MAX1837EUT50-T [MAXIM]
24V Internal Switch, 100% Duty Cycle, Step-Down Converters; 24V内部开关， 100 ％占空比，降压型转换器
| 型号： | MAX1837EUT50-T |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 24V Internal Switch, 100% Duty Cycle, Step-Down Converters |
| 文件： | 总13页 (文件大小：394K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-1919; Rev 1; 01/02
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
General Description
____________________________Features
ꢀ 4.5V to 24V Input Voltage Range
ꢀ Preset 3.3V or 5V Output
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
ꢀ Adjustable Output from 1.25V to V
IN
from 1.25V to V . An internal current-limited switching
IN
MOSFET delivers load currents up to 125mA
(MAX1836) or 250mA (MAX1837).
ꢀ Output 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.
ꢀ 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
The MAX1836/MAX1837 step-down converters with
internal switching MOSFETs are available in a 6-pin
SOT23 package, making them ideal for low-cost, low-
power, space-sensitive applications. For increased out-
put 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.
Ordering Information
TOP
MARK
PIN-
PACKAGE
PART
TEMP RANGE
6 SOT23-6
6 SOT23-6
6 SOT23-6
6 SOT23-6
MAX1836EUT50-T -40°C to +85°C
MAX1836EUT33-T -40°C to +85°C
MAX1837EUT50-T -40°C to +85°C
MAX1837EUT33-T -40°C to +85°C
AANW
AANY
AANX
AANZ
________________________Applications
9V Battery Systems
Note: The MAX1836/MAX1837 require special solder tempera-
ture profile described in the Absolute Maximum Ratings.
Notebook Computers
Distributed Power Systems
Backup Supplies
Selector Guide
4mA to 20mA Loop Power Supplies
Industrial Control Supplies
Hand-Held Devices
PRESET OUTPUT
VOLTAGE (V)
LOAD
CURRENT (mA)
PART
MAX1836EUT50
MAX1836EUT33
MAX1837EUT50
MAX1837EUT33
5
125
125
250
250
3.3
5
Typical Operating Circuit
3.3
OUTPUT
3.3V OR 5V
INPUT
4.5V TO 24V
Pin Configuration
IN
LX
TOP VIEW
SHDN
FB
GND
IN
1
2
3
6
5
4
OUT
SHDN
LX
OUT
MAX1836
MAX1837
MAX1836
MAX1837
GND
FB
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
ABSOLUTE MAXIMUM RATINGS
IN, SHDN to GND...................................................-0.3V to +25V
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
LX to GND.......................................................-2V to (V + 0.3V)
OUT, FB to GND.......................................................-0.3V to +6V
IN
Continuous Power Dissipation (T = +70°C) (Note 1)
A
6-Pin SOT23 (derate 8.7mW/°C above +70°C)............696mW
2
Note 1: Thermal properties are specified with product mounted on PC board with 1in 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), V = 12V, SHDN = IN, T = 0°C to +85°C. Typical values are at T = +25°C,
IN
A
A
unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
4.5
TYP
MAX
24
UNITS
Input Supply Range
V
V
IN
V
V
rising
falling
3.55
3.45
4.0
3.9
12
18
3
4.4
4.3
25
IN
IN
Input Undervoltage Lockout
Threshold
V
V
UVLO
Input Supply Current
I
µA
µA
µA
IN
Input Supply Current in Dropout
Input Shutdown Current
I
V
= 5V
IN(DROP)
IN
SHDN = GND
7
FB = GND,
MAX183_EUT50
MAX183_EUT33
4.80
3.168
1.25
5.00
3.30
5.20
I
= 0 to 125mA
LOAD
Output Voltage (Preset Mode)
V
V
V
OUT
OUT
(MAX1836) or
250mA (MAX1837)
3.432
Output Voltage Range
(Adjustable Mode)
(Note 2)
V
V
V
IN
Feedback Set Voltage
(Adjustable Mode)
V
1.200
1.25
2.5
1.300
FB
OUT Bias Current
V
V
V
= 5V
7.4
+25
150
0.6
13
µA
nA
mV
µs
µs
Ω
OUT
FB Bias Current
I
= 0 or 1.25V, T = +25°C
-25
50
0.2
7
FB
FB
FB
A
FB Dual ModeTM Threshold
LX Switch Minimum Off-Time
LX Switch Maximum On-Time
LX Switch On-Resistance
rising or falling
100
0.4
10
t
OFF(MIN)
t
V
V
= 1.3V
= 6V
ON(MAX)
FB
IN
R
1.1
312
625
2
LX
MAX1836
MAX1837
250
500
-75
450
850
+75
LX Current Limit
I
mA
mV
LIM
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), V = 12V, SHDN = IN, T = 0°C to +85°C. Typical values are at T = +25°C,
IN
A
A
unless otherwise noted.)
