View MAX1776_124912.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

19-1975; Rev 2; 7/03
24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter
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 0.4 MOSFET delivers load currents up to 600mA. Operation to 100% duty cycle minimizes dropout voltage (240mV at 600mA). The MAX1776 has a low 15A quiescent current to improve light-load efficiency and conserve battery life. The device draws only 3A 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 MAX 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. o Fixed 5V or Adjustable Output o 4.5V to 24V Input Voltage Range o Up to 600mA Output Current o Internal 0.4 P-Channel MOSFET o Efficiency Over 95% o 15A Quiescent Supply Current o 3A Shutdown Current o 100% Maximum Duty Cycle for Low Dropout o Current-Limited Architecture o Thermal Shutdown o Small 8-MAX Package
Features
MAX1776
Applications
Notebook Computers Distributed Power Systems Keep-Alive Supplies Hand-Held Devices
PART MAX1776EUA
Ordering Information
TEMP RANGE -40C to +85C PIN-PACKAGE 8 MAX
Typical Operating Circuit
TOP VIEW
SHDN IN VIN
Pin Configuration
ILIM MAX1776 ILIM2
LX
VOUT
FB GND ILIM
1 2 3 4 MAX1776EUA
8 7 6 5
OUT SHDN ILIM2 IN
OUT
LX
MAX
FB GND
MAX
________________________________________________________________ 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, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter MAX1776
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 = +70C) 8-Pin MAX (derate 4.1mW/C above +70C) .............330mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1, VIN = +12V, SHDN = IN, TA = 0C to +85C, unless otherwise noted.)
PARAMETER Input Voltage Range Input Supply Current Input Supply Current in Dropout Input Shutdown Current Input Undervoltage Lockout Threshold Output Voltage (Preset Mode) Feedback Set Voltage (Adjustable Mode) OUT Bias Current OUT Pin Maximum Voltage FB Bias Current FB Dual ModeTM Threshold Low LX Switch Minimum Off-Time LX Switch Maximum On-Time tOFF(MIN) tON(MAX) VFB = 1.3V ILIM = ILIM2 = GND VIN = 6V LX Switch On-Resistance RLX VIN = 4.5V ILIM = GND, ILIM2 = IN ILIM = IN, ILIM2 = GND ILIM = ILIM2 = IN ILIM = ILIM2 = GND ILIM = GND, ILIM2 = IN ILIM = IN, ILIM2 = GND ILIM = ILIM2 = IN ILIM = ILIM2 = GND LX Current Limit ILX(PEAK) ILIM = GND, ILIM2 = IN ILIM = IN, ILIM2 = GND ILIM = ILIM2 = IN LX Zero-Crossing Threshold Zero-Crossing Timeout LX Switch Leakage Current LX does not rise above the threshold VIN = 24V, LX = GND TA = +25C TA = 0C to +85C 120 240 480 960 -75 30 1 10 IFB VFB = 1.3V -25 50 0.22 8 100 0.42 10 1.6 0.8 0.4 0.4 1.9 1.0 0.5 0.5 150 300 600 1200 VUVLO VOUT VFB VOUT = 5.5V SYMBOL VIN IIN IIN(DROP) No load No load SHDN = GND VIN rising VIN falling FB = GND 3.6 3.5 4.80 1.212 1.65 CONDITIONS MIN 4.5 15 50 3 4.0 3.9 5.00 1.25 3.5 TYP MAX 24 28 70 7 4.4 4.3 5.20 1.288 6.25 5.5 +25 150 0.62 12 3.2 1.6 0.8 0.8 3.8 1.9 0.95 0.95 180 360 720 1440 +75 mV s A mA UNITS V A A A V V V A V nA mV s s
Dual Mode is a trademark of Maxim Integrated Products, Inc. 2 _______________________________________________________________________________________
24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VIN = +12V, SHDN = IN, TA = 0C to +85C, unless otherwise noted.)
