MAX748ACWE [MAXIM]
3.3V, Step-Down, Current-Mode PWM DC-DC Converters; 3.3V ，降压，电流模式PWM DC- DC转换器
| 型号： | MAX748ACWE |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 3.3V, Step-Down, Current-Mode PWM DC-DC Converters |
| 文件： | 总12页 (文件大小：136K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-0190; Rev 0; 9/93
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
48/MX763A
__________________Ge n e ra l De s c rip t io n
________________________________Fe a t u re s
The MAX748A/MAX763A are 3.3V-output CMOS, step-
d own s witc hing re g ula tors . The MAX748A a c c e p ts
inputs from 3.3V to 16V and delivers up to 500mA. The
MAX763A accepts inputs between 3.3V and 11V and
delivers up to 500mA. Typical efficiencies are 85% to
90%. Quiescent supply current is 1.4mA (MAX763A),
and only 0.2µA in shutdown.
♦ Up to 500mA Load Currents
♦ Guaranteed 159kHz to 219.5kHz Current-Mode
PWM
♦ 85% to 90% Efficiencies
♦ 1.7mA Quiescent Current (MAX748A)
1.4mA Quiescent Current (MAX763A)
Pulse-width-modulation (PWM) current-mode control pro-
vides precise output regulation and excellent transient
responses. Output voltage accuracy is guaranteed to be
± 5% ove r line , loa d , a nd te mp e ra ture va ria tions .
Fixed-frequency switching allows easy filtering of output
ripple and noise, as well as the use of small external
components. A 22µH inductor works in most applica-
tions, so no magnetics design is necessary.
♦ 0.2µA Shutdown Supply Current
♦ 22µH Preselected Inductor Value;
No Component Design Required
♦ Overcurrent, Soft-Start, and Undervoltage
Lockout Protection
♦ Cycle-by-Cycle Current Limiting
♦ 8-Pin DIP/SO Packages (MAX763A)
The MAX748A/MAX763A also feature cycle-by-cycle cur-
rent limiting, overcurrent limiting, undervoltage lockout,
and programmable soft-start protection. The MAX748A
is available in 8-pin DIP and 16-pin wide SO packages;
the MAX763A comes in 8-pin DIP and SO packages.
_________________Ord e rin g In fo rm a t io n
PART
TEMP. RANGE
0°C to +70°C
PIN-PACKAGE
8 Plastic DIP
16 Wide SO
Dice*
MAX748ACPA
MAX748ACWE
MAX748AC/D
MAX748AEPA
MAX748AEWE
MAX748AMJA
0°C to +70°C
___________________________Ap p lic a t io n s
5V-to-3.3V Converters
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
-55°C to +125°C
8 Plastic DIP
16 Wide SO
8 CERDIP
Cellular Phones
Portable Instruments
Hand-Held Computers
Ordering Information continued on last page.
Computer Peripherals
* Contact factory for dice specifications.
____________Typ ic a l Op e ra t in g Circ u it
____________________P in Co n fig u ra t io n s
INPUT
3.3V TO 16V
TOP VIEW
OUTPUT
3.3V
22µH
V+
LX
MAX748A
SHDN
REF
SS
V+
1
2
3
4
8
7
6
5
LX
ON/OFF
SHDN
OUT
CC
MAX748A
MAX763A
GND
OUT
100µF
CC
REF
SS
GND
DIP
Pin Configurations continued on last page.
_______________________________________________________________________ Maxim Integrated Products
1
Ca ll t o ll fre e 1 -8 0 0 -9 9 8 -8 8 0 0 fo r fre e s a m p le s o r lit e ra t u re .
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
ABSOLUTE MAXIMUM RATINGS
Pin Voltages:
16-Pin Wide SO (derate 9.52mW/°C above +70°C)....762mW
8-Pin CERDIP (derate 8.00mW/°C above +70°C).......640mW
Operating Temperature Ranges:
MAX7__AC__ ....................................................0°C to +70°C
MAX7__AE__ .................................................-40°C to +85°C
MAX7__AMJA__ ..........................................-55°C to +125°C
Junction Temperatures:
MAX7__AC/E ..............................................................+150°C
MAX7__AM .................................................................+175°C
Storage Temperature Range ............................-65°C to +160°C
Lead Temperature (soldering, 10sec) ............................+300°C
V+ (MAX748A)......................................................+17V, -0.3V
V+ (MAX763A)......................................................+12V, -0.3V
LX (MAX748A) .................................(V+ - 21V) to (V+ + 0.3V)
LX (MAX763A) .................................(V+ - 12V) to (V+ + 0.3V)
OUT.................................................................................±25V
SS, CC, SHDN ..........................................-0.3V to (V+ + 0.3V)
Peak Switch Current (ILX) .....................................................2.0A
Reference Current (IREF) ...................................................2.5mA
Continuous Power Dissipation (TA = +70°C)
8-Pin Plastic DIP (derate 6.90mW/°C above +70°C)...552mW
8-Pin SO (derate 5.88mW/°C above +70°C)...............471mW
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 3, V+ = 5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted.)