PARAMETER
Zero-Crossing Timeout
LX Switch Leakage Current
Dropout Voltage
SYMBOL
CONDITIONS
LX does not rise above the threshold
MIN
TYP
MAX
UNITS
µs
30
V
= 18V, LX = GND, T = +25°C
1
µA
IN
A
V
I
= 100mA, V = 5V
IN
120
mV
%
DROPOUT
OUT
Line Regulation
V
= 5V to 24V
0.05
IN
I
= 0 to 125mA (MAX1836) or 250mA
OUT
Load Regulation
0.3
%
(MAX1837)
Shutdown Input Threshold
Shutdown Leakage Current
Thermal Shutdown
V
V
V
= 4.5V to 24V (Note 3)
0.8
-1
2.4
+1
V
SHDN
IN
I
= 0 or 24V
µA
°C
SHDN
SHDN
10°C hysteresis (typ)
160
ELECTRICAL CHARACTERISTICS
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), V = 12V, SHDN = IN, T = -40°C to +85°C, unless otherwise noted.) (Note 4)
IN
A
PARAMETER
Input Supply Range
SYMBOL
CONDITIONS
MIN
4.5
TYP
MAX
24
UNITS
V
V
IN
V
V
rising
falling
3.55
3.45
4.4
4.3
25
IN
IN
Input Undervoltage Lockout
Threshold
V
V
UVLO
Input Supply Current
I
µA
µA
IN
Input Shutdown Current
SHDN = GND
7
FB = GND,
MAX183_EUT50
4.80
3.168
1.25
5.20
I
= 0 to 125mA
LOAD
Output Voltage (Preset Mode)
V
V
OUT
OUT
(MAX1836) or
250mA (MAX1837)
MAX183_EUT33
3.432
Output Voltage Range
(Adjustable Mode)
V
(Note 2)
V
V
V
IN
Feedback Set Voltage
(Adjustable Mode)
V
1.200
1.300
FB
OUT Bias Current
V
V
= 5V
7.4
150
0.6
13
µA
mV
µs
µs
Ω
OUT
FB Dual Mode Threshold
LX Switch Minimum Off-Time
LX Switch Maximum On-Time
LX Switch On-Resistance
rising or falling
50
0.2
7
FB
t
OFF(MIN)
t
V
V
= 1.3V
= 6V
ON(MAX)
FB
IN
R
LX
2
MAX1836
MAX1837
250
500
450
900
LX Current Limit
I
mA
LIM
_______________________________________________________________________________________
3
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
ELECTRICAL CHARACTERISTICS (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), V = 12V, SHDN = IN, T = -40°C to +85°C, unless otherwise noted.) (Note 4)
IN
A
PARAMETER
SYMBOL
CONDITIONS
MIN
-75
0.8
-1
TYP
MAX
+75
2.4
UNITS
mV
V
LX Zero-Crossing Threshold
Shutdown Input Threshold
Shutdown Leakage Current
V
V
V
= 4.5V to 24V (Note 3)
SHDN
IN
I
= 0 or 24V
+1
µA
SHDN
SHDN
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.
