutdown. Connect to IN for normal operation. When setting output voltages above 5.5V, permanently connect SHDN to IN. Regulated Output Voltage High-Impedance Sense Input. Internally connected to a resistive divider. Connect to the output when using the preset output voltage. Connect to GND when using an external resistive divider to adjust the output voltage.
MAX1836/MAX1837
1
FB
2 3 4 5
GND IN LX SHDN
6
OUT
INPUT 4.5V OR 12V CIN 10F 25V
L1 47H IN SHDN OUT LX D1
OUTPUT 3.3V OR 5V COUT 100F 6.3V
INPUT 4.5V OR 12V CIN 10F 25V
L1 22H IN SHDN OUT LX D1
OUTPUT 3.3V OR 5V COUT 150F 6.3V
MAX1836
GND FB GND
MAX1837
FB
CIN = TAIYO YUDEN TMK432BJ106KM L1 = SUMIDA CDRH5D28-470 COUT = SANYO POSCAP 6TPC100M (SMALLER CAPACITORS CAN BE USED FOR 5V) D1 = NIHON EP05Q03L NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
CIN = TAIYO YUDEN TMK432BJ106KM L1 = SUMIDA CDRH5D28-220 COUT = SANYO OS-CON 6SA150M (SMALLER CAPACITORS CAN BE USED FOR 5V) D1 = NIHON ED05Q03L NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
Figure 1. Typical MAX1836 Application Circuit
Figure 2. Typical MAX1837 Application Circuit
Detailed Description
The MAX1836/MAX1837 step-down converters are designed primarily for battery-powered devices, notebook computers, and industrial control applications. A unique current-limited control scheme provides high efficiency over a wide load range. Operation up to 100% duty cycle allows the lowest possible dropout voltage, increasing the useable supply voltage range. Under no-load, the MAX1836/MAX1837 draw only 12A, and in shutdown mode, they draw only 3A to further reduce power consumption and extend battery
life. Additionally, an internal 24V switching MOSFET, internal current sensing, and a high switching frequency minimize PC board space and component cost.
Current-Limited Control Architecture
The MAX1836/MAX1837 use a proprietary current-limited control scheme that operates with duty cycles up to 100%. These DC-DC converters pulse as needed to maintain regulation, resulting in a variable switching frequency that increases with the load. This eliminates the high supply currents associated with conventional con-
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24V Internal Switch, 100% Duty Cycle, Step-Down Converters MAX1836/MAX1837
INPUT 4.5V OR 24V CIN IN SHDN LX D1 L1 OUTPUT 3.3V OR 5V COUT
VSENSE
OUT R Q MAXIMUM OFF-TIME DELAY Q TRIG 100mV FB
S
Q
TRIG MAXIMUM ON-TIME DELAY
VSET 1.25V GND
MAX1836 MAX1837
Figure 3. Functional Diagram
stant-frequency pulse-width-modulation (PWM) controllers that switch the MOSFET unnecessarily. When the output voltage is too low, an error comparator sets a flip-flop, which turns on the internal P-channel MOSFET and begins a switching cycle (Figure 3). As shown in Figure 4, the inductor current ramps up linearly, charging the output capacitor and servicing the load. The MOSFET turns off when the current limit is reached, or when the maximum on-time is exceeded while the output voltage is in regulation. Otherwise, the MOSFET remains on, allowing a duty cycle up to 100% to ensure the lowest possible dropout voltage. Once the MOSFET turns off, the flip-flop resets, diode D1 turns on, and the current through the inductor ramps back down, transferring the stored energy to the output capacitor and load. The MOSFET remains off until the 0.5s minimum off-time expires and the inductor current ramps down to zero, and the output voltage drops back below the set point.