48/MX763A
MAX748A
TYP
MAX763A
TYP
PARAMETER
CONDITIONS
UNITS
MIN
MAX
MIN
MAX
Input Voltage Range
3.3
16.0
3.3
11.0
V
C/E temp. ranges,
V+ = 4.0V to 16V,
0mA < ILOAD < 300mA
3.135
3.135
3.135
3.135
3.3
3.3
3.3
3.3
3.465
3.465
3.465
3.465
M temp. range,
V+ = 4.0V to 16V,
0mA < ILOAD < 250mA
C/E temp. ranges,
V+ = 4.75V to 16V,
0mA < ILOAD < 500mA
M temp. range,
V+ = 4.75V to 16V,
0mA < ILOAD < 400mA
Output Voltage
V
C/E temp. ranges,
V+ = 4.0V to 11V,
0mA < ILOAD < 300mA
3.135
3.135
3.135
3.135
3.3
3.3
3.3
3.3
3.465
3.465
3.465
3.465
M temp. range,
V+ = 4.0V to 11V,
0mA < ILOAD < 250mA
C/E temp. ranges,
V+ = 4.75V to 11V,
0mA < ILOAD < 500mA
M temp. range,
V+ = 4.75V to 11V,
0mA < ILOAD < 400mA
Line Regulation
Load Regulation
0.13
0.13
%/V
ILOAD = 0mA to 500mA
0.001
0.001
%/mA
2
________________________________________________________________________________________________
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
48/MX763A
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 3, V+ = 5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted.)
MAX748A
TYP
MAX763A
TYP
PARAMETER
Efficiency
CONDITIONS
UNITS
MIN
MAX
MIN
MAX
ILOAD = 300mA
ILOAD = 100mA
88
88
90
V+ = 5V
%
90
Supply Current
Includes switch current
1.7
0.2
3.0
1.4
0.2
2.5
mA
µA
Shutdown Current
SHDN = 0V (Note 1)
100.0
100.0
V
2.0
2.0
IH
Shutdown Input
Threshold
V
V
0.25
1.0
0.25
1.0
IL
Shutdown Input
Leakage Current
µA
Short-Circuit Current
Undervoltage Lockout
LX On Resistance
LX Leakage Current
Reference Voltage
Reference Drift
1.2
2.7
1.0
10
1.2
2.7
1.0
10
A
V
V+ falling
3.0
3.0
ILX = 500mA
V+ = 12V, LX = 0
TA = +25°C
Ω
nA
1.15
159
1.22
50
1.30
1.15
159
1.22
50
1.30
V
TA = TMIN to TMAX
ppm/°C
kHz
Oscillator Frequency
180
212.5
200
212.5
Compensation Pin
Impedance
7500
7500
Ω
Note 1: The standby current typically settles to 10µA (over temperature) within 2 seconds; however, to decrease test time, the part
is guaranteed at a 100µA maximum value.
________________________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s
(Circuit of Figure 3, T = +25°C, VOUT = 3.3V, unless otherwise noted.)
A
MAXIMUM OUTPUT CURRENT
vs. SUPPLY VOLTAGE
MAX763A
EFFICIENCY vs. OUTPUT CURRENT
MAX748A
EFFICIENCY vs. OUTPUT CURRENT
100
95
1000
800
100
95
90
85
80
75
V
= 4.5V
IN
MAX748A
MAX763A
V
= 7.0V
V
= 5.0V
IN
IN
V
= 6V
IN
90
85
600
400
200
0
80
75
70
V
= 11.0V
V
IN
= 16V
IN
V
= 9V
IN
70
65
60
NOTE 2
NOTE 2
V
= 12V
NOTE 2
IN
65
60
0
100 200 300 400 500 600 700 800
OUTPUT CURRENT (mA)
0
2
4
6
8
10 12 14 16
0
100 200 300 400 500 600 700 800 900
OUTPUT CURRENT (mA)
SUPPLY VOLTAGE (V)
_________________________________________________________________________________________________
3
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(Circuit of Figure 3, T = +25°C, VOUT = 3.3V, unless otherwise noted.)