Typical Operating Characteristics
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), V = 12V, SHDN = IN, T = +25°C.)
IN
A
MAX1836EUT33
EFFICIENCY vs. LOAD CURRENT
MAX1837EUT33
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1836EUT33
OUTPUT VOLTAGE vs. LOAD CURRENT
100
95
3.33
3.32
3.31
3.30
3.29
3.28
3.27
3.33
3.32
3.31
3.30
3.29
3.28
3.27
FIGURE 1
= 3.3V
FIGURE 2
FIGURE 1
V
OUT
V
= 5V
V
= 5V
IN
IN
V
= 9V
IN
90
85
80
V
= 5V
IN
V
= 9V
IN
V
= 12V
IN
V
= 9V to 12V
IN
V
= 12V
100
IN
75
70
0.1
1
10
1000
0
50 100 150 200 250 300 350
LOAD CURRENT (mA)
0
50
100
150
200
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MAX1837EUT33
SWITCHING FREQUENCY vs. LOAD CURRENT
MAX1837EUT33
OUTPUT VOLTAGE vs. INPUT VOLTAGE
MAX1837EUT33
EFFICIENCY vs. LOAD CURRENT
100
180
3.33
3.32
3.31
3.30
3.29
3.28
3.27
FIGURE 2
OUT
FIGURE 2
= 3.3V
I
= 10mA
OUT
160
V
= 3.3V
V
OUT
95
V = 9V
IN
140
120
100
80
V
= 12V
IN
V
= 5V
IN
90
85
80
I
= 200mA
OUT
V
= 9V
IN
60
40
FIGURE 2
75
70
V
= 5V
IN
20
V
= 3.3V
OUT
L1 = 47µH
V
= 12V
IN
0
0.1
1
10
LOAD CURRENT (mA)
100
1000
0
50 100 150 200 250 300 350
LOAD CURRENT (mA)
0
4
8
12
16
20
24
INPUT VOLTAGE (V)
4
_______________________________________________________________________________________
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Typical Operating Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), V = 12V, SHDN = IN, T = +25°C.)
IN
A
MAX1837EUT33
EFFICIENCY vs. INPUT VOLTAGE
MAX1837EUT33
PEAK INDUCTOR CURRENT vs. INPUT VOLTAGE
MAX1837EUT33
SWITCHING FREQUENCY vs. INPUT VOLTAGE
100
95
90
85
80
75
70
100
10
1000
FIGURE 2
= 3.3V
FIGURE 2
OUT
L1 = 47µH
I
= 200mA
OUT
V
V
= 3.3V
OUT
L1 = 47µH
I
= 200mA
OUT
800
I
= 10mA
OUT
600
400
FIGURE 2
= 3.3V
V
OUT
I
= 200mA
OUT
L1 = 47µH
I
= 10mA
12
OUT
8
200
0
I
= 10mA
20
OUT
LIMITED BY
LIMITED BY
t
I
ON(MIN)
LIM
1
0
4
16
20
24
0
4
8
12
16
24
0
4
8
12
16
20
24
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
MAX1837EUT50
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1837EUT50
EFFICIENCY vs. LOAD CURRENT
5.04
5.02
5.00
4.98
100
95
FIGURE 6
= 5V
V
OUT
V
= 12V TO 24V
IN
V
= 7V
IN
V
= 9V
IN
V
= 9V
IN
V
= 12V
IN
90
85
80
V
= 7V
IN
V
= 24V
IN
75
70
V
= 18V
10
IN
FIGURE 6
50
4.96
0
100
150
200
250
300
0.1
1
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MAX1837EUT50
DROPOUT VOLTAGE vs. LOAD CURRENT
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
400
350
300
250
200
150
100
15
14
13
12
11
10
FIGURE 6
OUT
V
= 5V
50
0
0
100
200
300
0
4
8
12
16
20
24
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
_______________________________________________________________________________________
5
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Typical Operating Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), V = 12V, SHDN = IN, T = +25°C.)
