19-1975; Rev 2; 7/03
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
Features
♦ Fixed 5V or Adjustable Output
♦ 4.5V to 24V Input Voltage Range
♦ Up to 600mA Output Current
♦ Internal 0.4Ω P-Channel MOSFET
♦ Efficiency Over 95%
♦ 15µA Quiescent Supply Current
♦ 3µA Shutdown Current
♦ 100% Maximum Duty Cycle for Low Dropout
♦ Current-Limited Architecture
♦ Thermal Shutdown
♦ Small 8-µMAX Package
Ordering Information
Applications
Notebook Computers
Distributed Power Systems
PART
TEMP RANGE
PIN-PACKAGE
MAX1776EUA
-40°C to +85°C
8 µMAX
Keep-Alive Supplies
Hand-Held Devices
Pin Configuration
Typical Operating Circuit
TOP VIEW
SHDN
IN
ILIM
LX
MAX1776
ILIM2
OUT
FB
GND
VIN
VOUT
FB
1
8
GND
2
7
SHDN
ILIM
3
6
ILIM2
LX
4
5
IN
MAX1776EUA
OUT
µMAX
µMAX
________________________________________________________________ Maxim Integrated Products
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.
1
MAX1776
General Description
The MAX1776 high-efficiency step-down converter provides an adjustable output voltage from 1.25V to VIN from
supply voltages as high as 24V. An internal current-limited 0.4Ω MOSFET delivers load currents up to 600mA.
Operation to 100% duty cycle minimizes dropout voltage (240mV at 600mA).
The MAX1776 has a low 15µA quiescent current to
improve light-load efficiency and conserve battery life.
The device draws only 3µA while in shutdown.
High switching frequencies (up to 200kHz) allow the
use of tiny surface-mount inductors and output capacitors. The MAX1776 is available in an 8-pin µMAX package, which uses half the space of an 8-pin SO. For
increased output drive capability, use the MAX1626/
MAX1627 step-down controllers, which drive an external P-channel MOSFET to deliver up to 20W.
MAX1776
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
ABSOLUTE MAXIMUM RATINGS
IN, SHDN, ILIM, ILIM2 to GND .................................-0.3V to 25V
LX to GND.......................................................-2V to (VIN + 0.3V)
OUT, FB to GND .........................................................-0.3V to 6V
Peak Input Current .................................................................. 2A
Maximum DC Input Current.............................................. 500mA
Continuous Power Dissipation (TA = +70°C)
8-Pin µMAX (derate 4.1mW/°C above +70°C) .............330mW
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
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
(Circuit of Figure 1, VIN = +12V, SHDN = IN, TA = 0°C to +85°C, unless otherwise noted.)
PARAMETER
SYMBOL
Input Voltage Range
VIN
Input Supply Current
IIN
Input Supply Current in Dropout
IIN(DROP)
Input Shutdown Current
Input Undervoltage Lockout
Threshold
VUVLO
Output Voltage (Preset Mode)
VOUT
Feedback Set Voltage
(Adjustable Mode)
OUT Bias Current
CONDITIONS
MIN
TYP
4.5
28
No load
50
70
µA
SHDN = GND
3
7
µA
VIN rising
3.6
4.0
4.4
VIN falling
3.5
3.9
4.3
FB = GND
4.80
5.00
5.20
V
1.212
1.25
1.288
V
1.65
3.5
6.25
µA
VOUT = 5.5V
VFB = 1.3V
-25
FB Dual Mode™ Threshold Low
LX Switch Minimum Off-Time
tOFF(MIN)
LX Switch Maximum On-Time
tON(MAX)
VFB = 1.3V
VIN = 6V
LX Switch On-Resistance
RLX
VIN = 4.5V
ILX(PEAK)
V
+25
nA
mV
50
100
150
0.42
0.62
µs
8
10
12
µs
1.6
3.2
ILIM = GND, ILIM2 = IN
0.8
1.6
ILIM = IN, ILIM2 = GND
0.4
0.8
ILIM = ILIM2 = IN
0.4
0.8
ILIM = ILIM2 = GND
1.9
3.8
ILIM = GND, ILIM2 = IN
1.0
1.9
ILIM = IN, ILIM2 = GND
0.5
0.95
0.5
0.95
ILIM = ILIM2 = GND
120
150
180
ILIM = GND, ILIM2 = IN
240
300
360
ILIM = IN, ILIM2 = GND
480
600
720
ILIM = ILIM2 = IN
960
1200
1440
-75
Zero-Crossing Timeout
LX does not rise above the threshold
LX Switch Leakage Current
VIN = 24V,
LX = GND
+75
30
1
TA = 0°C to +85°C
10
_______________________________________________________________________________________
Ω
mA
mV
µs
TA = +25°C
Dual Mode is a trademark of Maxim Integrated Products, Inc.