A
MAX763A
QUIESCENT SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX748A
QUIESCENT SUPPLY CURRENT
vs. SUPPLY VOLTAGE
PEAK INDUCTOR CURRENT
vs. OUTPUT CURRENT
1000
800
5.0
4.0
4.5
4.0
I
= OmA
OUT
I
= 0mA
OUT
3.5
3.0
2.5
2.0
1.5
1.0
V
= 11V
T
= -55°C
IN
A
T = +125°C
A
3.0
2.0
1.0
0
600
400
T
= +125°C
A
T = -55°C
A
T
= +25°C
A
T = +25°C
A
V
= 6V
IN
200
0
V
= 8V
IN
0.5
0
0
2
4
6
8
10 12 14 16
0
50 100 150 200 250 300 350 400 450 500 550
OUTPUT CURRENT (mA)
0
1
2
3
4
5 6 7 8 9 10 11 12
48/MX763A
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
SHUTDOWN CURRENT
vs. TEMPERATURE
OUTPUT VOLTAGE
vs. SUPPLY VOLTAGE
OSCILLATOR FREQUENCY
vs. SUPPLY VOLTAGE
4.0
3.5
3.5
275
250
I
= 50mA
LOAD
NOTES 3, 4
3.0
I
= 200mA
LOAD
3.0
2.5
2.5
2.0
225
200
175
MAX748A
2.0
I
= 500mA
1.5
1.0
0.5
LOAD
MAX763A
1.5
1.0
0.5
I
= 300mA
LOAD
150
125
I
= 0mA
OUT
V+ = 5V
0
100
0
0
2
4
6
8
10 12 14 16
2.0
2.5
3.0
3.5
4.0
4.5
5.0
-60 -40 -20
0
20 40 60 80 100 120 140
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
OSCILLATOR FREQUENCY
vs. TEMPERATURE
OUTPUT VOLTAGE
vs. OUTPUT CURRENT
4.0
300
250
200
150
100
V
= 4.0V
I
= 100mA
IN
OUT
3.5
3.0
V+ = 5V
MAX748A
MAX763A
2.5
V
= 3.5V
IN
2.0
1.5
V
= 3.0V
IN
1.0
0.5
50
0
0
-60 -40 -20
0
20 40 60 80 100 120 140
0
50 100 150 200 250 300 350 400 450 500
OUTPUT CURRENT (mA)
TEMPERATURE (°C)
4
_______________________________________________________________________________________
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
48/MX763A
_________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(Circuit of Figure 3, T = +25°C, VOUT = 3.3V, unless otherwise noted.)
A
SWITCHING WAVEFORMS,
CONTINUOUS CONDUCTION
SWITCHING WAVEFORMS,
DISCONTINUOUS CONDUCTION
6V
6V
A
A
B
0V
0V
400mA
200mA
B
0mA
0mA
C
C
2µs/div
2µs/div
A: SWITCH VOLTAGE (LX PIN), 5V/div, 0V TO +6V
B: INDUCTOR CURRENT, 200mA/div
A: SWITCH VOLTAGE (LX PIN), 5V/div, 0V TO +6V
B: INDUCTOR CURRENT, 100mA/div
C: OUTPUT VOLTAGE RIPPLE, 50mV/div
C: OUTPUT VOLTAGE RIPPLE, 50mV/div
V+ = 6V, I
OUT
= 250mA
V+ = 6V, I = 75mA
OUT
LINE-TRANSIENT RESPONSE
LOAD-TRANSIENT RESPONSE
A
B
A
B
500mA
0mA
10V
7V
0V
5ms/div
5ms/div
A: V , 50mV/div
OUT
A: V , 50mV/div
OUT
B: V+, 5V/div, 7.0V TO 10.0V
B: I , 200mA/div, 0mA TO 500mA
OUT
I
= 350mA
V+ = 6V
OUT
Note 2: Operation beyond the specifications listed in the Electrical Characteristics may exceed the power dissipation ratings of
the device.
Note 3: Wide temperature range circuit of Figure 5 using Sprague surface-mount capacitors.