IN
A
MAX1837EUT50
LINE TRANSIENT
MAX1837EUT50
LOAD TRANSIENT
MAX1836/7 toc14
MAX1836/7 toc15
400mA
20V
10V
A
200mA
0
A
0
5.1V
5.0V
5.02V
5.00V
4.98V
B
B
4.9V
500mA
0
750mA
C
250mA
0
C
100µs/div
400µs/div
A: I
= 10mA to 250mA, 200mA/div
= 5V, 20mV/div
A: V = 9V to 18V, 10V/div
IN
OUT
B: V
B: V
= 5V, R
= 100Ω, 100mV/div
OUT
OUT
OUT
C: I , 500mA/div
C: I , 500mA/div
L
L
V
IN
= 12V, FIGURE 6
FIGURE 6
MAX1837EUT50
STARTUP WAVEFORM
MAX1837EUT50
LINE TRANSIENT NEAR DROPOUT
MAX1836/7 toc17
MAX1836/7 toc16
15V
10V
2V
A
A
B
0
5V
5.1V
5.0V
4V
2V
B
0
500mA
0
4.9V
500mA
0
C
C
200µs/div
= 0 to 2V, 2V/div
400µs/div
A: V = 5V to 12V, 5V/div
A: V
B: V
SHDN
IN
= 5V, R
= 100Ω, 2V/div
OUT
OUT
B: V
= 5V, R
= 100Ω, 100mV/div
OUT
OUT
C: I , 500mA/div
L
C: I , 500mA/div
L
V
= 12V, FIGURE 6
IN
FIGURE 6
6
_______________________________________________________________________________________
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
1
FB
IN
and V , and connect the OUT pin to GND. When setting output voltages above 5.5V, permanently connect
SHDN
to IN.
2
3
4
GND
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.
LX
Shutdown Input. A logic low shuts down the MAX1836/MAX1837 and reduces supply current to 3µA. LX is
SHDN
5
6
high impedance in shutdown. Connect to IN for normal operation. When setting output voltages above 5.5V,
SHDN
permanently connect
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.
OUT
L1
47µH
L1
22µH
OUTPUT
3.3V OR 5V
OUTPUT
3.3V OR 5V
INPUT
4.5V OR 12V
INPUT
4.5V OR 12V
IN
LX
IN
LX
C
C
OUT
C
OUT
100µF
C
IN
10µF
IN
10µF
D1
D1
SHDN
SHDN
150µF
6.3V
6.3V
25V
25V
OUT
OUT
MAX1836
MAX1837
GND
FB
GND
FB
C
= TAIYO YUDEN TMK432BJ106KM
C = TAIYO YUDEN TMK432BJ106KM
IN
IN
L1 = SUMIDA CDRH5D28-470
L1 = SUMIDA CDRH5D28-220
C
OUT
= SANYO POSCAP 6TPC100M (SMALLER CAPACITORS CAN BE USED FOR 5V)
C
OUT
= SANYO OS-CON 6SA150M (SMALLER CAPACITORS CAN BE USED FOR 5V)
D1 = NIHON EP05Q03L
D1 = NIHON ED05Q03L
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
Figure 1. Typical MAX1836 Application Circuit
Figure 2. Typical MAX1837 Application Circuit
life. Additionally, an internal 24V switching MOSFET,
internal current sensing, and a high switching frequen-
cy minimize PC board space and component cost.
Detailed Description
The MAX1836/MAX1837 step-down converters are
designed primarily for battery-powered devices, note-
book 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
Current-Limited Control Architecture
The MAX1836/MAX1837 use a proprietary current-limit-
ed 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 fre-
quency that increases with the load. This eliminates the
high supply currents associated with conventional con-
_______________________________________________________________________________________
7
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
OUTPUT
3.3V OR 5V
INPUT
4.5V OR 24V
L1
LX
IN
C
IN
SHDN
D1
C
OUT
V
SENSE
OUT
FB
R
S
Q
MAXIMUM
OFF-TIME
DELAY
Q
TRIG
100mV
V
SET
1.25V
Q
TRIG
MAXIMUM
ON-TIME
DELAY
GND
MAX1836
MAX1837
Figure 3. Functional Diagram
stant-frequency pulse-width-modulation (PWM) con-
trollers that switch the MOSFET unnecessarily.