2
5.5
ILIM = ILIM2 = GND
LX Zero-Crossing Threshold
V
0.22
ILIM = ILIM2 = IN
LX Current Limit
V
µA
15
VFB
IFB
UNITS
24
No load
OUT Pin Maximum Voltage
FB Bias Current
MAX
µA
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
(Circuit of Figure 1, VIN = +12V, SHDN = IN, TA = 0°C to +85°C, unless otherwise noted.)
PARAMETER
Dropout Voltage
SYMBOL
CONDITIONS
MIN
VDROPOUT IOUT = 525mA, ILIM = ILIM2 = IN
TYP
%/V
VIN = 8V/24V, 200Ω load
0.1
Load Regulation
No load/full load
0.9
Low
SHDN, ILIM2
High
2.4
Digital Input Leakage Current
V SHDN, VILIM, VILIM2 = 0 or 24V, VIN = 24V
-1
+1
Low
0.05
High
Thermal Shutdown
%
0.8
Digital Input Level
2.2
10°C hysteresis
UNITS
V
Line Regulation
ILIM Input Level
MAX
0.2
160
V
µA
V
°C
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1, VIN = +12V, SHDN = IN, TA = -40°C to +85°C, unless otherwise noted.) (Note 1)
PARAMETER
Input Voltage Range
Input Supply Current
Input Supply Current in Dropout
SYMBOL
MIN
MAX
UNITS
4.5
24
V
µA
IIN
No load
28
IIN(DROP)
No load
70
µA
SHDN = GND
7
µA
Input Shutdown Current
Input Undervoltage Lockout
Threshold
VUVLO
Output Voltage (Preset Mode)
VOUT
Feedback Set Voltage
(Adjustable Mode)
CONDITIONS
VIN
VIN rising
3.6
4.4
VIN falling
3.5
4.3
FB = GND
4.75
5.25
V
1.2
1.3
V
1.65
6.25
µA
5.5
V
VFB
OUT Bias Current
VOUT = 5.5V
OUT Pin Maximum Voltage
FB Bias Current
IFB
VFB = 1.3V
FB Dual Mode Threshold Low
LX Switch Minimum Off-Time
tOFF(MIN)
LX Switch Maximum On-Time
tON(MAX)
VFB = 1.3V
VIN = 6V
LX Switch On-Resistance
RLX
VIN = 4.5V
LX Current Limit
ILX(PEAK)
V
-25
+25
nA
45
155
mV
0.22
0.64
µs
7.5
12.5
µs
ILIM = ILIM2 = GND
3.2
ILIM = GND, ILIM2 = IN
1.6
ILIM = IN, ILIM2 = GND
0.8
ILIM = ILIM2 = IN
0.8
ILIM = ILIM2 = GND
3.8
ILIM = GND, ILIM2 = IN
1.9
ILIM = IN, ILIM2 = GND
0.95
ILIM = ILIM2 = IN
0.95
ILIM = ILIM2 = GND
100
200
ILIM = GND, ILIM2 = IN
200
400
ILIM = IN, ILIM2 = GND
400
800
ILIM = ILIM2 = IN
800
1600
Ω
mA
_______________________________________________________________________________________
3
MAX1776
ELECTRICAL CHARACTERISTICS (continued)
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VIN = +12V, SHDN = IN, TA = -40°C to +85°C, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
MAX
UNITS
-75
75
mV
10
µA
LX Zero-Crossing Threshold
LX Switch Leakage Current
VIN = 24V, LX = GND
Digital Input Level
SHDN, ILIM2
Digital Input Leakage Current
V SHDN, VILIM, VILIM2 = 0 or 24V, VIN = 24V
Low
0.8
High
V
2.4
-1
1
Low
ILIM Input Level
µA
0.05
High
V
2.2
Note 1: Specifications to -40°C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1, components from Table 3, VIN = +12V, SHDN = IN, TA = +25°C.)