Note 4: Standby current includes all external component leakage currents. Capacitor leakage currents dominate at T = +85°C.
A
_______________________________________________________________________________________
5
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
_______________________________________________________________________P in De s c rip t io n
PIN #
16-PIN WIDE SO NAME
(MAX748A)
FUNCTION
8-PIN DIP/SO
Shutdown—active low. Connect to ground to power down chip; tie to V+ for normal
operation. Output voltage falls to 0V when SHDN is low.
1
2
3
7
8
9
SHDN
REF
SS
Reference Voltage Output (+1.23V) supplies up to 100µA for external loads.
Bypass to GND with a 0.047µF capacitor.
2
3
4
5
Soft-Start. Capacitor between SS and GND provides soft-start and
short-circuit protection.
Compensation Capacitor Input externally compensates the outer (voltage)
feedback loop. Connect to OUT with a 330pF capacitor.
CC
Output-Voltage Sense Input provides regulation feedback sensing.
Connect to +3.3V output.
OUT
6
7
10, 11
GND
LX
Ground*
48/MX763A
12, 13, 14
Drain of internal P-channel power MOSFET*
Supply Voltage Input. Bypass to GND with 1µF ceramic and large-value
electrolytic capacitor in parallel. The 1µF capacitor must be as close
to the GND and V+ pins as possible.*
8
1,15,16
4, 5, 6
V+
N.C.
No Connect—no internal connections to these pins.
*16-pin wide SO package: All pins sharing the same name must be connected together externally.
P ro g ra m m a b le S o ft -S t a rt
_________________De t a ile d De s c rip t io n
Figure 2 shows a capacitor connected to the soft-start
(SS) pin to ensure orderly power-up. A typical value is
0.047µF. SS controls both the SS timing and the maxi-
mum output current that can be delivered while main-
taining regulation.
The MAX748A/MAX763A switch-mode regulators use a
current-mode pulse-width-modulation (PWM) control
system in a step-down (buck) regulator topography.
They convert an unregulated DC input voltage from 4V
to 11V (MAX763A) or from 4V to 16V (MAX748A) to a
regulated 3.3V output at 300mA. For loads less than
300mA, V+ may be less than 4.0V (see the Output
Volta g e vs . Sup p ly Volta g e g ra p h in the Typ ic a l
Op e ra ting Cha ra c te ris tic s ). The c urre nt-mod e
PWM architecture provides cycle-by-cycle current limit-
ing, improved load-transient response, and simpler
outerloop design.
The charging capacitor slowly raises the clamp on the
error-amplifier output voltage, limiting surge currents
at power-up by slowly increasing the cycle-by-cycle
current-limit threshold. Table 1 lists timing characteris-
tics for selected capacitor values and circuit conditions.
The overcurrent comparator trips when the load exceeds
approximately 1.2A. When either an undervoltage or over-
current fault condition is detected, an SS cycle is actively
initiated, which triggers an internal transistor to discharge
the SS capacitor to ground. An SS cycle is also enabled
at power-up and when coming out of shutdown mode.
The controller consists of two feedback loops: an inner
(current) loop that monitors the switch current via the
current-sense resistor and amplifier, and an outer (volt-
age) loop that monitors the output voltage through the
e rror a mp lifie r (Fig ure 1). The inne r loop p e rforms
cycle-by-cycle current limiting, truncating the power
transistor on-time when the switch current reaches a
predetermined threshold. This threshold is determined
by the outer loop. For example, a sagging output volt-
age produces an error signal that raises the threshold,
allowing the circuit to store and transfer more energy
during each cycle.
Ove rc u rre n t Lim it in g
The overcurrent comparator triggers when the load
current exceeds approximately 1.2A. On each clock
cycle, the output FET turns on and attempts to deliver
current until cycle-by-cycle or overcurrent limits are
exceeded. Note that the SS capacitor must be greater
than 0.01µF for overcurrent protection to function prop-
erly. A typical value is 0.047µF.