10V A 0 3.3V 500mA 0 4s/div CIRCUIT OF FIGURE 2, VIN = 12V A. VLX, 5V/div B. VOUT = 3.3V, 20mV/div, 200mA LOAD C. INDUCTOR CURRENT, 500mA/div C B
Figure 4. Discontinuous-Conduction Operation
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24V Internal Switch, 100% Duty Cycle, Step-Down Converters
Input-Output (Dropout) Voltage
A step-down converter's minimum input-to-output voltage differential (dropout voltage) determines the lowest useable input supply voltage. In battery-powered systems, this limits the useful end-of-life battery voltage. To maximize battery life, the MAX1836/MAX1837 operate with duty cycles up to 100%, which minimizes the inputto-output voltage differential. When the supply voltage approaches the output voltage, the P-channel MOSFET remains on continuously to supply the load. Dropout voltage is defined as the difference between the input and output voltages when the input is low enough for the output to drop out of regulation. For a step-down converter with 100% duty cycle, the dropout voltage depends on the MOSFET drain-to-source onresistance (RDS(ON)) and inductor series resistance; therefore, it is proportional to the load current: VDROPOUT = IOUT x RDS(ON) + RINDUCTOR (see the Selector Guide). For example, the MAX1836EUT33 has a preset 3.3V output voltage. The MAX1836/MAX1837 output voltage may be adjusted by connecting a voltage divider from the output to FB (Figure 5). When externally adjusting the output voltage, connect OUT to GND. Select R2 in the 10k to 100k range. Calculate R1 with the following equation: V R1 = R2 OUT - 1 VFB where VFB = 1.25V, and VOUT may range from 1.25V to VIN. When setting output voltages above 5.5V, the shutdown feature cannot be used, so SHDN must be permanently connected to IN.
MAX1836/MAX1837
Inductor Selection
When selecting the inductor, consider these four parameters: inductance value, saturation current rating, series resistance, and size. The MAX1836/MAX1837 operate with a wide range of inductance values. For most applications, values between 10H and 100H work best with the controller's switching frequency. Calculate the minimum inductance value as follows: L(MIN) = (VIN(MAX) - VOUT ) t ON(MIN) ILIM
(
)
Shutdown (SHDN)
A logic-level low voltage on SHDN shuts down the MAX1836/MAX1837. When shut down, the supply current drops to 3A to maximize battery life, and the internal P-channel MOSFET turns off to isolate the output from the input. The output capacitance and load current determine the rate at which the output voltage decays. A logic-level high voltage on SHDN activates the MAX1836/MAX1837. Do not leave SHDN floating. If unused, connect SHDN to IN. When setting output voltages above 5.5V, the shutdown feature cannot be used, so SHDN must be permanently connected to IN. The SHDN input voltage slew rate must be greater than 10V/ms.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation in the MAX1836/MAX1837. When the junction temperature exceeds TJ = +160C, a thermal sensor turns off the pass transistor, allowing the IC to cool. The thermal sensor turns the pass transistor on again after the IC's junction temperature cools by 10C, resulting in a pulsed output during continuous thermal-overload conditions.
where tON(MIN) = 1.0s. Inductor values up to six times L(MIN) are acceptable. Low-value inductors may be smaller in physical size and less expensive, but they result in higher peak-current overshoot due to currentsense comparator propagation delay (300ns). Peakcurrent overshoot reduces efficiency and could exceed the current ratings of the internal switching MOSFET and external components.