LOAD REGULATION,
CIRCUIT 1, VOUTPUT = 3.3V
VIN = 5V
0.4
LOAD REGULATION, CIRCUIT 2
0.2
MAX1776 toc02
0
0
0.2
VOUTPUT (%)
VIN = 12V
-0.4
-0.6
VIN = 24V
-0.8
-0.2
0
-0.2
VIN = 15V
-0.4
VIN = 12V
-0.6
-1.0
VOUTPUT (%)
-0.2
VOUTPUT (%)
0.6
MAX1776 toc01
0.2
MAX1776 toc03
LOAD REGULATION,
CIRCUIT 1, VOUTPUT = 5V
VIN = 24V
-0.4
-0.6
VIN = 12V
-0.8
VIN = 24V
VIN = 15V
-1.0
-0.8
VIN = 15V
-1.0
-1.2
100
200
300
400
500
600
-1.2
0
700
100
200
300
400
500
600
0
50
100 150 200 250 300 350 400
ILOAD (mA)
ILOAD (mA)
ILOAD (mA)
LOAD REGULATION, CIRCUIT 5
VOUTPUT vs. VIN,
CIRCUIT 5, VOUTPUT = 5V
VOUTPUT vs. VIN,
CIRCUIT 5, VOUTPUT = 3.3V
-0.2
2
VIN = 24V
VOUTPUT (%)
-0.3
-0.4
-0.5
-0.6
-0.7
VIN = 15V
-0.8
2.0
ILOAD = 1mA
1
ILOAD = 50mA
0
ILOAD = 500mA
1.5
VOUTPUT (%)
VIN = 12V
MAX1776 toc06
3
MAX1776 toc04
0
-0.1
MAX1776 toc05
0
VOUTPUT (% FROM VOUT(NOM))
MAX1776
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
1.0
ILOAD = 1mA
0.5
-1
0
-2
-0.5
-3
-1.0
ILOAD = 10mA
-0.9
ILOAD = 50mA
1.0
0
0.1
0.2
0.3
ILOAD (A)
4
0.4
0.5
0.6
5
7
9
11 13 15 17 19 21 23 25
5
7
9
11 13 15 17 19 21 23 25
VIN (V)
_______________________________________________________________________________________
VIN (V)
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
0.4
0.2
-0.8
1.0
ILOAD = 500mA
80
70
65
-0.8
60
-1.0
55
9
11 13 15 17 19 21 23 25
50
5
7
9
VIN (V)
VIN (V)
EFFICIENCY vs. ILOAD, CIRCUIT 5,
VOUTPUT = 3.3V
EFFICIENCY vs. ILOAD, CIRCUIT 1,
VOUTPUT = 3.3V
90
85
EFFICIENCY (%)
VIN = 12V
80
75
VIN = 15V
VIN = 24V
70
VIN = 6V
85
80
75
VIN = 24V
95
VIN = 12V
70
80
65
60
55
55
55
50
50
100
80
VIN = 12V
ILOAD = 500mA
100
ILOAD = 375mA
ILOAD = 250mA
ILOAD = 5mA
40
ILOAD = 50mA
40
0
100 200 300 400 500 600 700 800 900
ILOAD (mA)
12
13
14
15
16
1.0
0.5
0
-0.5
-1.0
0
0
11
ILOAD = 10mA
20
20
10
VOUTPUT ACCURACY vs. TEMPERATURE
80
60
9
1.5
VOUT ACCURACY (%)
VIN = 24V
FREQUENCY (kHz)
120
100
140
MAX1776 toc13
140
8
VIN (V)
SWITCHING FREQUENCY vs.
