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
12µA. 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.
________________________Applications
9V Battery Systems
Notebook Computers
Distributed Power Systems
Backup Supplies
4mA to 20mA Loop Power Supplies
Industrial Control Supplies
Hand-Held Devices
____________________________Features
♦ 4.5V to 24V Input Voltage Range
♦ Preset 3.3V or 5V Output
♦ Adjustable Output from 1.25V to V
IN
♦ Output Currents Up to 125mA (MAX1836) or
250mA (MAX1837)
♦ Internal P-Channel MOSFET
♦ Efficiency Over 90%
♦ 12µA Quiescent Current
♦ 3µA Shutdown Current
♦ 100% Maximum Duty Cycle for Low Dropout
♦ Current-Limiting and Overtemperature Protection
♦ Small 6-Pin SOT23 Package
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
________________________________________________________________ Maxim Integrated Products 1
19-1919; Rev 1; 01/02
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.
PART
PRESET OUTPUT
VOLTAGE (V)
LOAD
CURRENT (mA)
MAX1836EUT50 5 125
MAX1836EUT33 3.3 125
MAX1837EUT50 5 250
MAX1837EUT33 3.3 250
Selector Guide
MAX1836
MAX1837
IN
GND
LX
SHDN
FB
OUT
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
OUTPUT
3.3V OR 5V
INPUT
4.5V TO 24V
Typical Operating Circuit
GND
LXIN
16OUT
5 SHDN
FB
MAX1836
MAX1837
SOT23
TOP VIEW
2
34
Pin Configuration
Note: The MAX1836/MAX1837 require special solder temperature profile described in the Absolute Maximum Ratings.
PART
TEMP RANGE
PIN-
TOP
M ARK
MAX1836EUT50-T
AANW
MAX1836EUT33-T
AANY
MAX1837EUT50-T
AANX
MAX1837EUT33-T
AANZ
Ordering Information
PA C K A G E
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
6 SOT23-6
6 SOT23-6
6 SOT23-6
6 SOT23-6
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
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.
IN, SHDN to GND ...................................................-0.3V to +25V
LX to GND.......................................................-2V to (V
IN
+ 0.3V)
OUT, FB to GND.......................................................-0.3V to +6V
Continuous Power Dissipation (T
A
= +70°C) (Note 1)
6-Pin SOT23 (derate 8.7mW/°C above +70°C)............696mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Note 1: Thermal properties are specified with product mounted on PC board with 1in
2
of copper area and still air.
)
ELECTRICAL CHARACTERISTICS
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN= 12V, SHDN = IN, TA= 0°C to +85°C. Typical values are at TA= +25°C,
unless otherwise noted.)
Dual Mode is a trademark of Maxim Integrated Products, Inc.
Input Supply Range V
Input Undervoltage Lockout
Threshold
Input Supply Current I
Input Supply Current in Dropout I
Input Shutdown Current SHDN = GND 3 7 µA
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
IN
V
UVLO
IN
IN (D ROP) VIN
VIN rising 3.55 4.0 4.4
VIN falling 3.45 3.9 4.3
= 5V 18 µA
4.5 24 V
12 25 µA
V
FB = GND,
I
= 0 to 125mA
Output Voltage (Preset Mode) V
Output Voltage Range
(Adjustable Mode)
Feedback Set Voltage
(Adjustable Mode)
OUT Bias Current V
FB Bias Current I
FB Dual ModeTM Threshold VFB rising or falling 50 100 150 mV
LX Switch Minimum Off-Time t
LX Switch Maximum On-Time t
LX Switch On-Resistance R
LX Current Limit I
LX Zero-Crossing Threshold -75 +75 mV
OUT
V
OUT
V
FB
FB
OFF( M IN)
ON (M AX
LX
LIM
LOAD
(MAX1836) or
250mA (MAX1837)
(Note 2) 1.25 V
= 5V 2.5 7.4 µA
OUT
VFB = 0 or 1.25V, TA = +25°C -25 +25 nA
VFB = 1.3V 7 10 13 µs
VIN = 6V 1.1 2 Ω
MAX1836 250 312 450
MAX1837 500 625 850
MAX183_EUT50 4.80 5.00 5.20
MAX183_EUT33 3.168 3.30 3.432
IN
1.200 1.25 1.300 V
0.2 0.4 0.6 µs
V
V
mA
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN= 12V, SHDN = IN, TA= 0°C to +85°C. Typical values are at TA= +25°C,
unless otherwise noted.)