INPUT 4.5V OR 24V IN CIN SHDN FB LX D1
L1
OUTPUT 1.25V TO VIN COUT
R1
Design Information
Output Voltage Selection
The feedback input features dual-mode operation. Connect the output to OUT and FB to GND for the preset output voltage. The MAX1836/MAX1837 are supplied with factory-set output voltages of 3.3V or 5V. The two-digit part number suffix identifies the output voltage
GND
MAX1836 MAX1837
OUT
R2
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
Figure 5. Adjustable Output Voltage
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24V Internal Switch, 100% Duty Cycle, Step-Down Converters MAX1836/MAX1837
The inductor's saturation current rating must be greater than the peak switching current, which is determined by the switch current limit plus the overshoot due to the 300ns current-sense comparator propagation delay: IPEAK = ILIM + (VIN - VOUT ) 300ns L able for initial capacitor selection, but final values should be set by testing a prototype or evaluation circuit. As a general rule, a smaller amount of charge delivered in each pulse results in less output ripple. Since the amount of charge delivered in each oscillator pulse is determined by the inductor value and input voltage, the voltage ripple increases with larger inductance but decreases with lower input voltages. With low-cost aluminum electrolytic capacitors, the ESR-induced ripple can be larger than that caused by the current into and out of the capacitor. Consequently, high-quality low-ESR aluminum-electrolytic, tantalum, polymer, or ceramic filter capacitors are required to minimize output ripple. Best results at reasonable cost are typically achieved with an aluminum-electrolytic capacitor in the 100F range, in parallel with a 0.1F ceramic capacitor.
where the switch current-limit (ILIM) is typically 312mA (MAX1836) or 625mA (MAX1837). Saturation occurs when the inductor's magnetic flux density reaches the maximum level the core can support, and the inductance starts to fall. Inductor series resistance affects both efficiency and dropout voltage (see the Input-Output Voltage section). High series resistance limits the maximum current available at lower input voltages and increases the dropout voltage. For optimum performance, select an inductor with the lowest possible DC resistance that fits in the allotted dimensions. Typically, the inductor's series resistance should be significantly less than that of the internal P-channel MOSFET's on-resistance (1.1 typ). Inductors with a ferrite core, or equivalent, are recommended. The maximum output current of the MAX1836/MAX1837 current-limited converter is limited by the peak inductor current. For the typical application, the maximum output current is approximately: IOUT(MAX) = 1 IPEAK 2
Input Capacitor
The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on the input caused by the circuit's switching. The input capacitor must meet the ripple-current requirement (IRMS) imposed by the switching currents defined by the following equation: IRMS = ILOAD VOUT (VIN - VOUT ) VIN
Output Capacitor
Choose the output capacitor to supply the maximum load current with acceptable voltage ripple. The output ripple has two components: variations in the charge stored in the output capacitor with each LX pulse, and the voltage drop across the capacitor's equivalent series resistance (ESR) caused by the current into and out of the capacitor: VRIPPLE VRIPPLE(ESR) + VRIPPLE(C) The output voltage ripple as a consequence of the ESR and output capacitance is: VRIPPLE(ESR) = IPEAKESR VRIPPLE(C) = L(IPEAK - IOUT ) VIN V -V 2COUT VOUT IN OUT
2
For most applications, nontantalum chemistries (ceramic, aluminum, polymer, or OS-CON) are preferred due to their robustness with high inrush currents typical of systems with low-impedance battery inputs. Alternatively, two (or more) smaller-value low-ESR capacitors can be connected in parallel for lower cost. Choose an input capacitor that exhibits <+10C temperature rise at the RMS input current for optimal circuit longevity.
Diode Selection
The current in the external diode (D1) changes abruptly from zero to its peak value each time the LX switch turns off. To avoid excessive losses, the diode must have a fast turn-on time and a low forward voltage. Use a diode with an RMS current rating of 0.5A or greater, and with a breakdown voltage >VIN. Schottky diodes are preferred. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents. In such cases, ultra-high-speed silicon rectifiers are recommended, although a Schottky diode with a higher reverse voltage rating can often provide acceptable performance.