VIN, CIRCUIT 1
120
7
1000
MAX1776 toc14
10
SWITCHING FREQUENCY vs.
LOAD CURRENT, CIRCUIT 1
160
60
1
ILOAD (mA)
VIN = 15V
CIRCUIT 5, 3.3V
50
0.10
ILOAD (mA)
200
180
1000
CIRCUIT 1, 3.3V
70
60
100
CIRCUIT 1, 5V
75
60
10
1000
85
65
1
CIRCUIT 5, 5V
90
65
0.10
100
EFFICIENCY vs. VIN, ILOAD = 500mA
95
90
10
100
MAX1776 toc11
VIN = 6V
95
1
ILOAD (mA)
100
MAX1776 toc10
100
0.10
11 13 15 17 19 21 23 25
EFFICIENCY (%)
7
VIN = 24V
VIN = 15V
75
-1.2
5
EFFICIENCY (%)
ILOAD = 50mA
-0.6
-0.6
FREQUENCY (kHz)
-0.4
85
MAX1776 toc12
ILOAD = 500mA
-0.2
VIN = 12V
90
MAX1776 toc15
ILOAD = 50mA
-0.4
0
VIN = 6V
95
EFFICIENCY (%)
VOUTPUT (%)
VOUTPUT (%)
0
-0.2
ILOAD = 1mA
ILOAD = 10mA
100
MAX1776 toc08
ILOAD = 1mA
0.2
0.6
MAX1776 toc07
0.4
ILOAD = 10mA
EFFICIENCY vs. ILOAD, CIRCUIT 1,
VOUT = 5V
VOUTPUT vs. VIN,
CIRCUIT 1, VOUTPUT = 3.3V
MAX1776 toc09
VOUTPUT vs. VIN,
CIRCUIT 1, VOUTPUT = 5V
-1.5
5
10
15
VIN (V)
20
25
-40
-20
0
20
40
60
80
100
TEMPERATURE (°C)
_______________________________________________________________________________________
5
MAX1776
Typical Operating Characteristics (continued)
(Circuit of Figure 1, components from Table 3, VIN = +12V, SHDN = IN, TA = +25°C.)
Typical Operating Characteristics (continued)
(Circuit of Figure 1, components from Table 3, VIN = +12V, SHDN = IN, TA = +25°C.)
QUIESCENT SUPPLY CURRENT
vs. TEMPERATURE
QUIESCENT SUPPLY CURRENT
vs. SUPPLY VOLTAGE
QUIESCENT SUPPLY CURRENT (µA)
17.5
17.0
16.5
16.0
15.5
MAX1776 toc17
14.20
MAX1776 toc16
QUIESCENT SUPPLY CURRENT (µA)
18.0
14.15
14.10
14.05
14.00
13.95
13.90
13.85
13.80
13.75
15.0
13.70
-40
-20
0
20
40
60
80
5
TEMPERATURE (°C)
9
11 13 15 17 19 21 23 25
LOAD-TRANSIENT RESPONSE,
CIRCUIT 5
MAX1776 toc19
MAX1776 toc18
0.8
L = 10µH
0.7
7
SUPPLY VOLTAGE (V)
PEAK SWITCH CURRENT
vs. INPUT VOLTAGE, CIRCUIT 3, 0.3A
PEAK SWITCH CURRENT (A)
MAX1776
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
1A
IL
0.6
0
L = 22µH
0.5
10V
VLX
0.4
0
L = 47µH
0.3
AC COUPLED
50mV/div
VOUT
L = 100µH
0.2
500mA
ILOAD
0.1
10mA
0
0
5
10
15
20
25
10µs/div
VIN (V)
LINE-TRANSIENT RESPONSE,
CIRCUIT 5, ILOAD = 50mA
LINE-TRANSIENT RESPONSE,
CIRCUIT 5, ILOAD = 500mA
MAX1776 toc21
MAX1776 toc20
AC-COUPLED
200mv/div
VOUT
AC-COUPLED
200mv/div
VOUT
15V
VIN
10V
10V
VIN
5V
10V
5V
VLX
VLX
5V
0
0
200µs/div
6
200µs/div
_______________________________________________________________________________________
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
LX WAVEFORM, CIRCUIT 1
VIN = 15V, ILOAD = 500mA
STARTUP WAVEFORM, CIRCUIT 1,
RLOAD = 100Ω
MAX1776 toc22
MAX1776 toc23
5V
0
VSHDN
1A
IL
0
1A
IL
10V
VLX
0
0
6V
4V
VOUT
2V
VOUT
50mV/div
0
2µs/div
2µs/div
EFFICIENCY vs. ILOAD, CIRCUIT 3, VIN = 12V
95
L = 22µH
EFFICIENCY (%)
L = 22µH, 0.6A
90
85
L = 47µH
MAX1776 toc25
95
EFFICIENCY (%)
EFFICIENCY vs. ILOAD, CIRCUIT 3, VIN = 12V
100
MAX1776 toc24
100