6
_______________________________________________________________________________________
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
48/MX763A
MAX748A 3.3V to 16.0V
MAX763A 3.3V to 11.0V
C2
1.0µF
C3
150µF
V
IN
SHDN
V+
OVERCURRENT COMPARATOR
SLOPE COMPENSATION
RAMP
BIAS
GEN
GEN
Σ
OUT
CC
R
SENSE
CURRENT
SENSE AMP
ERROR AMP
C5
330pF
F/F
R
Q
L1
22µH
V
OUT
S
LX
3.3V
1.23V
BANDGAP
PWM
COMPARATOR
D1
1N5817
C4
150µF
REF
C6
200kHz
OSC
1000pF
MAX748A
MAX763A
1M
±35%
SS CLAMP
UNDERVOLTAGE
LOCKOUT
SS
C1
0.047µF
V
UVLO
GND
Figure 1. Detailed Block Diagram with External Components
Table 1. Typical Soft-Start Times
(Circuit of Figure 3, C4 = 150µF)
FROM SHDN
Circuit Cond.
Soft-Start Time (ms) vs. C1 (µF)
V+ (V) IOUT (mA) C1 = 0.01 C1 = 0.047 C1 = 0.1 C1 = 0.47
SS CLAMP
8
0
1
1
4
7
12
6
12*
8
0
2
3
SS
200
200
300
300
10
7
33
17
44
25
50
20
65
35
200
80
12*
8
1M
±35%
13
8
325
140
C1
12*
1.23V
* MAX748A only
MAX748A
MAX763A
Figure 2. Soft-Start Circuitry Block Diagram
_______________________________________________________________________________________
7
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
Table 3. External Component Suppliers
Production Method
Inductors
Capacitors
Sumida
Matsuo
CD105 series
267 series
Surface Mount
Coiltronics
CTX series
Sprague
595D/293D series
Coilcraft
DT series
High Performance/
Sumida
Sanyo
Miniature Through-Hole
RCH895 series
OS-CON series (very low ESR)
Renco
RL1284 series
Nichicon
PL series (low ESR)
Through-Hole
Phone and FAX Numbers:
Coilcraft
Coiltronics USA:
Matsuo
USA:
(708) 639-6400, FAX: (708) 639-1469
(305) 781-8900, FAX: (305) 782-4163
(714) 969-2491, FAX: (714) 960-6492
Renco
Sanyo
USA: (516) 586-5566, FAX: (516) 586-5562
USA: (0720) 70-1005, FAX: (0720) 70-1174
USA:
Sprague Elec. Co. USA: (603) 224-1961, FAX: (603) 224-1430
Sumida USA: (708) 956-0666, FAX: (708) 956-0702
48/MX763A
Japan: (06) 332-0871
Nichicon
USA:
(708) 843-7500, FAX: (708) 843-2798
Japan: (03) 3607-5111, FAX: (03) 3607-5428
Un d e rvo lt a g e Lo c k o u t
normal operation, connect SHDN to V+. Coming out of
The undervoltage lockout feature monitors the supply
voltage at V+ and allows operation to start when V+
rises above 2.95V. When V+ falls, operation continues
until the supply voltage falls below 2.7V (typ). When an
undervoltage condition is detected, control logic turns
off the output power FET and discharges the SS capac-
itor to ground. This prevents partial turn-on of the power
MOSFET and avoids excessive power dissipation. The
control logic holds the output power FET off until the
supply voltage rises above approximately 2.95V, at
which time an SS cycle begins. When the input voltage
exceeds the undervoltage lockout threshold, switching
action will occur, but the output will not be regulated
until the input voltage exceeds 3.3V (no load). The
exact input voltage required for regulation depends on
load conditions (see the Output Voltage vs. Supply
Voltage graph in the Typical Operating Characteristics).
shutdown mode initiates an SS cycle.
Co n t in u o u s -/Dis c o n t in u o u s -
Co n d u c t io n Mo d e s
The input voltage, output voltage, load current, and
inductor value determine whether the IC operates in
continuous or discontinuous mode. As the inductor
value or load current decreases, or the input voltage
increases, the MAX748A/MAX763A tend to operate in
discontinuous-conduction mode (DCM). In DCM, the
inductor current slope is steep enough so it decays to
zero before the end of the transistor off-time. In contin-
uous-conduction mode (CCM), the inductor current
never decays to zero, which is typically more efficient
than DCM. CCM allows the MAX748A/MAX763A to
deliver maximum load current, and is also slightly less
noisy than DCM, because it doesn’t exhibit the ringing
that occurs when the inductor current reaches zero.