)
ELECTRICAL CHARACTERISTICS
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN= 12V, SHDN = IN, TA= -40°C to +85°C, unless otherwise noted.) (Note 4)
Zero-Crossing Timeout LX does not rise above the threshold 30 µs
LX Switch Leakage Current VIN = 18V, LX = GND, TA = +25°C1µA
Dropout Voltage V
Line Regulation VIN = 5V to 24V 0.05 %
Load Regulation
Shutdown Input Threshold V
Shutdown Leakage Current I
Thermal Shutdown 10°C hysteresis (typ) 160 °C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
DROPOUTIOUT
SHDN
SHDN
= 100mA, VIN = 5V 120 mV
I
= 0 to 125mA (MAX1836) or 250mA
OUT
(MAX1837)
VIN = 4.5V to 24V (Note 3) 0.8 2.4 V
V
= 0 or 24V -1 +1 µA
SHDN
0.3 %
Input Supply Range V
Input Undervoltage Lockout
Threshold
Input Supply Current I
Input Shutdown Current SHDN = GND 7 µA
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
4.5 24 V
V
25 µA
V
IN
UVLO
IN
VIN rising 3.55 4.4
VIN falling 3.45 4.3
FB = GND,
I
= 0 to 125mA
Output Voltage (Preset Mode) V
Output Voltage Range
(Adjustable Mode)
Feedback Set Voltage
(Adjustable Mode)
OUT Bias Current V
FB Dual Mode Threshold VFB rising or falling 50 150 mV
LX Switch Minimum Off-Time t
LX Switch Maximum On-Time t
LX Switch On-Resistance R
LX Current Limit I
OUT
V
OUT
V
FB
OFF( M IN)
ON (M AX
LX
LIM
LOAD
(MAX1836) or
250mA (MAX1837)
(Note 2) 1.25 V
= 5V 7.4 µA
OUT
VFB = 1.3V 7 13 µs
VIN = 6V 2 Ω
MAX1836 250 450
MAX1837 500 900
MAX183_EUT50 4.80 5.20
MAX183_EUT33 3.168 3.432
IN
1.200 1.300 V
0.2 0.6 µs
V
V
mA
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
4 _______________________________________________________________________________________
Typical Operating Characteristics
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN= 12V, SHDN = IN, TA = +25°C.)
3.27
3.29
3.28
3.31
3.30
3.32
3.33
0 10050 150 200
MAX1836EUT33
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1836/7 toc01
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
VIN = 5V
VIN = 9V to 12V
FIGURE 1
100
95
90
85
80
70
0.1 10 1001 1000
MAX1836EUT33
EFFICIENCY vs. LOAD CURRENT
MAX1836/7 toc02
LOAD CURRENT (mA)
EFFICIENCY (%)
75
VIN = 9V
VIN = 12V
VIN = 5V
FIGURE 1
V
OUT
= 3.3V
3.27
3.29
3.28
3.31
3.30
3.32
3.33
0 150 20050 100 250 300 350
MAX1837EUT33
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1836/7 toc03
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
VIN = 9V
VIN = 5V
VIN = 12V
FIGURE 2
100
95
90
85
80
70
0.1 10 1001 1000
MAX1837EUT33
EFFICIENCY vs. LOAD CURRENT
MAX1836/7 toc04
LOAD CURRENT (mA)
EFFICIENCY (%)
75
VIN = 9V
VIN = 12V
VIN = 5V
FIGURE 2
V
OUT
= 3.3V
0
40
20
100
80
60
160
140
120
180
0 100 15050 200 250 300 350
MAX1837EUT33
SWITCHING FREQUENCY vs. LOAD CURRENT
MAX1836/7 toc05
LOAD CURRENT (mA)
FREQUENCY (kHz)
VIN = 9V
VIN = 5V
VIN = 12V
FIGURE 2
V
OUT
= 3.3V
3.27
3.29
3.28
3.31
3.30
3.32
3.33
0 4 8 12162024
MAX1837EUT33
OUTPUT VOLTAGE vs. INPUT VOLTAGE
MAX1836/7 toc06
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
I
OUT
= 10mA
I
OUT
= 200mA
FIGURE 2
V
OUT
= 3.3V
L1 = 47µH
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 -40°C are guaranteed by design, not production tested.