L = 100µH
80
L = 47µH, 0.3A
90
85
L = 10µH, 1.2A
80
75
75
0.10
1
10
ILOAD (mA)
100
1000
0.10
1
10
100
1000
ILOAD (mA)
_______________________________________________________________________________________
7
MAX1776
Typical Operating Characteristics (continued)
(Circuit of Figure 1, components from Table 3, VIN = +12V, SHDN = IN, TA = +25°C.)
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
MAX1776
Pin Description
PIN
NAME
FUNCTION
1
FB
2
GND
Ground
3
ILIM
Peak Current Control Input. Connect to IN or GND to set peak current limit. ILIM and ILIM2 together set
the peak current limit. See Setting Current Limit.
4
LX
Inductor Connection. Connect LX to external inductor and diode as shown in Figure 1.
5
IN
Input Supply Voltage. Input voltage range is 4.5V to 24V.
6
ILIM2
Peak Current Control Input 2. Connect to IN or GND. ILIM and ILIM2 together set the peak current limit.
See Setting Current Limit.
7
SHDN
Shutdown Input. A logic low shuts down the MAX1776 and reduces the supply current to 3µA. LX is high
impedance in shutdown. Connect to IN for normal operation.
8
OUT
Dual-Mode Feedback Input. Connect to GND for the preset 5V output. Connect to a resistive divider
between OUT and GND to adjust the output voltage between 1.25V and VIN.
Regulated Output Voltage High-Impedance Sense Input. Internally connected to a resistive divider.
Do not connect for output voltages higher than 5.5V. Connect to GND when not used.
Detailed Description
The MAX1776 step-down converter is designed primarily for battery-powered devices and notebook computers. The 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 usable supply
voltage range. Under no load, the MAX1776 draws only
15µA, and in shutdown mode, it draws 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 costs.
INPUT
4.5V TO 24V
L1
IN
CIN
OUTPUT
5V
LX
D1
SHDN
COUT
J1
J2
ILIM
J3
ILIM2
J4
MAX1776
OUT
FB
GND
CIN: 10µF, 25V CERAMIC
Current-Limited Control Architecture
The MAX1776 uses a proprietary current-limited control
scheme with operation to 100% duty cycle. This DC-DC
converter pulses 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 constant-frequency
pulse-width-modulation (PWM) controllers that switch
the MOSFET unnecessarily.
When the output voltage is too low, the error comparator
sets a flip-flop, which turns on the internal P-channel
MOSFET and begins a switching cycle (Figure 2). As
shown in Figure 3, the inductor current ramps up linearly, storing energy in a magnetic field while charging the
output capacitor and servicing the load. The MOSFET
turns off when the peak current limit is reached, or when
the maximum on-time of 10µs is exceeded and the output voltage is in regulation. If the output is out of regulation and the peak current is never obtained, the
MOSFET remains on, allowing a duty cycle up to 100%.