S h u t d o w n Mo d e
The MAX748A/MAX763A are held in shutdown mode
by keeping SHDN at ground. In shutdown mode, the
output drops to 0V and the output power FET is held in
an off state. The internal reference also turns off, which
causes the SS capacitor to discharge. Typical supply
current in shutdown mode is 0.2µA. The actual design
limit for shutdown current is much less than the 100µA
specified in the Electrical Characteristics. However,
testing to tighter limits is prohibitive because the cur-
rent takes several seconds to settle to a final value. For
In t e rn a l Re fe re n c e
The +1.23V bandgap reference supplies up to 100µA
at REF. A 1000pF bypass capacitor from REF to GND
is required.
Os c illa t o r
The MAX748A/MAX763A’s internal oscillator is guaran-
teed to operate in the 159kHz to 212.5 kHz range over
temperature for V+ = 5V. Temperature stability over the
military temperature range is about 0.04%/°C.
8
_______________________________________________________________________________________
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
48/MX763A
INPUT
MAX748A 3.3V TO 16.0V
MAX763A 3.3V TO 11.0V
C2
1.0µF
V
IN
C3
150µF
8
V+
1
7
SHDN
LX
OPTIONAL 21kHz LOWPASS OUTPUT FILTER
D1
1N5817
L1
22µH
MAX748A
MAX763A
L2
25µH
FILTER
OUTPUT
5
4
OUTPUT
3.3V
OUT
CC
OUTPUT
C7
C5
330pF
3
2.2µF
SS
C4
150µF
C1
0.047µF
GND
REF
6
2
C6
1000pF
Figure 3. Standard 3.3V Step-Down Application Circuit Using Through-Hole Components (commercial temperture range)
input voltages of 4.75V to 16V with 800mA typical out-
put currents. The MAX763A delivers a guaranteed
300mA for input voltages of 4V to 11V, a guaranteed
Table 2. Component Table for
Wide Temperature Applications
500mA for input voltages of 4.75V to 11V, and has
700mA typ ic a l outp ut c urre nts . The MAX748A/
MAX763A operate from an input down to 3V (the upper
limit of undervoltage lockout), but with some reduction
in output voltage and maximum output current.
C1(µF) C2(µF) C3(µF) C4(µF) C5(pF) C6(pF) L1(µH)
Through-
Hole
0.047
0.047
1.0
1.0
150* 220* 330
68** 100*** 330
1000
1000
22
22
SO
* Sanyo OS-CON Series (very low ESR)
** 16V or greater maximum voltage rating.
*** 6.3V or greater maximum voltage rating.
In d u c t o r S e le c t io n
The MAX748A/MAX763A require no inductor design
because they are tested in-circuit, and are guaranteed
to d e live r the p owe r s p e c ifie d in the Ele c tric a l
Cha ra c te ris tic s with hig h e ffic ie nc y us ing a s ing le
22µH inductor. The 22µH inductor’s incremental satu-
ration current rating should be greater than 1A for
500mA load operation. Table 3 lists inductor types and
suppliers for various applications. The surface-mount
inductors have nearly equivalent efficiencies to the
larger through-hole inductors.
____________Ap p lic a t io n s In fo rm a t io n
Fix e d +3 .3 V S t e p -Do w n
Co n ve rt e r Ap p lic a t io n
Figure 3 shows the standard 3.3V step-down circuit with
components shown for commercial temperature range
applications. Figures 4, 5, and Table 2 suggest external
component values for both SO and through-hole wide
temperature range applications. These circuits are use-
ful in systems that require high current and high efficien-
cy and are powered by an unregulated supply, such as
a battery or wall-plug AC-DC adapter.
Ou t p u t Filt e r Ca p a c it o r S e le c t io n
The p rima ry c rite rion for s e le c ting the outp ut filte r
capacitor is low effective series resistance (ESR). The
product of the inductor-current variation and the output
capacitor’s ESR determines the amplitude of the saw-
tooth ripple seen on the output voltage. Minimize the
output filter capacitor’s ESR to maintain AC stability.
The MAX748A delivers a guaranteed 300mA for input
voltages of 4V to 16V, and a guaranteed 500mA for
_______________________________________________________________________________________
9
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
INPUT
MAX748A 3.3V TO 16.0V
MAX763A 3.3V TO 11.0V
INPUT
MAX748A 3.3V TO 16.0V
MAX763A 3.3V TO 11.0V
C2
1.0µF
C2
1.0µF
V
IN
V
IN
C3*
68µF
(16V)
C3*
150µF
(16V)
8
8
V+
1
7
V+
1
7
SHDN
LX
SHDN
LX
D1
1N5817
L1
22µH
D1
1N5817
L1
22µH
MAX748A
MAX763A
MAX748A
MAX763A
5
4
OUTPUT
3.3V
5
4
OUT
CC
OUTPUT
3.3V
OUT
CC
C5
330pF
C5
330pF
3
3
SS
C4*
100µF
(6.3V)
SS
C4*
220µF
(10V)
C1
C1
0.047µF
0.047µF
GND
REF
GND
REF
6
2
C6
6
2
48/MX763A
C6
1000pF
1000pF
*Sprague 293D or 595D Series-16V. See Table 3
for alternative suppliers.