10V
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 linear-
ly, 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 cur-
rent ramps down to zero, and the output voltage drops
back below the set point.
A
0
B
3.3V
500mA
C
0
4µs/div
CIRCUIT OF FIGURE 2, V = 12V
IN
A. V , 5V/div
OUT
LX
B. V
= 3.3V, 20mV/div, 200mA LOAD
C. INDUCTOR CURRENT, 500mA/div
Figure 4. Discontinuous-Conduction Operation
8
_______________________________________________________________________________________
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
(see the Selector Guide). For example, the
Input-Output (Dropout) Voltage
A step-down converter’s minimum input-to-output volt-
age differential (dropout voltage) determines the lowest
useable input supply voltage. In battery-powered sys-
tems, 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 input-
to-output voltage differential. When the supply voltage
approaches the output voltage, the P-channel MOSFET
remains on continuously to supply the load.
MAX1836EUT33 has a preset 3.3V output voltage.
The MAX1836/MAX1837 output voltage may be adjust-
ed by connecting a voltage divider from the output to
FB (Figure 5). When externally adjusting the output volt-
age, connect OUT to GND. Select R2 in the 10kΩ to
100kΩ range. Calculate R1 with the following equation:
V
V
OUT
R1=R2
-1
FB
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 on-
where V = 1.25V, and V
IN
down feature cannot be used, so SHDN must be per-
manently connected to IN.
may range from 1.25V to
FB
OUT
V . When setting output voltages above 5.5V, the shut-
resistance (R
) and inductor series resistance;
DS(ON)
therefore, it is proportional to the load current:
Inductor Selection
When selecting the inductor, consider these four para-
meters: 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:
V
=I
× R
(
+R
)
DROPOUT OUT
DS(ON) INDUCTOR
Shutdown (SHDN)
A logic-level low voltage on SHDN shuts down the
MAX1836/MAX1837. When shut down, the supply cur-
rent drops to 3µA to maximize battery life, and the inter-
nal P-channel MOSFET turns off to isolate the output
from the input. The output capacitance and load cur-
rent 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 volt-
ages 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.
(V
-V
)t
IN(MAX) OUT ON(MIN)
L
=
(MIN)
I
LIM
where t
(MIN)
= 1.0µs. Inductor values up to six times
ON(MIN)
L
are acceptable. Low-value inductors may be
smaller in physical size and less expensive, but they
result in higher peak-current overshoot due to current-
sense comparator propagation delay (300ns). Peak-
current overshoot reduces efficiency and could exceed
the current ratings of the internal switching MOSFET
and external components.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipa-
tion in the MAX1836/MAX1837. When the junction tem-
perature exceeds T = +160°C, a thermal sensor turns
J
OUTPUT
1.25V TO V
INPUT
4.5V OR 24V
L1
off the pass transistor, allowing the IC to cool. The ther-
mal 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.
IN
IN
LX
C
IN
D1
C
OUT
SHDN
R1
R2
FB
MAX1836
MAX1837
Design Information
GND
OUT
Output Voltage Selection
The feedback input features dual-mode operation.
Connect the output to OUT and FB to GND for the pre-
set output voltage. The MAX1836/MAX1837 are sup-
plied with factory-set output voltages of 3.3V or 5V. The
two-digit part number suffix identifies the output voltage
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
Figure 5. Adjustable Output Voltage
_______________________________________________________________________________________
9
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
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:
able for initial capacitor selection, but final values
should be set by testing a prototype or evaluation cir-
cuit. 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 induc-
tance but decreases with lower input voltages.
