MAX651 [MAXIM]
5V/3.3V/3V or Adjustable.High-Efficiency.Low-I＜sub＞Q＜/sub＞.Step-Down DC-DC Controllers ; 5V / 3.3V / 3V或Adjustable.High - Efficiency.Low -I \u003cSUB \u003e Q \u003c / SUB\u003e .STEP ，降压型DC -DC控制器\n
| 型号： | MAX651 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 5V/3.3V/3V or Adjustable.High-Efficiency.Low-I＜sub＞Q＜/sub＞.Step-Down DC-DC Controllers
|
| 文件： | 总16页 (文件大小：267K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-0225; Rev 3; 9/97
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Do w n DC-DC Co n t ro lle rs
Q
491/MAX652
_______________Ge n e ra l De s c rip t io n
____________________________Fe a t u re s
♦ More than 90% Efficiency (10mA to 1.5A Loads)
♦ More than 12.5W Output Power
The MAX649/MAX651/MAX652 BiCMOS, step-down DC-
DC switching controllers provide high efficiency over
three decades of load current. A unique, current-limited
pulse-frequency-modulated (PFM) control scheme gives
these devices the benefits of pulse-width-modulation
(PWM) converters (high efficiency at heavy loads), while
using only 100µA of supply current (vs. 2mA to 10mA for
PWM converters). The result is high efficiency over loads
ranging from 10mA to more than 2.5A.
♦ 100µA Max Quiescent Supply Current
♦ 5µA Max Shutdown Supply Current
♦ Less than 1.0V Dropout Voltage
♦ 16.5V Max Input Voltage
♦ 5V (MAX649), 3.3V (MAX651), 3V (MAX652),
or Adjustable Output Voltage
These devices use miniature external components.
Their high switching frequency (up to 300kHz) allows
for less than 9mm diameter surface-mount inductors.
♦ Current-Limited Control Scheme
♦ Up to 300kHz Switching Frequency
The MAX649/MAX651/MAX652 have dropout voltages
less than 1V and accept input voltages up to 16.5V.
Outp ut volta g e s a re p re s e t a t 5V (MAX649), 3.3V
(MAX651), and 3V (MAX652). These controllers can
also be adjusted to any voltage from 1.5V to the input
voltage by using two resistors.
______________Ord e rin g In fo rm a t io n
PART
TEMP. RANGE
0°C to +70°C
PIN-PACKAGE
8 Plastic DIP
8 SO
These step-down controllers drive external P-channel
MOSFETs at loads greater than 10W. If less power is
required, use the MAX639/MAX640/MAX653 step-down
c onve rte rs with on-c hip FETs , whic h a llow up to a
225mA load current.
MAX649CPA
MAX649CSA
MAX649C/D
MAX649EPA
MAX649ESA
MAX649MJA
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
________________________Ap p lic a t io n s
5V-to-3.3V Green PC Applications
8 CERDIP**
Ordering Information continued at end of data sheet.
* Dice are tested at T = +25°C.
**Contact factory for availability and processing to MIL-STD-883.
High-Efficiency Step-Down Regulation
Minimum-Component DC-DC Converters
Battery-Powered Applications
A
__________Typ ic a l Op e ra t in g Circ u it
__________________P in Co n fig u ra t io n
INPUT
4V TO 16.5V
TOP VIEW
V+
OUT
FB
1
2
3
4
8
7
6
5
GND
EXT
CS
MAX651
SHDN
CS
ON/OFF
MAX649
MAX651
MAX652
SHDN
REF
EXT
P
OUTPUT
3.3V
V+
DIP/SO
OUT
REF
FB GND
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Do w n DC-DC Co n t ro lle rs
Q
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, V+ to GND.......................................-0.3V, +17V
REF, SHDN, FB, CS, EXT, OUT.......................-0.3V, (V+ + 0.3V)
Operating Temperature Ranges
MAX649C_A, MAX65_C_A ..................................0°C to +70°C
MAX649E_A, MAX65_E_A ................................-40°C to +85°C
MAX649MJA, MAX65_MJA ............................-55°C to +125°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
Continuous Power Dissipation (T = +70°C)
A
Plastic DIP (derate 9.09mW/°C above +70°C) .............727mW
SO (derate 5.88mW/°C above +70°C)..........................471mW
CERDIP (derate 8.00mW/°C above +70°C)..................640mW
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
(V+ = 5V, T = T
to T , unless otherwise noted. Typical values are at T = +25°C.)
MAX A
A
MIN
PARAMETER
SYMBOL
V+
CONDITIONS
MIN
TYP
MAX
16.5
100
UNITS
V+ Input Voltage Range
4.0
V
V+ = 16.5V, SHDN ≤ 0.4V (operating, switch off)
V+ = 16.5V, SHDN ≥ 1.6V (shutdown)
V+ = 10V, SHDN ≥ 1.6V (shutdown)
MAX649C, MAX65_C
80
4
Supply Current
I
Q
µA
V
2
5
1.470
1.4625
1.455
1.5
1.5
1.5
1.530
1.5375
1.545
±50
FB Trip Point
MAX649E, MAX65_E
MAX649M, MAX65_M
MAX649C, MAX65_C
491/MAX652
FB Input Current
Output Voltage
Reference Voltage
I
MAX649E, MAX65_E
±70
nA
V
FB
MAX649M, MAX65_M
±90
MAX649, V+ = 6V to 16.5V
4.80
3.17
5.0
3.3
3.0
1.5
1.5
1.5
4
5.20
3.43
3.12
1.530
1.5375
1.545
10
Circuit of
Figure 1
V
OUT
MAX651, V+ = 4V to 16.5V
MAX652, V+ = 4V to 16.5V
2.88
MAX649C, MAX65_C, I
= 0
= 0
1.470
1.4625
1.455
REF
REF
V
REF
MAX649E, MAX65_E, I
V
MAX649M, MAX65_M, I
= 0
REF
MAX649C/E, MAX65_C/E
MAX649M, MAX65_M
0 ≤ I
≤ 100µA,
REF
REF Load Regulation
REF Line Regulation
mV
sourcing only
4
15
4V ≤ V+ ≤ 16.5V
40
100
µV/V
MAX649, 6V ≤ V+ ≤ 16V,
2.6
1.7
1.9
I
= 1A
LOAD
MAX651, 4.5V ≤ V+ ≤ 16V,
= 1A
Output Voltage
Line Regulation
Circuit of
Figure 1
mV/V
I
LOAD
MAX652, 4V ≤ V+ ≤ 16V,
= 1A
I
LOAD
2
_______________________________________________________________________________________
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Do w n DC-DC Co n t ro lle rs
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491/MAX652
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, T = T
to T , unless otherwise noted. Typical values are at T = +25°C.)
MAX A
A
MIN
PARAMETER
SYMBOL
CONDITIONS
MAX649, 0 ≤ I
MIN
TYP
MAX
UNITS
≤ 1.5A,
≤ 1.5A,
≤ 1.5A,
LOAD
LOAD
LOAD
-47
V
IN
= 10V
MAX651, 0 ≤ I
Output Voltage
Load Regulation
Circuit of
Figure 1
-45
-45
92
mV/A
V
IN
= 5V
MAX652, 0 ≤ I
= 5V
V
IN
MAX649, V+ = 10V,
= 1A
I
LOAD
MAX651, V+ = 5V,
= 1A
Circuit of
Figure 1
89
Efficiency
%
I
LOAD
MAX652, V+ = 5V,
= 1A
88
I
LOAD
SHDN Input Current
V+ = 16.5V, SHDN = 0V or V+
4V ≤ V+ ≤ 16.5V
1
µA
V
SHDN Input Voltage High
SHDN Input Voltage Low
V
1.6
IH
V
IL
4V ≤ V+ ≤ 16.5V
0.4
240
260
±1
V
MAX649C/E, MAX65_C/E
MAX649M, MAX65_M
180
160
210
210
Current-Limit Trip
Level (V+ to CS)
V
CS
4V ≤ V+ ≤ 16.5V
4V ≤ V+ ≤ 16.5V
V+ = 12V
mV
µA
µs
CS Input Current
Switch Maximum
On-Time
t
ON
(max)
12
16
20
Switch Minimum
Off-Time
t
OFF
(min)
V+ = 12V
1.8
2.3
2.8
µs
EXT Rise Time
EXT Fall Time
C
C
= 0.001µF, V+ = 12V
= 0.001µF, V+ = 12V
50
50
ns
ns
EXT
EXT
_______________________________________________________________________________________
3
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
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Q
__________________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s
(T = +25°C, unless otherwise noted.)
A
SHUTDOWN CURRENT
vs. TEMPERATURE
MAX649 MAXIMUM LOAD CURRENT
vs. SUPPLY VOLTAGE
SUPPLY CURRENT vs. TEMPERATURE
80
4.0
2500
3.5
3.0
2.5
2.0
1.5
1.0
0.5
78
76
74
72
70
68
V+ = 16.5V
V+ = 10V
2000
1500
1000
500
0
V+ = 16.5V
V+ = 8V
V
= 5V
OUT
CIRCUIT OF FIGURE 1
V+ = 4V
V+ = 4V
-60 -40 -20
66
0
-60 -40 -20
0
20 40 60 80 100 120 140
0
20 40 60 80 100 120 140
0
1 2 3 4 5 6 7 8 9 10 11 12 13 1415
TEMPERATURE (°C)
TEMPERATURE (°C)
INPUT VOLTAGE (V)
MAX649
MAX652
MAX651
EFFICIENCY vs. LOAD CURRENT
EFFICIENCY vs. LOAD CURRENT
EFFICIENCY vs. LOAD CURRENT
100
100
100
90
80
90
80
90
80
491/MAX652
TOP TO BOTTOM:
TOP TO BOTTOM:
TOP TO BOTTOM:
70
60
70
60
70
60
V
IN
= 4.3V
V
IN
= 6V
V
V
IN
= 4.3V
= 5V
IN
V
IN
= 5V
V
IN
= 8V
V
IN
= 8V
V
= 10V
= 12V
= 15V
V
IN
= 8V
IN
50
40
50
40
50
40
V
= 10V
= 12V
= 15V
IN
V
V
= 10V
= 12V
= 15V
IN
IN
V
IN
V
IN
V
IN
V
IN
V
IN
30
20
30
20
30
20
10
0
V
= 5V
10
0
OUT
10
0
V = 3V
OUT
V
= 3.3V
1m
OUT
100µ
1m
10m
100m
1
100µ
1m
10m
100m
1
100µ
10m
100m
1
LOAD CURRENT (A)
LOAD CURRENT (A)
LOAD CURRENT (A)
SWITCH ON-TIME
vs. TEMPERATURE
SWITCH ON-TIME/OFF-TIME RATIO
vs. TEMPERATURE
SWITCH OFF-TIME
vs. TEMPERATURE
17
8.0
2.5
2.0
1.5
V+ = 5V
V+ = 5V
V+ = 5V
7.8
7.6
7.4
7.2
7.0
6.8
6.6
6.4
6.2
6.0
16
15
-60 -40 -20
0
20 40 60 80 100 120
-60 -40 -20
0
20 40 60 80 100 120 140
-60 -40 -20
0
20 40 60 80 100 120
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
4
_______________________________________________________________________________________
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Do w n DC-DC Co n t ro lle rs
Q
491/MAX652
____________________________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 )
(T = +25°C, unless otherwise noted.)
A
DROPOUT VOLTAGE
vs. LOAD CURRENT
EXT RISE AND FALL TIMES
vs. TEMPERATURE (1nF)
EXT RISE AND FALL TIMES
vs. TEMPERATURE (5nF)
130
500
450
1000
900
800
700
600
MAX649, V = 5V
OUT
C
EXT
= 5nF
C
EXT
= 1nF
V+ = 5V, t
120
RISE
MAX652, V = 3V
OUT
110
100
90
V+ = 5V, t
RISE
400
350
300
250
MAX651, V = 3.3V
OUT
V+ = 12V, t
RISE
80
V+ = 5V, t
FALL
500
400
300
200
100
0
70
V+ = 5V, t
FALL
60
50
40
30
20
200
150
100
50
V+ = 12V, t
RISE
V+ = 12V, t
FALL
V+ = 12V, t
FALL
-60 -40 -20
0
20 40 60 80 100 120 140
-60 -40 -20
0
20 40 60 80 100 120 140
0
0.2 0.4 0.6 0.8 1.0
LOAD CURRENT (A)
1.4 1.6
1.2
TEMPERATURE (°C)
TEMPERATURE (°C)
REFERENCE OUTPUT VOLTAGE
vs. TEMPERATURE
DROPOUT VOLTAGE
vs. TEMPERATURE
CS TRIP LEVEL
vs. TEMPERATURE
1.506
1.504
1.502
1.500
1.498
1.496
1.494
1100
235
230
225
220
215
210
205
200
195
190
185
MAX649
1000
900
800
700
600
MAX652
MAX651
I
= 1A
LOAD
CIRCUIT OF FIGURE 1
1.492
-60 -40 -20
0
20 40 60 80 100 120 140
-60 -40 -20
0
20 40 60 80 100 120 140
-60 -40 -20
0
20 40 60 80 100 120 140
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
REFERENCE OUTPUT RESISTANCE
vs. TEMPERATURE
250
I
= 10µA
200
150
100
50
REF
I
REF
= 50µA
I
= 100µA
REF
0
-60 -40 -20
0
20 40 60 80 100 120 140
TEMPERATURE (°C)
_______________________________________________________________________________________
5
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Do w n DC-DC Co n t ro lle rs
Q
_____________________________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 )
MAX649
MAX649
LOAD-TRANSIENT RESPONSE
LINE-TRANSIENT RESPONSE
A
A
B
B
250µs/div
250µs/div
A: LOAD CURRENT (100mA & 1A), 500mA/div
B: 5V OUTPUT VOLTAGE, AC COUPLED, 50mV/div
I
= 1A
LOAD
A: INPUT VOLTAGE (7V & 12V), 5V/div
B: 5V OUT, AC COUPLED, 100mV/div
MAX649
SHUTDOWN RESPONSE TIME
A
491/MAX652
B
1ms/div
I
= 1A
LOAD
A: SHDN INPUT VOLTAGE (0V & 5V), 2V/div
B: 5V OUTPUT VOLTAGE, 2V/div
______________________________________________________________P in De s c rip t io n
PIN
NAME
FUNCTION
Sense input for fixed 5V, 3.3V, or 3V output operation. OUT is internally connected to the on-chip voltage divider.
Although it is connected to the output of the circuit, the OUT pin does not supply current.
OUT
1
Feedback input. Connect to GND for fixed-output operation. Connect a resistor divider between OUT, FB,
and GND for adjustable-output operation. See Setting the Output Voltage section.
2
3
FB
Active-high TTL/CMOS logic-level input. Part is placed in shutdown when SHDN is driven high. In shutdown mode,
the reference and the external MOSFET are turned off, and OUT = 0V. Connect to GND for normal operation.
SHDN
4
5
REF
V+
1.5V reference output that can source 100µA. Bypass with 0.1µF.
Positive power-supply input
Current-sense input. Connect current-sense resistor between V+ and CS. When the voltage across the
resistor equals the current-limit trip level, the external MOSFET is turned off.
6
CS
7
8
EXT
Gate drive for external P-channel MOSFET. EXT swings between V+ and GND.
Ground
GND
6
_______________________________________________________________________________________
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Do w n DC-DC Co n t ro lle rs
Q
491/MAX652
is out of regulation. However, unlike traditional PFM
c onve rte rs , s witc hing is a c c omp lis he d throug h the
combination of a peak current limit and a pair of one-
shots that set the maximum switch on-time (16µs) and
minimum switch off-time (2.3µs). Once off, the minimum
off-time one-shot holds the switch off for 2.3µs. After
this minimum time, the switch either 1) stays off if the
output is in regulation, or 2) turns on again if the output
is out of regulation.
V
IN
C4
C1
0.1µF 100µF
5
V+
R1
0.1Ω
MAX649
MAX651
MAX652
6
7
CS
P1
Si9430*
OUTPUT
@ 1.5A
3
4
The MAX649/MAX651/MAX652 also limit the peak induc-
tor current, which allows them to run in continuous-con-
duction mode and maintain high efficiency with heavy loads
(Figure 3a). This current-limiting feature is a key compo-
nent of the control circuitry. Once turned on, the switch
stays on until either 1) the maximum on-time one-shot turns
it off (16µs later), or 2) the current limit is reached.
SHDN
EXT
L1
22µH**
1
REF
OUT
FB
GND
2
8
C3
0.1µF
D1
C2
330µF
NSQ03A02L
To increase light-load efficiency, the current limit for
the first two pulses is set to half the peak current limit.
If those pulses bring the output voltage into regulation,
the voltage comparator holds the MOSFET off and the
current limit remains at half its peak. If the output vol-
ta g e is s till out of re g ula tion a fte r two p uls e s , the
current limit for the next pulse is raised to its peak (Figure
3b). Calculate the peak current limit by dividing the
Current-Limit Trip Level (see Electrical Characteristics)
by the value of the current-sense resistor.
*SILICONIX SURFACE-MOUNT MOSFET
**SUMIDA CDR125-220
Figure 1. Test Circuit
_______________De t a ile d De s c rip t io n
The MAX649/MAX651/MAX652 a re BiCMOS, s te p -
down, switch-mode power-supply controllers that pro-
vide fixed outputs of 5V, 3.3V, and 3V, respectively.
Their unique control scheme combines the advantages
of pulse-frequency-modulation (low supply current)
and pulse-width-modulation (high efficiency at high
loads). An external P-channel power MOSFET allows
peak currents in excess of 3A, increasing the output
current capability over previous PFM devices. Figure 2
is the block diagram.
S h u t d o w n Mo d e
When SHDN is high, the MAX649/MAX651/MAX652 enter
shutdown mode. In this mode, the internal biasing circuit-
ry is turned off (including the reference) and the supply
current drops to less than 5µA. EXT goes high, turning off
the external MOSFET. SHDN is a TTL/CMOS logic-level
input. Connect SHDN to GND for normal operation.
Qu ie s c e n t Cu rre n t
In normal operation, the quiescent current is less than
100µA. However, this current is measured by forcing
the external transistor switch off. In an actual applica-
tion, even with no load, additional current is drawn to
supply external feedback resistors (if used) and the
diode and capacitor leakage currents. In the circuit of
The MAX649/MAX651/MAX652 offe r thre e ma in
improvements over prior solutions:
1) The converters operate with tiny (less than 9mm
d ia me te r) s urfa c e -mount ind uc tors , d ue to the ir
300kHz switching frequency.
Figure 1, with V+ at 5V and V
quiescent current is 90µA.
at 3.3V, the typical
OUT
2) The c urre nt-limite d PFM c ontrol s c he me a llows
greater than 90% efficiencies over a wide range of
load currents (1.0mA to 1.5A).
EXT Drive Vo lt a g e Ra n g e
EXT swings from V+ to GND and provides the drive out-
put for an external P-channel power MOSFET.
3) The maximum supply current is only 100µA.
P FM Co n t ro l S c h e m e
The MAX649/MAX651/MAX652 use a proprietary, cur-
rent-limited PFM control scheme. As with traditional
PFM converters, the external power MOSFET is turned
on when the voltage comparator senses that the output
Mo d e s o f Op e ra t io n
Whe n d e live ring hig h outp ut c urre nts , the MAX649/
MAX651/MAX652 op e ra te in c ontinuous-c ond uc tion
mode (CCM). In this mode, current always flows in the
_______________________________________________________________________________________
7
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Do w n DC-DC Co n t ro lle rs
Q
V+
FB
DUAL-MODE™
COMPARATOR
MAX649
MAX651
MAX652
SHDN
REF
ERROR
COMPARATOR
OUT
1.5V
REFERENCE
N
MINIMUM
Q
OFF-TIME TRIG
ONE-SHOT
FROM V+
EXT
491/MAX652
S
Q
MAXIMUM
TRIG ON-TIME
ONE-SHOT
Q
R
CURRENT
COMPARATOR
CS
CURRENT
CONTROL CIRCUITS
0.2V
(FULL CURRENT)
0.1V
(HALF CURRENT)
FROM V+
GND
Figure 2. Block Diagram
8
_______________________________________________________________________________________
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Do w n DC-DC Co n t ro lle rs
Q
491/MAX652
2.5A
2.0A
1.5A
1A
1.5A
1.0A
0.5A
0A
0A
2µs/div
CIRCUIT OF FIGURE 1, R1 = 150mΩ
5µs/div
CIRCUIT OF FIGURE 1, R1 = 100mΩ
V+ = 10V, I
= 1.3A
V+ = 10V, I
= 1.4A
LOAD
LOAD
Figure 3a. MAX649 Continuous-Conduction Mode, Heavy
Load-Current Waveform (500mA/div)
Figure 3b. MAX649 Light/Medium Load-Current Waveform
(500mA/div)
inductor, and the control circuit adjusts the switch duty
c yc le to ma inta in re g ula tion without e xc e e d ing the
s witc h c urre nt c a p a b ility (Fig ure 3a ). This p rovid e s
excellent load-transient response and high efficiency.
V
IN
C4
C1
0.1µF 100µF
5
V+
In d is c ontinuous -c ond uc tion mod e (DCM), c urre nt
through the inductor starts at zero, rises to a peak
value, then ramps down to zero. Although efficiency is
still excellent, the output ripple increases slightly, and
the switch waveforms exhibit ringing (the self-resonant
frequency of the inductor). This ringing is to be expect-
ed and poses no operational problems.
R1
0.1Ω
MAX649
MAX651
MAX652
6
7
CS
P1
Si9430
L1
22µH
3
4
OUTPUT
@ 1.5A
SHDN
EXT
1
2
REF
OUT
FB
Dro p o u t
The MAX649/MAX651/MAX652 a re s a id to b e in
d rop out whe n the inp ut volta g e (V+) is low e noug h
tha t the outp ut d rop s b e low the minimum outp ut
voltage specification (see Electrical Characteristics ).
The d rop out volta g e is the d iffe re nc e b e twe e n the
inp ut a nd outp ut volta g e whe n d rop out oc c urs .
Se e the Typ ic a l Op e ra ting Cha ra c te ris tic s for the
Dropout Voltage vs. Load Current and Dropout Voltage
vs. Temperature graphs.
R2
GND
8
C2
C3
0.1µF
330µF
D1
1N5820
R3
150k
V
OUT
R2 = R3
– 1
(
)
V
REF
V
= 1.5V
REF
Figure 4. Adjustable-Output Operation
_______________________________________________________________________________________
9
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
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Q
3.0
2.5
3.0
2.5
2.0
1.5
1.0
R = 0.06Ω
S
R = 0.06Ω
S
R = 0.07Ω
S
R = 0.07Ω
S
2.0
1.5
R = 0.08Ω
S
R = 0.08Ω
S
R = 0.10Ω
S
R = 0.10Ω
S
R = 0.12Ω
S
R = 0.12Ω
S
R = 0.14Ω
S
1.0
R = 0.14Ω
S
0.5
0
0.5
0
MAX649
MAX651
V
OUT
= 5V
V
OUT
= 3.3V
4
5
6
7
8
9
10 11 12 13 14 15 16
3
3 4 5 7 8 9
6 10 11 12 13 14 15 16
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Figure 5a. MAX649 Current-Sense Resistor Graph
Figure 5b. MAX651 Current-Sense Resistor Graph
__________________De s ig n P ro c e d u re
S e t t in g t h e Ou t p u t Vo lt a g e
The MAX649/MAX651/MAX652 are preset for 5V, 3.3V,
and 3V output voltages, respectively. Tie FB to GND
for fixed-output operation. They may also be adjusted
from 1.5V (the reference voltage) to the input voltage,
us ing e xte rna l re s is tors R2 a nd R3 c onfig ure d a s
shown in Figure 4. For adjustable-output operation,
150kΩ is recommended for resistor R3. 150kΩ is a
good value—high enough to avoid wasting energy, yet
low enough to avoid RC delays caused by parasitic
capacitance at FB. R2 is given by:
3.0
491/MAX652
R = 0.06Ω
S
2.5
2.0
1.5
1.0
R = 0.07Ω
S
R = 0.08Ω
S
R = 0.10Ω
S
R = 0.12Ω
S
R = 0.14Ω
S
0.5
0
MAX652
V
OUT
V
OUT
= 3.0V
——— -1
R2 = R3 x
[
]
V
REF
3
4
5
6
7
8
9
10 11 12 13 14 15 16
INPUT VOLTAGE (V)
where V
= 1.5V.
REF
When using external resistors, it does no harm to con-
nect OUT and the output together, or to leave OUT
unconnected.
Figure 5c. MAX652 Current-Sense Resistor Graph
To choose the proper current-sense resistor for a par-
ticular output voltage, determine the minimum input
voltage and the maximum load current. Next, referring
to Figures 5a, 5b, or 5c, using the minimum input volt-
age, find the curve with the largest sense resistor that
provides sufficient output current. It is not necessary
to perform worst-case calculations. These curves take
into account the worst-case values for sense resistor
(±5%), inductor (22µH ±10%), diode drop (0.6V), and
the IC’s current-sense trip level; an external MOSFET
Cu rre n t -S e n s e Re s is t o r S e le c t io n
The current-sense resistor limits the peak switch cur-
rent to 210mV/R
, where R
is the value of
SENSE
SENSE
the current-sense resistor, and 210mV is the current-
limit trip level (see Electrical Characteristics).
To maximize efficiency and reduce the size and cost
of external components, minimize the peak current.
However, since the available output current is a func-
tion of the p e a k c urre nt, the p e a k c urre nt mus t not
be too low.
on-resistance of 0.13Ω is assumed for V = -4.5V.
GS
10 ______________________________________________________________________________________
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Q
491/MAX652
Standard wire-wound and metal-film resistors have an
ind uc ta nc e hig h e noug h to d e g ra d e p e rforma nc e .
Surface-mount (chip) resistors have very little induc-
tance and are well suited for use as current-sense
resistors. A wire resistor made by IRC works well in
through-hole applications. Because this resistor is a
band of metal shaped as a “U”, its inductance is less
than 10nH (an order of magnitude less than metal film
resistors). Resistance values between 5mΩ and 0.1Ω
are available (see Table 1).
tor’s saturation-current rating is greater than I (max).
LIM
However, it is generally acceptable to bias the inductor
into s a tura tion b y a b out 20% (the p oint whe re the
inductance is 20% below its nominal value).
The peak current of Figure 1 is 2.35A for a 1.5A output.
The inductor used in this circuit is specified to drop by
10% at 2.2A (worst case); a curve provided by the
manufacturer shows that the inductance typically drops
by 20% at 3.1A. Using a slightly underrated inductor
can sometimes reduce size and cost, with only a minor
impact on efficiency. The MAX649/MAX651/MAX652
current limit prevents any damage from an underrated
inductor’s low inductance at high currents.
In d u c t o r S e le c t io n
Practical inductor values range from 10µH to 50µH or more.
The circuit operates in discontinuous-conduction mode if:
Table 1 lists inductor types and suppliers for various
applications. The efficiencies of the listed surface-
mount inductors are nearly equivalent to those of the
larger size through-hole versions.
V
x (R + 1)
V
D
OUT
V + ≤ ———————— + —— + V
SW
R
R
R, the switch on-time/off-time ratio, equals 6.7. V is the
D
diode’s drop, and V
is the voltage drop across the
Dio d e S e le c t io n
The MAX649/MAX651/MAX652’s high switching fre-
quency demands a high-speed rectifier (commonly
called a catch diode when used in switching-regulator
circuits). Schottky diodes, such as the 1N5817 through
1N5822 families (and their surface-mount equivalents),
are recommended. Choose a diode with an average
SW
P-c ha nne l FET. To g e t the full outp ut c a p a b ility in
discontinuous-conduction mode, choose an inductor
value no larger than:
R
x 12µs x (V+ - V - V
)
SENSE
SW
OUT
L(max) = —————————————————
V
CS
current rating equal to or greater than I (max) and a
LIM
where V is the current-sense voltage.
CS
voltage rating higher than V+(max). For high-tempera-
ture a p p lic a tions , whe re Sc hottky d iod e s c a n b e
ina d e q ua te b e c a use of high le a ka g e c urre nts, use
high-speed silicon diodes instead. At heavy loads and
high temperatures, the disadvantages of a Schottky
diode’s high leakage current may outweigh the benefits
of its low forward voltage. Table 1 lists diode types and
suppliers for various applications.
In both the continuous and discontinuous modes, the
lowe r limit of the ind uc tor is more imp orta nt. With a
small inductor value, the current rises faster and over-
shoots the desired peak current limit because the cur-
rent-limit comparator cannot respond fast enough. This
reduces efficiency slightly and, more importantly, could
cause the current rating of the external components
to be exceeded. Calculate the minimum inductor value
as follows:
Ex t e rn a l S w it c h in g Tra n s is t o r
The MAX649/MAX651/MAX652 d rive P-c ha nne l
e nha nc e me nt-mod e MOSFET tra ns is tors only. The
choice of power transistor is primarily dictated by the
input voltage and the peak current. The transistor's
on-re s is ta nc e , g a te -s ourc e thre s hold , a nd g a te
capacitance must also be appropriately chosen. The
drain-to-source and gate-to-source breakdown voltage
ratings must be greater than V+. The total gate-charge
specification is normally not critical, but values should
be less than 100nC for best efficiency. The MOSFET
should be capable of handling the peak current and,
for maximum efficiency, have a very low on-resistance
at that current. Also, the on-resistance must be low for
(V+(max) - V - V
L(min) = ————————————––——
) x 0.3µs
OUT
SW
∆I x I (min)
LIM
whe re ∆I is the pe rc e nta ge of induc tor-c urre nt ove r-
shoot, where I
= V /R
and 0.3µs is the time
LIM
CS SENSE
it takes the comparator to switch. An overshoot of 10%
is usually not a problem. Inductance values above the
minimum work well if the maximum value defined above
is not e xc e e d e d . Sma lle r ind uc ta nc e va lue s c a us e
higher output ripple because of overshoot. Larger val-
ues tend to produce physically larger coils.
For highest efficiency, use a coil with low DC resis-
tance; a value smaller than 0.1V/I
works best. To
the minimum available V , which equals V+(min).
LIM
GS
minimize ra d ia te d nois e , us e a toroid , p ot c ore , or
shielded-bobbin inductor. Inductors with a ferrite core
or equivalent are recommended. Make sure the induc-
Select a transistor with an on-resistance between 50%
and 100% of the current-sense resistor. The Si9430
transistor chosen for the Typical Operating Circuit has
______________________________________________________________________________________ 11
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Do w n DC-DC Co n t ro lle rs
Q
a drain-to-source rating of -20V and a typical on-resis-
tance of 0.115Ω at 2A with VGS = -4.5V. Tables 1 and 2
list suppliers of switching transistors suitable for use
with these devices.
amount of noise at the voltage source caused by the
s witc hing a c tion of the MAX649/MAX651/MAX652.
The input voltage source impedance determines the
s ize of the c a p a c itor re q uire d a t the V+ inp ut. As
with the outp ut filte r c a p a c itor, a low-ESR c a p a c itor
is re c omme nd e d . Byp a s s the IC s e p a ra te ly with a
0.1µF ceramic capacitor placed close to the V+ and
GND p ins .
Ca p a c it o r S e le c t io n
Output Filter Capacitor
The p rima ry c rite rion for s e le c ting the outp ut filte r
capacitor is low equivalent series resistance (ESR),
rather than high capacitance. An electrolytic capacitor
with low e noug h ESR will a utoma tic a lly ha ve hig h
enough capacitance. The product of the inductor-cur-
rent variation and the ESR of the output filter capacitor
determines the amplitude of the high-frequency ripple
s e e n on the outp ut volta g e . Whe n a 330µF, 10V
Sprague surface-mount capacitor (595D series) with
ESR = 0.15Ωis used, 40mV of output ripple is typically
observed when stepping down from 10V to 5V at 1A.
Reference Capacitor
Bypass REF with a 0.1µF or larger capacitor. REF can
source at least 100µA.
La yo u t Co n s id e ra t io n s
Proper PC board layout is essential because of high
current levels and fast switching waveforms that radiate
noise. Minimize ground noise by connecting the anode
of the catch diode, the input bypass capacitor ground
lead, and the output filter capacitor ground lead to a
single point (“star” ground configuration). A ground
plane is recommended. Also minimize lead lengths to
reduce stray capacitance, trace resistance, and radiat-
ed noise. In particular, the traces connected to FB (if an
e xte rna l re sistor d ivid e r is use d ) a nd EXT must be
short. Place the 0.1µF ceramic bypass capacitor as
close as possible to V+ and GND.
The output filter capacitor's ESR also affects efficiency.
Use low-ESR capacitors for best performance. The
smallest low-ESR SMT tantalum capacitors currently
available are from the Sprague 595D series. Sanyo OS-
CON organic semiconductor through-hole capacitors
and the Nichicon PL series also exhibit very low ESR.
Table 1 lists some suppliers of low-ESR capacitors.
491/MAX652
Input Bypass Capacitor
The inp ut b yp a s s c a p a c itor re d uc e s p e a k c urre nts
drawn from the voltage source, and also reduces the
Table 1. Component Selection Guide
PRODUCTION
METHOD
CURRENT-SENSE
RESISTORS
INDUCTORS
CAPACITORS
DIODES
MOSFETS
Siliconix
Sumida
Matsuo
Little Foot series
CDR125-220 (22µH) 267 series
Nihon
NSQ series
IRC
LRC series
Surface Mount
Motorola
medium-power
surface-mount products
Coiltronics
CTX 100 series
Sprague
595D series
Sanyo
Miniature
Through-Hole
Sumida
RCH855-220M
OS-CON series
low-ESR organic
semiconductor
IRC
OAR series
Motorola
Nichicon
PL series
low-ESR electrolytics
Motorola
1N5820,
1N5823
Low-Cost
Renco
Motorola
Through-Hole
RL 1284-22
TMOS power MOSFETs
United Chemi-Con
LXF series
12 ______________________________________________________________________________________
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
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Q
491/MAX652
Table 2. Component Suppliers
COMPANY
Coiltronics
PHONE
FAX
USA
USA
USA
USA
(407) 241-7876
(800) 442-7747
(310) 322-3331
(704) 264-8861
(407) 241-9339
(407) 724-3937
(310) 322-3332
(704) 264-8866
Harris
International Rectifier
IRC
Matsuo
USA
Japan
(714) 969-2491
81-6-337-6450
(714) 960-6492
81-6-337-6456
Motorola
Nichicon
USA
(800) 521-6274
(602) 244-4015
USA
Japan
(708) 843-7500
81-7-5231-8461
(708) 843-2798
81-7-5256-4158
Nihon
USA
Japan
(805) 867-2555
81-3-3494-7411
(805) 867-2556
81-3-3494-7414
Renco
Sanyo
USA
(516) 586-5566
(516) 586-5562
USA
Japan
(619) 661-6835
81-7-2070-6306
(619) 661-1055
81-7-2070-1174
Siliconix
Sprague
Sumida
USA
USA
(408) 988-8000
(603) 224-1961
(408) 970-3950
(603) 224-1430
USA
Japan
(708) 956-0666
81-3-3607-5111
(708) 956-0702
81-3-3607-5144
United Chemi-Con
USA
(714) 255-9500
(714) 255-9400
__Ord e rin g In fo rm a t io n (c o n t in u e d )
___________________Ch ip To p o g ra p h y
PART
TEMP. RANGE
0°C to +70°C
PIN-PACKAGE
8 Plastic DIP
8 SO
GND
OUT
MAX651CPA
MAX651CSA
MAX651C/D
MAX651EPA
MAX651ESA
MAX651MJA
MAX652CPA
MAX652CSA
MAX652C/D
MAX652EPA
MAX652ESA
MAX652MJA
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
0°C to +70°C
8 Plastic DIP
8 SO
EXT
8 CERDIP**
8 Plastic DIP
8 SO
FB
0.109"
0°C to +70°C
(2.769mm)
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
CS
SHDN
REF
8 CERDIP**
* Dice are tested at T = +25°C.
**Contact factory for availability and processing to MIL-STD-883.
A
V+
0. 080"
(2. 032mm)
TRANSISTOR COUNT: 442;
SUBSTRATE CONNECTED TO V+.
______________________________________________________________________________________ 13
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
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Q
________________________________________________________P a c k a g e In fo rm a t io n
491/MAX652
14 ______________________________________________________________________________________
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , S t e p -Do w n DC-DC Co n t ro lle rs
Q
491/MAX652
___________________________________________P a c k a g e In fo rm a t io n (c o n t in u e d )
______________________________________________________________________________________ 15
5 V/3 .3 V/3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
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Q
NOTES
491/MAX652
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.
16 ____________________Ma x im In t e g ra t e d P ro d u c t s , 1 2 0 S a n Ga b rie l Drive , S u n n yva le , CA 9 4 0 8 6 4 0 8 -7 3 7 -7 6 0 0
© 1997 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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