*OS-CON Series (very low ESR)
Figure 5. Standard 3.3V Step-Down Application Circuit Using
Surface-Mount Components (Commercial and Extended
Industrial Temperature Ranges)
Figure 4. Standard 3.3V Step-Down Application Circuit Using
Through-Hole Components (all temperature ranges)
The capacitor’s ESR should be less than 0.25Ω to keep
the output ripple less than 50mV over the entire cur-
rent range (using a 22µH inductor). Capacitor ESR
usually rises at low temperatures, but OS-CON capaci-
tors provide very low ESR below 0°C. Table 3 lists
capacitor suppliers.
in Figures 6-9 have been tested successfully over a
wide range of operating conditions. The 1µF input
bypass capacitor must be positioned as close to
the V+ and GND pins as possible. Also, place the
output capacitor as close to the OUT and GND pins as
possible. The traces connecting ground to the input
and output filter capacitors and to the catch diode
must be short to reduce inductance. Use an uninter-
rupted ground plane if possible.
p-p
Ot h e r Co m p o n e n t s
The catch diode should be a Schottky or high-speed
silicon rectifier with a peak current rating of at least
1.0A for full-load (500mA) operation. The 1N5817 is a
g ood c hoic e . The 330p F oute r-loop c omp e ns a tion
capacitor provides the widest input voltage range and
best transient characteristics.
Ou t p u t -Rip p le Filt e rin g
A simple lowpass pi-filter (Figure 3) can be added to
p-p
the output to reduce output ripple to about 5mV
.
The cutoff frequency shown is 21kHz. Since the filter
inductor is in series with the circuit output, minimize the
filter inductor’s resistance so the voltage drop across it
is not excessive.
P rin t e d Circ u it La yo u t s
A good layout is essential for stable, low-noise opera-
tion. The layouts and component placement diagrams
10 ______________________________________________________________________________________
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
48/MX763A
Figure 6. DIP PC Layout, Through-Hole Component Placement
Figure 7. DIP PC Layout, Component Side (1X Scale)
Diagram (1X Scale)
Figure 8. DIP PC Layout, Solder Side (1X Scale)
Figure 9. DIP PC Layout, Drill Guide (1X Scale)
______________________________________________________________________________________ 11
3 .3 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
_____P in Co n fig u ra t io n s (c o n t in u e d )
____________________Ch ip To p o g ra p h ie s
0.131"
TOP VIEW
3.327mm
SHDN
REF
SS
V+
1
2
3
4
8
7
6
5
SS
REF
LX
MAX748A
MAX763A
GND
OUT
CC
SHDN
CC
0.116"
2.946mm
SO
V+
V+
V+
SHDN
REF
V+
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
V+
V+
V+
OUT
GND
LX
N.C.
LX
MAX748A
48/MX763A
N.C.
N.C.
SS
LX
GND
LX
LX
LX
GND
GND
OUT
MAX748A
CC
TRANSISTOR COUNT: 298
SUBSTRATE CONNECTION: V+
Wide SO
0.072"
1.829mm
V+ V+ V+
SHDN
_Ord e rin g In fo rm a t io n (c o n t in u e d )
PART
TEMP. RANGE
0°C to +70°C
PIN-PACKAGE
8 Plastic DIP
8 SO
REF
MAX763ACPA
MAX763ACSA
MAX763AC/D
MAX763AEPA
MAX763AESA
MAX763AMJA
LX
LX
0°C to +70°C
0°C to +70°C
Dice*
-40°C to +85°C
-40°C to +85°C
-55°C to +125°C
8 Plastic DIP
8 SO
LX
8 CERDIP
* Contact factory for dice specifications.
0.116"
2.946mm
SS
GND
CC
OUT
MAX763A
TRANSISTOR COUNT: 281
SUBSTRATE CONNECTION: V+
12 ______________________________________________________________________________________
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