(V -V
) 300ns
IN OUT
I
=I
+
PEAK LIM
L
where the switch current-limit (I ) is typically 312mA
LIM
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.
(MAX1836) or 625mA (MAX1837). Saturation occurs
when the inductor’s magnetic flux density reaches the
maximum level the core can support, and the induc-
tance 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 avail-
able 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 recom-
mended.
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
) imposed by the switching currents
RMS
defined by the following equation:
The maximum output current of the MAX1836/MAX1837
current-limited converter is limited by the peak inductor
current. For the typical application, the maximum out-
put current is approximately:
V
V -V
(
)
OUT IN OUT
I
=I
RMS LOAD
V
IN
For most applications, nontantalum chemistries (ceram-
ic, 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 tem-
perature rise at the RMS input current for optimal circuit
longevity.
1
2
I
=
I
PEAK
OUT(MAX)
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:
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,
V
≈ V
+ V
RIPPLE
RIPPLE(ESR) RIPPLE(C)
The output voltage ripple as a consequence of the ESR
and output capacitance is:
and with a breakdown voltage >V . Schottky diodes
IN
V
=I
ESR
are preferred. For high-temperature applications,
Schottky diodes may be inadequate due to their high
leakage currents. In such cases, ultra-high-speed sili-
con rectifiers are recommended, although a Schottky
diode with a higher reverse voltage rating can often
provide acceptable performance.
RIPPLE(ESR) PEAK
2
L I
- I
(
)
V
PEAK OUT
IN
V
=
RIPPLE(C)
2C
V
V -V
IN OUT
OUT OUT
where I
is the peak inductor current (see the
Inductor Selection section). These equations are suit-
PEAK
10 ______________________________________________________________________________________
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Table 1. Component Suppliers
SUPPLIER
INDUCTORS
PHONE
FAX
WEBSITE
Coilcraft
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
Coiltronics
Sumida USA
Toko
CAPACITORS
AVX
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
Kemet
Panasonic
Sanyo
Taiyo Yuden
DIODES
www.t-yuden.com
Central Semiconductor
International
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
Nihon
www.niec.co.jp
www.onsemi.com
www.zetex.com
On Semiconductor
Zetex
coupling. The MAX1837 evaluation kit shows the rec-
ommended layout.
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.
Applications Information
High-Voltage Step-Down Converter
The typical application circuits’ (Figures 1 and 2) com-
ponents 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 applica-
tions. When using higher input voltages, verify that the
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 degrad-
ed performance. High-power traces, bolded in the typi-
cal application circuits (Figures 1 and 2), should be as
short and wide as possible. Additionally, the current
input capacitor’s voltage rating exceeds V
and
IN(MAX)
that the inductor value exceeds the minimum induc-
tance recommended in the Inductor Selection section.
Inverter Configuration
Figure 7 shows the MAX1836/MAX1837 in a floating
ground configuration. By connecting what would nor-
mally 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.
loops formed by the power components (C , C
,
OUT
IN
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
______________________________________________________________________________________ 11
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
L1
47µH
L1
47µH
OUTPUT
5V
INPUT
INPUT
4.5V TO 24V
3.6V TO 18V
IN
LX
IN
LX
C
OUT
68µF
C
C
IN
IN
D1
OUT
SHDN
SHDN
10µF
10µF
C
10V
OUT
25V
D1
100µF
OUT
MAX1836
MAX1837
MAX1837
OUTPUT
-3.3V OR -5V
GND
FB
GND
FB
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
C
IN
= TAIYO YUDEN TMK432BJ106KM
L1 = SUMIDA CDRH5D28-470
= SANYO POSCAP 10TPC68M
C
OUT
Figure 7. MAX1836/MAX1837 Inverter Configuration
D1 = NIHON EP05Q03L
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
Chip Information
Figure 6. High-Voltage Application
TRANSISTOR COUNT: 731
PROCESS: BiCMOS
12 ______________________________________________________________________________________
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.)
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.
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