PARAMETER
CONDITIONS
UNITS
LX Zero-Crossing Threshold -75
mV
Shutdown Input Threshold
VIN = 4.5V to 24V (Note 3) 0.8 2.4 V
Shutdown Leakage Current I
SHDN
V
SHDN
= 0 or 24V -1 +1 µA
ELECTRICAL CHARACTERISTICS (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN= 12V, SHDN = IN, TA= -40°C to +85°C, unless otherwise noted.) (Note 4)
SYMBOL
V
SHDN
MIN TYP MAX
+75
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
_______________________________________________________________________________________ 5
Typical Operating Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN= 12V, SHDN = IN, TA = +25°C.)
MAX1837EUT33
EFFICIENCY vs. INPUT VOLTAGE
100
V
I
OUT
OUT
L1 = 47µH
= 200mA
95
90
85
EFFICIENCY (%)
80
I
= 10mA
75
70
0 4 8 12162024
OUT
INPUT VOLTAGE (V)
OUTPUT VOLTAGE vs. LOAD CURRENT
5.04
VIN = 12V TO 24V
5.02
FIGURE 2
= 3.3V
MAX1836/7 toc07
MAX1837EUT50
VIN = 9V
SWITCHING FREQUENCY vs. INPUT VOLTAGE
100
10
FREQUENCY (kHz)
1
0812
MAX1837EUT33
4
INPUT VOLTAGE (V)
MAX1836/7 toc10
I
= 200mA
OUT
FIGURE 2
= 3.3V
V
OUT
L1 = 47µH
I
= 10mA
OUT
16 2420
MAX1837EUT50
EFFICIENCY vs. LOAD CURRENT
100
FIGURE 6
= 5V
V
OUT
95
VIN = 12V
90
PEAK INDUCTOR CURRENT vs. INPUT VOLTAGE
MAX1837EUT33
1000
FIGURE 2
= 3.3V
V
OUT
MAX1836/7 toc08
L1 = 47µH
800
600
400
PEAK INDUCTOR CURRENT (mA)
200
LIMITED BY
t
ON(MIN)
0
0 4 8 12162024
VIN = 9V
VIN = 7V
INPUT VOLTAGE (V)
LIMITED BY
MAX1836/7 toc11
I
OUT
I
= 200mA
I
LIM
OUT
MAX1836/7 toc09
= 10mA
5.00
OUTPUT VOLTAGE (V)
4.98
FIGURE 6
4.96
0 10050 150 200 250 300
VIN = 7V
LOAD CURRENT (mA)
MAX1837EUT50
DROPOUT VOLTAGE vs. LOAD CURRENT
400
FIGURE 6
= 5V
V
350
OUT
300
250
200
150
DROPOUT VOLTAGE (mV)
100
50
0
0 100 200 300
LOAD CURRENT (mA)
MAX1836/7 toc12
85
EFFICIENCY (%)
80
75
70
0.1 10 1001 1000
VIN = 18V
LOAD CURRENT (mA)
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
15
14
13
12
SUPPLY CURRENT (µA)
11
10
0 4 8 12 16 20 24
INPUT VOLTAGE (V)
VIN = 24V
MAX1836/7 toc13
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN= 12V, SHDN = IN, TA = +25°C.)