PARAMETER Dropout Voltage Line Regulation Load Regulation Digital Input Level Digital Input Leakage Current ILIM Input Level Thermal Shutdown SYMBOL CONDITIONS VIN = 8V/24V, 200 load No load/full load SHDN, ILIM2 Low High 2.4 -1 2.2 160 +1 0.05 MIN TYP 0.2 0.1 0.9 0.8 MAX UNITS V %/V % V A V C
MAX1776
VDROPOUT IOUT = 525mA, ILIM = ILIM2 = IN
V SHDN, VILIM, VILIM2 = 0 or 24V, VIN = 24V Low High 10C hysteresis
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1, VIN = +12V, SHDN = IN, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage Range Input Supply Current Input Supply Current in Dropout Input Shutdown Current Input Undervoltage Lockout Threshold Output Voltage (Preset Mode) Feedback Set Voltage (Adjustable Mode) OUT Bias Current OUT Pin Maximum Voltage FB Bias Current FB Dual Mode Threshold Low LX Switch Minimum Off-Time LX Switch Maximum On-Time tOFF(MIN) tON(MAX) VFB = 1.3V ILIM = ILIM2 = GND VIN = 6V LX Switch On-Resistance RLX VIN = 4.5V ILIM = GND, ILIM2 = IN ILIM = IN, ILIM2 = GND ILIM = ILIM2 = IN ILIM = ILIM2 = GND ILIM = GND, ILIM2 = IN ILIM = IN, ILIM2 = GND ILIM = ILIM2 = IN ILIM = ILIM2 = GND LX Current Limit ILX(PEAK) ILIM = GND, ILIM2 = IN ILIM = IN, ILIM2 = GND ILIM = ILIM2 = IN 100 200 400 800 IFB VFB = 1.3V -25 45 0.22 7.5 VUVLO VOUT VFB VOUT = 5.5V SYMBOL VIN IIN IIN(DROP) No load No load SHDN = GND VIN rising VIN falling FB = GND 3.6 3.5 4.75 1.2 1.65 CONDITIONS MIN 4.5 MAX 24 28 70 7 4.4 4.3 5.25 1.3 6.25 5.5 +25 155 0.64 12.5 3.2 1.6 0.8 0.8 3.8 1.9 0.95 0.95 200 400 800 1600 mA UNITS V A A A V V V A V nA mV s s
_______________________________________________________________________________________
3
24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter MAX1776
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VIN = +12V, SHDN = IN, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER LX Zero-Crossing Threshold LX Switch Leakage Current Digital Input Level Digital Input Leakage Current ILIM Input Level VIN = 24V, LX = GND SHDN, ILIM2 Low High 2.4 -1 2.2 1 0.05 SYMBOL CONDITIONS MIN -75 MAX 75 10 0.8 UNITS mV A V A V
V SHDN, VILIM, VILIM2 = 0 or 24V, VIN = 24V Low High
Note 1: Specifications to -40C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1, components from Table 3, VIN = +12V, SHDN = IN, TA = +25C.)
LOAD REGULATION, CIRCUIT 1, VOUTPUT = 5V
MAX1776 toc01
LOAD REGULATION, CIRCUIT 1, VOUTPUT = 3.3V
MAX1776 toc02
LOAD REGULATION, CIRCUIT 2
MAX1776 toc03
0.2 0 -0.2 VOUTPUT (%) -0.4 -0.6 -0.8 -1.0 VIN = 15V -1.2 0 100 200 300 400 500 600 VIN = 24V VIN = 12V
0.6 0.4 0.2 VOUTPUT (%) VIN = 5V
0.2 0 -0.2 VOUTPUT (%) -0.4 -0.6 -0.8 -1.0 -1.2 VIN = 12V VIN = 15V VIN = 24V
0 -0.2 -0.4 -0.6 -0.8 -1.0 0 100 200 300 ILOAD (mA) 400 500 600 VIN = 12V VIN = 24V VIN = 15V
700
0
50
100 150 200 250 300 350 400 ILOAD (mA)
ILOAD (mA)
LOAD REGULATION, CIRCUIT 5
-0.1 VOUTPUT (% FROM VOUT(NOM)) -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 1.0 0 0.1 0.2 0.3 ILOAD (A) 0.4 0.5 0.6 -3 5 7 VIN = 15V VIN = 24V VOUTPUT (%) 1 0 VIN = 12V
MAX1776 toc04
VOUTPUT vs. VIN, CIRCUIT 5, VOUTPUT = 5V
MAX1776 toc05
VOUTPUT vs. VIN, CIRCUIT 5, VOUTPUT = 3.3V
MAX1776 toc06
0
3 2 ILOAD = 1mA ILOAD = 50mA
2.0 1.5 1.0 ILOAD = 1mA 0.5 0 -0.5 ILOAD = 50mA -1.0 ILOAD = 10mA
ILOAD = 500mA -1 -2
9
11 13 15 17 19 21 23 25 VIN (V)
VOUTPUT (%)
5
7
9
11 13 15 17 19 21 23 25 VIN (V)
4
_______________________________________________________________________________________
24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1, components from Table 3, VIN = +12V, SHDN = IN, TA = +25C.)