This feature ensures the lowest possible dropout voltage. Once the MOSFET turns off, the flip-flop resets, the
inductor current is pulled through D1, 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 0.42µs minimum off-time
expires, and the output voltage drops out of regulation.
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
SEE TABLE 3 FOR OTHER COMPONENT VALUES
Figure 1. Typical Application Circuit
8
_______________________________________________________________________________________
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
MAX1776
MAX1776
D
L1
LX
CIN
D1
OUTPUT
COUT
OUT
Q
SHDN
R
FB
S
MAXIMUM
ON-TIME
DELAY
ILIM
ILIM
SET
100mV
VSET
1.25V
ILIM2
MINIIMUM
OFF-TIME
DELAY
GND
Figure 2. Simplified Functional Diagram
Input-Output (Dropout) Voltage
LX WAVEFORM, CIRCUIT 1
VIN = 15V, ILOAD = 500mA
1A
IL
0
10V
VLX
0
VOUT
50mV/div
2µs/div
Figure 3. Discontinuous-Conduction Operation
A step-down converter’s minimum input-to-output voltage differential (dropout voltage) determines the lowest
usable supply voltage. In battery-powered systems,
this limits the useful end-of-life battery voltage. To maximize battery life, the MAX1776 operates with duty
cycles up to 100%, which minimizes the dropout voltage and eliminates switching losses while in dropout.
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, dropout
depends on the MOSFET drain-to-source on-resistance
and inductor series resistance; therefore, it is proportional to the load current:
VDROPOUT = IOUT ✕ (RDS(ON) + RINDUCTOR)
_______________________________________________________________________________________
9
MAX1776
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
Shutdown (SHDN)
A logic low level on SHDN shuts down the MAX1776
converter. When in shutdown, 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 on SHDN activates the MAX1776. Do not leave
SHDN floating. If unused, connect SHDN to IN.
Table 1. Current-Limit Configuration
CURRENT
LIMIT (mA)
ILIM
CONNECTED TO
ILIM2
CONNECTED TO
150
GND
GND
300
GND
IN
600
IN
GND
1200
IN
IN
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation in the MAX1776. 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.
Choose a current limit that realistically reflects the maximum load current. The maximum output current is half
of the peak current limit. Although choosing a lower
current limit allows using an inductor with a lower current rating, it requires a higher inductance (see
Inductor Selection) and does little to reduce inductor
package size.
Design Information
Output Voltage Selection
The feedback input features dual-mode operation.
Connect FB to GND for the 5.0V preset output voltage.
Alternatively, adjust the output voltage by connecting a
voltage-divider from the output to GND (Figure 4).
Select a value for R2 between 10kΩ and 100kΩ.
Calculate R1 with the following equation:
V
R1 = R2 × OUTPUT - 1
VFB
where V FB = 1.25V, and V OUTPUT may range from
1.25V to VIN.
Setting Current Limit
The MAX1776 has an adjustable peak current limit.
Configure this peak current limit by connecting ILIM
and ILIM2 as shown in Table 1.
Inductor Selection
When selecting the inductor, consider these four parameters: inductance value, saturation rating, series
resistance, and size. The MAX1776 operates with a
wide range of inductance values. For most applications, values between 10µH and 100µH work best with
the controller’s high switching frequency. Larger inductor values will reduce the switching frequency and
thereby improve efficiency and EMI. The trade-off for
improved efficiency is a higher output ripple and slower
transient response. On the other hand, low-value inductors respond faster to transients, improve output ripple,
offer smaller physical size, and minimize cost. If the
inductor value is too small, the peak inductor current
exceeds the current limit due to current-sense comparator propagation delay, potentially exceeding the
inductor’s current rating. Calculate the minimum inductance value as follows:
L(MIN) =
INPUT
4.5V TO 24V
IN
CIN
OUTPUT
1.25V TO VIN
L1
LX
D1
SHDN
ILIM
R1
MAX1776
FB
ILIM2
R2
GND
OUT
COUT
(VIN(MAX) - VOUTPUT ) × tON(MIN)
ILX (PEAK )
where tON(MIN) = 1µs.
The inductor’s saturation current rating must be greater
than the peak switch current limit, plus the overshoot
due to the 250ns current-sense comparator propagation delay. Saturation occurs when the inductor’s magnetic flux density reaches the maximum level the core
can support and the inductance starts to fall. Choose
an inductor with a saturation rating greater than IPEAK
in the following equation:
IPEAK = ILX(PEAK) + (VIN - VOUTPUT) ✕ 250ns / L
Figure 4. Adjustable Output Voltage
10
______________________________________________________________________________________
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
Maximum Output Current
The MAX1776 converter’s output current determines
the regulator’s switching frequency. When the converter approaches continuous mode, the output voltage
falls out of regulation. For the typical application, the
maximum output current is approximately:
ILOAD(MAX) = 1/2 ILX (PEAK)(MIN)
For low-input voltages, the maximum on-time may be
reached and the load current is limited by:
ILOAD = 1/2 (VIN - VOUT) ✕ 10µs / L
Output Capacitor
Choose the output capacitor to service 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) = ESR × IPEAK
L × (IPEAK - IOUTPUT )
VIN
2COUT × VOUTPUT VIN - VOUTPUT
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 current
defined by the following equation:
I
V
IRMS = LOAD OUTPUT
VIN
For most applications, nontantalum chemistries (ceramic, aluminum, polymer, or OS-CON) are preferred due to
their robustness to high inrush currents typical of systems with low-impedance battery inputs. Alternatively,
connect two (or more) smaller value low-ESR capacitors
in parallel to reduce cost. Choose an input capacitor
that exhibits less than +10°C temperature rise at the
RMS input current for optimal circuit longevity.
Table 2. Component Suppliers
SUPPLIER
where IPEAK is the peak inductor current (see Inductor
Selection). The worst-case ripple occurs at no-load.
These equations are suitable 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, and as the input voltage decreases. See Table 3 for recommended capacitor values and Table 2 for recommended component
manufacturers.
WEBSITE
DIODES
Central Semiconductor
www.centralsemi.com
Fairchild
www.fairchildsemi.com
General Semiconductor
www.gensemi.com
International Rectifier
www.irf.com
Nihon
www.niec.co.jp/engver2/
niec.co.jp_eg.htm
On Semi
www.onsemi.com
Vishay-Siliconix
www.vishay.com/brands/siliconix/
main.html
Zetex
www.zetex.com
2
VRIPPLE(C) =
4
VIN
− 1
3× V
OUTPUT
CAPACITORS
AVX
www.avxcorp.com
Kemet
www.kemet.com
Nichicon
www.nichicon-us.com
Sanyo
www.sanyo.com
Taiyo Yuden
www.t-yuden.com
INDUCTORS
Coilcraft
www.coilcraft.com
Coiltronics
www.cooperet.com
Pulse Engineering
www.pulseeng.com
Sumida USA
www.sumida.com
Toko
www.tokoam.com
______________________________________________________________________________________
11
MAX1776
Inductor series resistance affects both efficiency and
dropout voltage (see Input-Output (Dropout) Voltage).
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. Some recommended component
manufacturers are listed in Table 2.