100µs/div
MAX1837EUT50
LOAD TRANSIENT
A: I
OUT
= 10mA to 250mA, 200mA/div
B: V
OUT
= 5V, 20mV/div
C: I
L
, 500mA/div
V
IN
= 12V, FIGURE 6
400mA
200mA
5.02V
0
B
A
C
MAX1836/7 toc14
5.00V
4.98V
750mA
250mA
0
400
µs/div
MAX1837EUT50
LINE TRANSIENT
A: VIN = 9V to 18V, 10V/div
B: V
OUT
= 5V, R
OUT
= 100Ω, 100mV/div
C: I
L
, 500mA/div
FIGURE 6
20V
10V
5.1V
0
B
A
C
MAX1836/7 toc15
5.0V
4.9V
500mA
0
400
µs/div
MAX1837EUT50
LINE TRANSIENT NEAR DROPOUT
A: VIN = 5V to 12V, 5V/div
B: V
OUT
= 5V, R
OUT
= 100Ω, 100mV/div
C: I
L
, 500mA/div
FIGURE 6
15V
10V
5.1V
5V
B
A
C
MAX1836/7 toc16
5.0V
4.9V
500mA
0
200
µs/div
MAX1837EUT50
STARTUP WAVEFORM
A: V
SHDN
= 0 to 2V, 2V/div
B: V
OUT
= 5V, R
OUT
= 100Ω, 2V/div
C: I
L
, 500mA/div
V
IN
= 12V, FIGURE 6
2V
0
2V
4V
B
A
C
MAX1836/7 toc17
0
500mA
0
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
_______________________________________________________________________________________ 7
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
12µA, and in shutdown mode, they draw only 3µA 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-
Pin Description
Figure 1. Typical MAX1836 Application Circuit
Figure 2. Typical MAX1837 Application Circuit
PIN NAME FUNCTION
Dual-Mode Feedback Input. Connect to GND for the preset 3.3V (MAX183_EUT33) or 5.0V (MAX183_EUT50)
1FB
output. Connect to a resistive divider between the output and FB to adjust the output voltage between 1.25V
IN
, and connect the OUT pin to GND. When setting output voltages above 5.5V, permanently connect
and V
SHDN
to IN.
2 GND Ground
3 IN Input Voltage. 4.5V to 24V input range. Connected to the internal P-channel power MOSFET’s source.
4 LX 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 3µA. LX is
5
SHDN
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
6 OUT
to the output when using the preset output voltage. Connect to GND when using an external resistive divider
to adjust the output voltage.
INPUT
4.5V OR 12V
C
IN
10µF
25V
IN
SHDN
MAX1836
OUT
L1
LX
47µH
D1
OUTPUT
3.3V OR 5V
C
OUT
100µF
6.3V
INPUT
4.5V OR 12V
C
IN
10µF
25V
IN
SHDN
LX
OUT
MAX1837
L1
22µH
D1
OUTPUT
3.3V OR 5V
C
OUT
150µF
6.3V
GND
= TAIYO YUDEN TMK432BJ106KM
C
IN
L1 = SUMIDA CDRH5D28-470
= SANYO POSCAP 6TPC100M (SMALLER CAPACITORS CAN BE USED FOR 5V)
C
OUT
D1 = NIHON EP05Q03L
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
FB
GND
= TAIYO YUDEN TMK432BJ106KM
C
IN
L1 = SUMIDA CDRH5D28-220
= SANYO OS-CON 6SA150M (SMALLER CAPACITORS CAN BE USED FOR 5V)
C
OUT
D1 = NIHON ED05Q03L
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
FB
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
8 _______________________________________________________________________________________
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
0.5µs minimum off-time expires and the inductor current ramps down to zero, and the output voltage drops
back below the set point.
Figure 4. Discontinuous-Conduction Operation
Figure 3. Functional Diagram
INPUT
4.5V OR 24V
C
IN
IN
SHDN
V
SENSE
QR
S
Q
MAXIMUM
MAX1836
MAX1837
MAXIMUM
OFF-TIME
Q
TRIG
ON-TIME
DELAY
DELAY
TRIG
V
SET
1.25V
100mV
OUT
GND
LX
FB
L1
D1
OUTPUT
3.3V OR 5V
C
OUT
10V
0
3.3V
500mA
0
CIRCUIT OF FIGURE 2, V
, 5V/div
A. V
LX
= 3.3V, 20mV/div, 200mA LOAD
B. V
OUT
C. INDUCTOR CURRENT, 500mA/div
4µs/div
IN
= 12V
A
B
C
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
_______________________________________________________________________________________ 9
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 (R
DS(ON)
) and inductor series resistance;
therefore, it is proportional to the load current:
Shutdown (
SHDN
)
A logic-level low voltage on SHDN shuts down the
MAX1836/MAX1837. When shut down, the supply current drops to 3µA 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= +160°C, 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 10°C, resulting in
a pulsed output during continuous thermal-overload
conditions.
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
(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 10kΩ to
100kΩ range. Calculate R1 with the following equation:
where V
FB
= 1.25V, and V
OUT
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.
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 10µH and 100µH
work best with the controller’s switching frequency.
Calculate the minimum inductance value as follows:
where t
ON(MIN)
= 1.0µs. 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.
Figure 5. Adjustable Output Voltage
V
DROPOUT OUT DS(ON) INDUCTOR
=× +
IR R
()
R1 R2
V
OUT
V
=
-
1
FB
(V - V )
L
(MIN)
IN(MAX) OUT ON(MIN)
=
I
LIM
t
R1
R2
OUTPUT
1.25V TO V
IN
C
OUT
INPUT
4.5V OR 24V
C
IN
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
IN
SHDN
GND
MAX1836
MAX1837
LX
FB
OUT
L1
D1
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
10 ______________________________________________________________________________________
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:
where the switch current-limit (I
LIM
) 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 (1.1Ω 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:
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:
The output voltage ripple as a consequence of the ESR
and output capacitance is:
where I
PEAK
is the peak inductor current (see the
Inductor Selection section). These equations are suit-
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 100µF range, in parallel with a 0.1µF
ceramic capacitor.
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 (I
RMS
) imposed by the switching currents
defined by the following equation:
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 <+10°C 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.
(V - V )
I
=+I
PEAK LIM
II
IN OUT
L
1
OUT(MAX) PEAK
=
2
ns
300
VV V
RIPPLE RIPPLE(ESR) RIPPLE(C)
V ESR
RIPPLE(ESR) PEAK
V
RIPPLE(C)
≈+
I
=
LI
()
PEAK OUT
=
2C V
OUT OUT
-I
2
V
V-V
IN OUT
IN
II
RMS LOAD
VV-V
()
=
OUT IN OUT
V
IN
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.
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, C
OUT
,
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
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 V
IN(MAX)
and
that the inductor value exceeds the minimum inductance recommended in the Inductor Selection section.
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.
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
______________________________________________________________________________________ 11
Table 1. Component Suppliers
INDUCTORS
Coilcraft 847-639-6400 847-639-1469 www.coilcraft.com
Coiltronics 561-241-7876 561-241-9339 www.coiltronics.com
Sumida USA 847-956-0666 847-956-0702 www.sumida.com
Toko 847-297-0070 847-699-1194 www.tokoam.com
CAPACITORS
AVX 803-946-0690 803-626-3123 www.avxcorp.com
Kemet 408-986-0424 408-986-1442 www.kemet.com
Panasonic 847-468-5624 847-468-5815 www.panasonic.com
Sanyo 619-661-6835 619-661-1055 www.secc.co.jp
Taiyo Yuden 408-573-4150 408-573-4159 www.t-yuden.com
DIODES
Central Semiconductor 516-435-1110 516-435-1824 www.centralsemi.com
International
Nihon 847-843-7500 847-843-2798 www.niec.co.jp
On Semiconductor 602-303-5454 602-994-6430 www.onsemi.com
Zetex 516-543-7100 516-864-7630 www.zetex.com
SUPPLIER PHONE FAX WEBSITE
310-322-3331 310-322-3332 www.irf.com
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
12 ______________________________________________________________________________________
Figure 7. MAX1836/MAX1837 Inverter Configuration
Figure 6. High-Voltage Application
Chip Information
TRANSISTOR COUNT: 731
PROCESS: BiCMOS
INPUT
4.5V TO 24V
C
IN
10µF
25V
= TAIYO YUDEN TMK432BJ106KM
C
IN
L1 = SUMIDA CDRH5D28-470
= SANYO POSCAP 10TPC68M
C
OUT
D1 = NIHON EP05Q03L
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
IN
SHDN
GND
LX
OUT
MAX1837
FB
L1
47µH
D1
OUTPUT
5V
C
68µF
10V
OUT
INPUT
3.6V TO 18V
C
IN
10µF
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
IN
SHDN
MAX1836
MAX1837
LX
OUT
GND
FB
L1
47µH
D1
OUTPUT
-3.3V OR -5V
C
OUT
100µF
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
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
© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
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
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