MAX1776
VOUTPUT vs. VIN, CIRCUIT 1, VOUTPUT = 5V
MAX1776 toc07
VOUTPUT vs. VIN, CIRCUIT 1, VOUTPUT = 3.3V
MAX1776 toc08
EFFICIENCY vs. ILOAD, CIRCUIT 1, VOUT = 5V
95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 VIN = 15V VIN = 24V VIN = 6V VIN = 12V
MAX1776 toc09
0.4 ILOAD = 10mA 0.2 0 VOUTPUT (%) -0.2 -0.4 -0.6 -0.8 1.0 5 7 9 ILOAD = 50mA ILOAD = 500mA ILOAD = 1mA
0.6 0.4 0.2 VOUTPUT (%) 0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 ILOAD = 50mA ILOAD = 500mA ILOAD = 10mA ILOAD = 1mA
100
11 13 15 17 19 21 23 25 VIN (V)
5
7
9
11 13 15 17 19 21 23 25 VIN (V)
0.10
1
10 ILOAD (mA)
100
1000
EFFICIENCY vs. ILOAD, CIRCUIT 5, VOUTPUT = 3.3V
MAX1776 toc10
EFFICIENCY vs. ILOAD, CIRCUIT 1, VOUTPUT = 3.3V
MAX1776 toc11
EFFICIENCY vs. VIN, ILOAD = 500mA
95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 CIRCUIT 1, 3.3V CIRCUIT 5, 3.3V CIRCUIT 5, 5V
MAX1776 toc12
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 0.10 1 10 ILOAD (mA) 100 VIN = 15V VIN = 24V VIN = 6V VIN = 12V
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 VIN = 24V VIN = 12V VIN = 6V
100 CIRCUIT 1, 5V
1000
0.10
1
10 ILOAD (mA)
100
1000
7
8
9
10
11
12
13
14
15
16
VIN (V)
SWITCHING FREQUENCY vs. LOAD CURRENT, CIRCUIT 1
MAX1776 toc13
SWITCHING FREQUENCY vs. VIN, CIRCUIT 1
MAX1776 toc14
VOUTPUT ACCURACY vs. TEMPERATURE
MAX1776 toc15
200 180 160 FREQUENCY (kHz) 140 120 100 80 60 40 20 0 0 VIN = 12V VIN = 24V VIN = 15V
140 120 FREQUENCY (kHz) 100 80 60 40 20 0 ILOAD = 250mA ILOAD = 5mA ILOAD = 50mA ILOAD = 10mA ILOAD = 500mA ILOAD = 375mA
1.5 1.0 VOUT ACCURACY (%) 0.5 0 -0.5 -1.0 -1.5
100 200 300 400 500 600 700 800 900 ILOAD (mA)
5
10
15 VIN (V)
20
25
-40
-20
0
20
40
60
80
100
TEMPERATURE (C)
_______________________________________________________________________________________
5
24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter MAX1776
Typical Operating Characteristics (continued)
(Circuit of Figure 1, components from Table 3, VIN = +12V, SHDN = IN, TA = +25C.)
QUIESCENT SUPPLY CURRENT vs. TEMPERATURE
MAX1776 toc16
QUIESCENT SUPPLY CURRENT vs. SUPPLY VOLTAGE
14.15 14.10 14.05 14.00 13.95 13.90 13.85 13.80 13.75
MAX1776 toc17
18.0 QUIESCENT SUPPLY CURRENT (A) 17.5 17.0 16.5 16.0 15.5 15.0 -40 -20 0 20 40 60 80 TEMPERATURE (C)
14.20 QUIESCENT SUPPLY CURRENT (A)
13.70 5 7 9 11 13 15 17 19 21 23 25 SUPPLY VOLTAGE (V)
PEAK SWITCH CURRENT vs. INPUT VOLTAGE, CIRCUIT 3, 0.3A
0.7 PEAK SWITCH CURRENT (A) 0.6 0.5 0.4 0.3 L = 100H 0.2 0.1 0 0 5 10 VIN (V) 15 20 25 ILOAD L = 47H VOUT L = 10H
MAX1776 toc18
0.8
LOAD-TRANSIENT RESPONSE, CIRCUIT 5
MAX1776 toc19
1A IL 0 10V 0 AC COUPLED 50mV/div 500mA 10mA 10s/div
L = 22H VLX
LINE-TRANSIENT RESPONSE, CIRCUIT 5, ILOAD = 500mA
LINE-TRANSIENT RESPONSE, CIRCUIT 5, ILOAD = 50mA
AC-COUPLED 200mv/div VOUT
MAX1776 toc20
MAX1776 toc21
AC-COUPLED 200mv/div
VOUT
VIN 10V VIN 5V
15V 10V
10V VLX 200s/div 5V 0 200s/div VLX 5V 0
6
_______________________________________________________________________________________
24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter MAX1776
Typical Operating Characteristics (continued)
(Circuit of Figure 1, components from Table 3, VIN = +12V, SHDN = IN, TA = +25C.)