where I PEAK is the peak inductor current (see the Inductor Selection section). These equations are suit10
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24V Internal Switch, 100% Duty Cycle, Step-Down Converters MAX1836/MAX1837
Table 1. Component Suppliers
SUPPLIER INDUCTORS Coilcraft Coiltronics Sumida USA Toko CAPACITORS AVX Kemet Panasonic Sanyo Taiyo Yuden DIODES Central Semiconductor International Nihon On Semiconductor Zetex 516-435-1110 310-322-3331 847-843-7500 602-303-5454 516-543-7100 516-435-1824 310-322-3332 847-843-2798 602-994-6430 516-864-7630 www.centralsemi.com www.irf.com www.niec.co.jp www.onsemi.com www.zetex.com 803-946-0690 408-986-0424 847-468-5624 619-661-6835 408-573-4150 803-626-3123 408-986-1442 847-468-5815 619-661-1055 408-573-4159 www.avxcorp.com www.kemet.com www.panasonic.com www.secc.co.jp www.t-yuden.com 847-639-6400 561-241-7876 847-956-0666 847-297-0070 847-639-1469 561-241-9339 847-956-0702 847-699-1194 www.coilcraft.com www.coiltronics.com www.sumida.com www.tokoam.com PHONE FAX WEBSITE
MAX1836/MAX1837 Stability
Commonly, instability is caused by excessive noise on the feedback signal or ground due to poor layout or improper component selection. When seen, instability typically manifests itself as "motorboating," which is characterized by grouped switching pulses with large gaps and excessive low-frequency output ripple during no-load or light-load conditions.
coupling. The MAX1837 evaluation kit shows the recommended layout.
Applications Information
High-Voltage Step-Down Converter
The typical application circuits' (Figures 1 and 2) components were selected for 9V battery applications. However, the MAX1836/MAX1837 input voltage range allows supply voltages up to 24V. Figure 6 shows a modified application circuit for high-voltage applications. When using higher input voltages, verify that the input capacitor's voltage rating exceeds VIN(MAX) and that the inductor value exceeds the minimum inductance recommended in the Inductor Selection section.
PC Board Layout and Grounding
High switching frequencies and large peak currents make PC board layout an important part of the design. Poor layout may introduce switching noise into the feedback path, resulting in jitter, instability, or degraded performance. High-power traces, bolded in the typical application circuits (Figures 1 and 2), should be as short and wide as possible. Additionally, the current loops formed by the power components (CIN, COUT, L1, and D1) should be as tight as possible to avoid radiated noise. Connect the ground pins of these power components at a common node in a star-ground configuration. Separate the noisy traces, such as the LX node, from the feedback network with grounded copper. Furthermore, keep the extra copper on the board, and integrate it into a pseudoground plane. When using external feedback, place the resistors as close to the feedback pin as possible to minimize noise
Inverter Configuration
Figure 7 shows the MAX1836/MAX1837 in a floating ground configuration. By connecting what would normally be the output to the supply-voltage ground, the IC's ground pin is forced to regulate to -5V (MAX183_EUT50) or -3.3V (MAX183_EUT33). Avoid exceeding the maximum ratings of 24V between IN and GND, and 5.5V between OUT and GND. Other negative voltages may be generated by placing a resistive divider across the output capacitor and connecting the tap to FB in the same manner as the normal step-down configuration.
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24V Internal Switch, 100% Duty Cycle, Step-Down Converters MAX1836/MAX1837
INPUT 4.5V TO 24V CIN 10F 25V L1 47H IN SHDN OUT LX D1 COUT 68F 10V OUTPUT 5V INPUT 3.6V TO 18V CIN 10F L1 47H IN SHDN LX OUT D1 COUT 100F OUTPUT -3.3V OR -5V
MAX1837
GND FB
MAX1836 MAX1837
GND FB
CIN = TAIYO YUDEN TMK432BJ106KM L1 = SUMIDA CDRH5D28-470 COUT = SANYO POSCAP 10TPC68M D1 = NIHON EP05Q03L NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
Figure 7. MAX1836/MAX1837 Inverter Configuration
Figure 6. High-Voltage Application
Chip Information
TRANSISTOR COUNT: 731 PROCESS: BiCMOS
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24V Internal Switch, 100% Duty Cycle, Step-Down Converters
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)
6LSOT.EPS
MAX1836/MAX1837
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) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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