MAX1776
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
Table 3. Recommended Components
CIRCUIT
INPUT
VOLTAGE
(V)
MAXIMUM
LOAD
CURRENT
(mA)
ILX(PEAK)
CURRENT
(A)
INDUCTOR
CAPACITOR
10µH, 1.56A, 70mΩ
Toko D75F 646FY-100M,
10µH, 1.70A, 48mΩ
Sumida CDRH6D28-100NC,
or 10µH, 1.63A, 55mΩ
Toko D75C 646CY-100M 0.055
22µH, 1.17A, 120mΩ
Toko D75F 646FY-220M,
22µH, 1.09A, 115mΩ
Toko D75C 646CY-220M,
or 22µH, 1.20A, 95mΩ
Sumida CDRH6D28-220NC
100µF, 6.3V
Sanyo POSCAP 6TPC100M
1
10 to 24
600
1.20
2
10 to 24
300
0.60
3
10 to 24
150
0.30
47µH, 0.54A, 440mΩ
Sumida CDRH5D18-470
22µF, 6.3V, 1210 case
Taiyo Youden JMK325BJ226MM
4
10 to 24
75
0.15
100µH, 0.29A, 766mΩ
Sumida CDRH4D28-101
10µF, 6.3V, X7R, 1206 case
Taiyo Youden JMK316BJ106ML
5
5 to 15
600
1.20
5.4µH, 1.6A, 56mΩ
Sumida CDRH5D18-5R4
100µF, 6.3V
Sanyo POSCAP 6TPC100m
6
5 to 15
300
0.60
10µH, 1.04A, 80mΩ
Toko D73LC 817CY-100M
47µF, 6.3V
Sanyo POSCAP 6TPA47M
7
5 to 15
150
0.30
22µH, 0.41A, 294mΩ
Sumida CDRH4D18-220
22µF, 6.3V, 1210 case
Taiyo Youden JMK325BJ226MM
8
5 to 15
75
0.15
47µH, 0.33A, 230mΩ
Coilcraft DS1608C-473
10µF, 6.3V, X7R, 1206 case
Taiyo Youden JMK316BJ106ML
47µF, 6.3V
Sanyo POSCAP 6TPA47M
Diode Selection
MAX1776 Stability
The current in the external diode (D1 in Figure 1)
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.
Make sure that the diode’s peak current rating exceeds
the peak current limit set by the current limit, and that
its breakdown voltage exceeds V IN . Use Schottky
diodes when possible.
Instability is frequently caused by excessive noise on
OUT, FB, or GND due to poor layout or improper component selection. Instability typically manifests itself as
“motorboating,” which is characterized by grouped
switching pulses with large gaps and excessive lowfrequency output ripple during no-load or light-load
conditions.
12
PC Board Layout and Grounding
High switching frequencies and large peak currents
make PC board layout an important part of the design.
Poor layout introduces switching noise into the feedback path, resulting in jitter, instability, or degraded
performance. High-power traces, highlighted in the
______________________________________________________________________________________
24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
board and integrate it into a pseudo-ground plane.
When using external feedback, place the resistors as
close to the feedback pin as possible to minimize noise
coupling.
Chip Information
TRANSISTOR COUNT: 932
PROCESS: BiCMOS
4X S
8
8
INCHES
DIM
A
A1
A2
b
E
ÿ 0.50±0.1
H
c
D
e
E
H
0.6±0.1
L
1
1
α
0.6±0.1
S
BOTTOM VIEW
D
MIN
0.002
0.030
MAX
0.043
0.006
0.037
0.014
0.010
0.007
0.005
0.120
0.116
0.0256 BSC
0.120
0.116
0.198
0.188
0.026
0.016
6∞
0∞
0.0207 BSC
8LUMAXD.EPS
Package Information
MILLIMETERS
MAX
MIN
0.05
0.75
1.10
0.15
0.95
0.25
0.36
0.13
0.18
2.95
3.05
0.65 BSC
2.95
3.05
5.03
4.78
0.41
0.66
0∞
6∞
0.5250 BSC
TOP VIEW
A1
A2
A
α
c
e
b
L
SIDE VIEW
FRONT VIEW
PROPRIETARY INFORMATION
TITLE:
PACKAGE OUTLINE, 8L uMAX/uSOP
APPROVAL
DOCUMENT CONTROL NO.
21-0036
REV.
J
1
1
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
© 2003 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
MAX1776
Typical Application Circuit (Figure 1), 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 short 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
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