LX WAVEFORM, CIRCUIT 1 VIN = 15V, ILOAD = 500mA
MAX1776 toc22
STARTUP WAVEFORM, CIRCUIT 1, RLOAD = 100
1A VSHDN
MAX1776 toc23
IL 0 VLX 10V 0 IL
5V 0 1A 0 6V 4V
VOUT
50mV/div
VOUT
2V 0
2s/div
2s/div
EFFICIENCY vs. ILOAD, CIRCUIT 3, VIN = 12V
MAX1776 toc24
EFFICIENCY vs. ILOAD, CIRCUIT 3, VIN = 12V
MAX1776 toc25
100
100
95 EFFICIENCY (%)
L = 22H EFFICIENCY (%)
95 L = 22H, 0.6A 90 L = 47H, 0.3A
90
85
L = 47H
L = 100H
85 L = 10H, 1.2A 80
80
75 0.10 1 10 ILOAD (mA) 100 1000
75 0.10 1 10 ILOAD (mA) 100 1000
_______________________________________________________________________________________
7
24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter MAX1776
Pin Description
PIN 1 2 3 4 5 6 7 8 NAME FB GND ILIM LX IN ILIM2 SHDN OUT FUNCTION 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. Ground 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. Inductor Connection. Connect LX to external inductor and diode as shown in Figure 1. Input Supply Voltage. Input voltage range is 4.5V to 24V. Peak Current Control Input 2. Connect to IN or GND. ILIM and ILIM2 together set the peak current limit. See Setting Current Limit. Shutdown Input. A logic low shuts down the MAX1776 and reduces the supply current to 3A. LX is high impedance in shutdown. Connect to IN for normal operation. 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 15A, and in shutdown mode, it draws 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 costs.
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 10s 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 0.42s minimum off-time expires, and the output voltage drops out of regulation.
INPUT 4.5V TO 24V IN CIN J1 J2 ILIM J3 ILIM2 J4 GND MAX1776 OUT SHDN LX D1
L1
OUTPUT 5V COUT
FB
CIN: 10F, 25V CERAMIC
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 CIN LX D1 L1 OUTPUT COUT
OUT SHDN Q R FB S
ILIM ILIM SET ILIM2
MAXIMUM ON-TIME DELAY
100mV
VSET
1.25V
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 VLX 10V 0
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)
VOUT
50mV/div
2s/div
Figure 3. Discontinuous-Conduction Operation
_______________________________________________________________________________________
9
24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter MAX1776
S Shutdown (SHDN)
A logic low level on SHDN shuts down the MAX1776 converter. When in shutdown, 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 on SHDN activates the MAX1776. Do not leave SHDN floating. If unused, connect SHDN to IN.
Table 1. Current-Limit Configuration
CURRENT LIMIT (mA) 150 300 600 1200 ILIM CONNECTED TO GND GND IN IN ILIM2 CONNECTED TO GND IN GND IN
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation in the MAX1776. 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. 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 10k and 100k. Calculate R1 with the following equation: V R1 = R2 x OUTPUT - 1 VFB where V FB = 1.25V, and V OUTPUT may range from 1.25V to VIN.
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 10H and 100H 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) =
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.
(VIN(MAX) - VOUTPUT ) x tON(MIN)
ILX (PEAK )
INPUT 4.5V TO 24V IN CIN SHDN ILIM ILIM2 MAX1776 FB LX D1
L1
OUTPUT 1.25V TO VIN COUT
where tON(MIN) = 1s. 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
R1
R2 GND OUT
Figure 4. Adjustable Output Voltage 10 ______________________________________________________________________________________
24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter
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.
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 4 VIN - 1 3x V OUTPUT
MAX1776
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) 10s / L
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 +10C temperature rise at the RMS input current for optimal circuit longevity.
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 x IPEAK VRIPPLE(C) = L x (IPEAK - IOUTPUT ) VIN 2COUT x VOUTPUT VIN - VOUTPUT
2
Table 2. Component Suppliers
SUPPLIER DIODES Central Semiconductor Fairchild General Semiconductor International Rectifier Nihon On Semi Vishay-Siliconix Zetex CAPACITORS AVX Kemet Nichicon Sanyo Taiyo Yuden INDUCTORS Coilcraft Coiltronics Pulse Engineering Sumida USA Toko www.coilcraft.com www.cooperet.com www.pulseeng.com www.sumida.com www.tokoam.com www.avxcorp.com www.kemet.com www.nichicon-us.com www.sanyo.com www.t-yuden.com www.centralsemi.com www.fairchildsemi.com www.gensemi.com www.irf.com www.niec.co.jp/engver2/ niec.co.jp_eg.htm www.onsemi.com www.vishay.com/brands/siliconix/ main.html www.zetex.com WEBSITE
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.
______________________________________________________________________________________
11
24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter MAX1776
Table 3. Recommended Components
CIRCUIT INPUT VOLTAGE (V) MAXIMUM LOAD CURRENT (mA) ILX(PEAK) CURRENT (A) INDUCTOR CAPACITOR
1
10 to 24
600
1.20
2
10 to 24
300
0.60
10H, 1.56A, 70m Toko D75F 646FY-100M, 10H, 1.70A, 48m Sumida CDRH6D28-100NC, or 10H, 1.63A, 55m Toko D75C 646CY-100M 0.055 22H, 1.17A, 120m Toko D75F 646FY-220M, 22H, 1.09A, 115m Toko D75C 646CY-220M, or 22H, 1.20A, 95m Sumida CDRH6D28-220NC 47H, 0.54A, 440m Sumida CDRH5D18-470 100H, 0.29A, 766m Sumida CDRH4D28-101 5.4H, 1.6A, 56m Sumida CDRH5D18-5R4 10H, 1.04A, 80m Toko D73LC 817CY-100M 22H, 0.41A, 294m Sumida CDRH4D18-220 47H, 0.33A, 230m Coilcraft DS1608C-473
100F, 6.3V Sanyo POSCAP 6TPC100M
47F, 6.3V Sanyo POSCAP 6TPA47M
3 4 5 6 7 8
10 to 24 10 to 24 5 to 15 5 to 15 5 to 15 5 to 15
150 75 600 300 150 75
0.30 0.15 1.20 0.60 0.30 0.15
22F, 6.3V, 1210 case Taiyo Youden JMK325BJ226MM 10F, 6.3V, X7R, 1206 case Taiyo Youden JMK316BJ106ML 100F, 6.3V Sanyo POSCAP 6TPC100m 47F, 6.3V Sanyo POSCAP 6TPA47M 22F, 6.3V, 1210 case Taiyo Youden JMK325BJ226MM 10F, 6.3V, X7R, 1206 case Taiyo Youden JMK316BJ106ML
Diode Selection
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.
MAX1776 Stability
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.
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
12
______________________________________________________________________________________
24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter
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 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.
MAX1776
Chip Information
TRANSISTOR COUNT: 932 PROCESS: BiCMOS
Package Information
4X S
8LUMAXD.EPS
8
8
INCHES DIM A A1 A2 b MIN 0.002 0.030 MAX 0.043 0.006 0.037
MILLIMETERS MAX MIN 0.05 0.75 1.10 0.15 0.95
y 0.500.1
E
H
0.60.1
c D e E H L
1
1
0.60.1
S
D
BOTTOM VIEW
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
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
A2
A1
A
c e b L
SIDE VIEW
FRONT VIEW
PROPRIETARY INFORMATION TITLE:
PACKAGE OUTLINE, 8L uMAX/uSOP
APPROVAL DOCUMENT CONTROL NO. REV.
21-0036
1 1
J
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) 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

▲Up To Search▲

Price & Availability of MAX1776

	To Download MAX1776 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .