MAX1651CSA [MAXIM]
5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers; 5V / 3.3V或可调，高效率，低压差，降压型DC -DC控制器
| 型号： | MAX1651CSA |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers |
| 文件： | 总12页 (文件大小：169K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-0305; Rev 2; 9/95
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
49/MAX651
_______________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 MAX1649/MAX1651 BiCMOS, step-down, DC-DC
switching controllers provide high efficiency over loads
ranging from 1mA to more than 2.5A. A unique, current-
limited pulse-frequency-modulated (PFM) control scheme
gives these devices the benefits of pulse-width-modula-
tion (PWM) converters (high efficiency at heavy loads),
while using only 100µA of supply current (vs. 2mA to
10mA for PWM converters). Dropout performance down
to 300mV is provided by a high switch duty cycle (96.5%)
and a low current-sense threshold (110mV).
♦ Less than 0.3V Dropout Voltage at 500mA
♦ 100µA Max Quiescent Supply Current
♦ 5µA Max Shutdown Supply Current
♦ 16V Max Input Voltage
♦ 5V (MAX1649), 3.3V (MAX1651), or Adjustable
Output Voltage
A high switching frequency (up to 300kHz) allows these
devices to use miniature external components.
♦ Current-Limited Control Scheme
♦ Up to 300kHz Switching Frequency
♦ Up to 96.5% Duty Cycle
The MAX1649/MAX1651 have dropout voltages less
than 0.3V at 500mA and accept input voltages up to
16V. Output voltages are preset at 5V (MAX1649), or
3.3V (MAX1651). They 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
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
8 Plastic DIP
8 SO
MAX1649CPA
MAX1649CSA
MAX1649C/D
MAX1649EPA
MAX1649ESA
MAX1651CPA
MAX1651CSA
MAX1651C/D
MAX1651EPA
MAX1651ESA
These step-down controllers drive external P-channel
MOSFETs at loads greater than 12.5W. 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.
Dice*
8 Plastic DIP
8 SO
________________________Ap p lic a t io n s
PDAs
8 Plastic DIP
8 SO
Dice*
High-Efficiency Step-Down Regulation
5V-to-3.3V Green PC Applications
Battery-Powered Applications
8 Plastic DIP
8 SO
* Dice are tested at T = +25°C.
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
3.6V TO 16V
TOP VIEW
V+
OUT
FB
1
2
3
4
8
7
6
5
GND
EXT
CS
MAX1651
SHDN
CS
ON/OFF
MAX1649
MAX1651
SHDN
REF
EXT
P
OUTPUT
3.3V
V+
OUT
REF
DIP/SO
FB GND
________________________________________________________________ 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 .
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, V+ to GND.......................................-0.3V, +17V
Operating Temperature Ranges
REF, SHDN, FB, CS, EXT, OUT.......................-0.3V, (V+ + 0.3V)
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
MAX1649C_A, MAX1651C_A ..............................0°C to +70°C
MAX1649E_A, MAX1651E_A............................-40°C to +85°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+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
(V+ = 5V, T = T
to T , unless otherwise noted. Typical values are at T = +25°C.)
MAX A
A
MIN
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
16
UNITS
V+ Input Voltage Range
V+
V
OUT
< V+
3.0
V
V+ = 16V, SHDN ≤ 0.4V (operating, switch off)
V+ = 16V, SHDN ≥ 1.6V (shutdown)
V+ = 10V, SHDN ≥ 1.6V (shutdown)
MAX1649C, MAX1651C
78
2
100
Supply Current
I+
µA
49/MAX651
1
5
1.470
1.5
1.5
1.530
1.5375
±50
FB Trip Point
V
nA
V
MAX1649E, MAX1651E
1.4625
MAX1649C, MAX1651C
FB Input Current
Output Voltage
Reference Voltage
I
FB
MAX1649E, MAX1651E
±70
MAX1649, V+ = 5.5V to 16V
MAX1651, V+ = 3.6V to 16V
4.80
3.17
5.0
3.3
1.5
1.5
4
5.20
3.43
1.530
1.5375
10
Circuit of
Figure 1
V
OUT
MAX1649C, MAX1651C, I
= 0µA
= 0µA
1.470
1.4625
REF
REF
V
REF
V
MAX1649E, MAX1651E, I
REF Load Regulation
REF Line Regulation
0µA ≤ I
≤ 100µA, sourcing only
mV
REF
3V ≤ V+ ≤ 16V
40
100
µV/V
MAX1649, 5.5V ≤ V+ ≤ 16V,
= 1A
2.6
1.7
-47
-45
90
I
LOAD
Output Voltage
Line Regulation
Circuit of
Figure 1
mV/V
mV/A
%
MAX1651, 3.6V ≤ V+ ≤ 16V,
= 1A
I
LOAD
MAX1649, 0A ≤ I
≤ 1.5A,
LOAD
V
IN
= 10V
Output Voltage
Load Regulation
Circuit of
Figure 1
MAX1651, 0A ≤ I
= 5V
≤ 1.5A,
LOAD
V
IN
MAX1649, V+ = 10V,
= 1A
I
LOAD
Circuit of
Figure 1
Efficiency
MAX1651, V+ = 5V,
= 1A
90
I
LOAD
SHDN Input Current
V+ = 16V, SHDN = 0V or V+
3V ≤ V+ ≤ 16V
1
µA
V
SHDN Input Voltage High
SHDN Input Voltage Low
V
1.6
IH
V
IL
3V ≤ V+ ≤ 16V
0.4
V
2
_______________________________________________________________________________________
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
49/MAX651
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
MIN
TYP
MAX
UNITS
Current-Limit Trip Level
(V+ to CS)
V
3V ≤ V+ ≤ 16V
3V ≤ V+ ≤ 16V
80
110
140
mV
CS
CS Input Current
±1
40
µA
µs
µs
ns
ns
Switch Maximum On-Time
Switch Minimum Off-Time
EXT Rise Time
t
(max) V+ = 12V
(min) V+ = 12V
24
32
1.1
25
25
ON
t
0.8
1.8
OFF
C
C
= 0.001µF, V+ = 12V
EXT
EXT
EXT Fall Time
= 0.001µF, V+ = 12V
t
ON
+ t
OFF
x 100%
Maximum Duty Cycle
95
96.5
%
t
ON
__________________________________________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
EXT RISE AND FALL TIMES
vs. TEMPERATURE (1nF)
SUPPLY CURRENT vs. TEMPERATURE
80
4.0
60
55
C
EXT
= 1nF
3.5
3.0
2.5
2.0
1.5
1.0
0.5
78
76
74
72
70
68
V+ = 16V
V+ = 10V
50
45
V+ = 5V, t
RISE
V+ = 16V
V+ = 8V
40
35
V+ = 5V, t
FALL
30
25
V+ = 15V, t
RISE
V+ = 4V
20
15
V+ = 15V, t
FALL
V+ = 4V
-60 -40 -20 0 20 40 60 80 100 120 140
66
0
-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)
EXT RISE AND FALL TIMES
vs. TEMPERATURE (5nF)
EFFICIENCY
vs. LOAD CURRENT (V = 5V)
OUT
EFFICIENCY
vs. LOAD CURRENT (V = 3.3V)
OUT
240
220
200
180
160
140
120
100
80
100
90
100
90
V
= 5V
OUT
V
= 3.3V
C
= 5nF
OUT
EXT
CIRCUIT OF
FIGURE 1
CIRCUIT OF
FIGURE 1
V+ = 5V, t
RISE
80
80
V+ = 5V, t
FALL
TOP TO
70
60
TOP TO
70
60
BOTTOM:
BOTTOM:
V
V
IN
= 4.3V
= 5V
IN
V+ = 15V, t
RISE
V
IN
= 6V
V
IN
= 8V
V
IN
= 8V
V
= 10V
= 12V
= 15V
IN
V
= 10V
= 12V
= 15V
IN
V
50
40
IN
50
40
V
IN
V
IN
60
V
IN
V+ = 15V, t
FALL
40
0.1
1
10
100
1k
10k
-60 -40 -20
0
20 40 60 80 100 120 140
0.1
1
10
100
1k
10k
LOAD CURRENT (mA)
LOAD CURRENT (mA)
TEMPERATURE (°C)
_______________________________________________________________________________________
3
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle 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 )
(T = +25°C, unless otherwise noted.)
A
SWITCH ON-TIME
vs. TEMPERATURE
SWITCH OFF-TIME
vs. TEMPERATURE
MAXIMUM DUTY CYCLE
vs. TEMPERATURE
34.0
1.5
1.4
100
99
33.5
33.0
32.5
32.0
31.5
31.0
30.5
30.0
1.3
1.2
98
1.1
1.0
97
96
0.9
0.8
0.7
0.6
95
94
93
0.5
-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)
49/MAX651
DROPOUT VOLTAGE
vs. LOAD CURRENT
CS TRIP LEVEL
vs. TEMPERATURE
600
500
120
115
CIRCUIT OF
FIGURE 1
400
300
V
OUT
= 4.80V
110
105
100
V
OUT
= 3.17V
200
100
0
95
0
0.5
1.0
1.5
2.0
-60 -40 -20
0
20 40 60 80 100 120 140
LOAD CURRENT (A)
TEMPERATURE (°C)
REFERENCE OUTPUT VOLTAGE
vs. TEMPERATURE
REFERENCE OUTPUT RESISTANCE
vs. TEMPERATURE
1.506
1.504
1.502
1.500
1.498
1.496
1.494
250
I
= 10µA
200
150
100
50
REF
I
REF
= 10µA
I
REF
= 50µA
I
= 100µA
REF
1.492
0
-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)
4
_______________________________________________________________________________________
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
49/MAX651
____________________________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
MAX1649
MAX1649
LOAD-TRANSIENT RESPONSE
LINE-TRANSIENT RESPONSE
A
B
A
B
16V
6V
1.6A
0A
200µs/div
5ms/div
CIRCUIT OF FIGURE 1, V+ = 10V
A: V = 5V, 100mV/div, AC-COUPLED
CIRCUIT OF FIGURE 1, I
A: V = 5V, 100mV/div, AC-COUPLED
OUT
= 1A
LOAD
OUT
B: I
LOAD
= 30mA TO 1.6A, 1A/div
B: V+ = 6V TO 16V, 5V/div
MAX1649
SHDN RESPONSE TIME
5V
OUTPUT
0V
4V
SHDN
INPUT
0V
1ms/div
CIRCUIT OF FIGURE 1, V+ = 10V, I
LOAD
= 1A
_______________________________________________________________________________________
5
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
______________________________________________________________P in De s c rip t io n
PIN NAME
FUNCTION
Sense Input for fixed 5V or 3.3V 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. Leave OUT unconnected
for adjustable-output operation.
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 Shutdown Input. Part is placed in shutdown when SHDN is driven high. In shutdown mode, the refer-
ence, output, and external MOSFET are turned off. 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
49/MAX651
The MAX1649/MAX1651 offer four main improvements
over prior solutions:
V
IN
C4
C1
1) The converters operate with miniature surface-mount
inductors, due to their 300kHz switching frequency.
0.1µF 100µF
5
V+
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 (10mA to 1.5A).
R1
0.05Ω
MAX1649
MAX1651
6
CS
3) Dropout volta ge ha s be e n re duc e d to le ss tha n
300mV for many applications.
P1
Si9430*
OUTPUT
@ 1.5A
3
4
7
1
SHDN
EXT
4) The quiescent supply current is only 100µA.
L1
47µH**
P FM Co n t ro l S c h e m e
The MAX1649/MAX1651 use a proprietary, current-limit-
ed PFM control scheme. As with traditional PFM con-
verters, the external power MOSFET is turned on when
the voltage comparator senses that the output is out of
regulation. However, unlike traditional PFM converters,
switching is accomplished through the combination of a
peak current limit and a pair of one-shots that set the
maximum switch on-time (32µs) and minimum switch
off-time (1.1µs). Once off, the off-time one-shot holds
the switch off for 1.1µs. 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.
REF
OUT
FB
GND
2
8
C3
0.1µF
D1
NSQ03A02L
C2
330µF
*SILICONIX SURFACE-MOUNT MOSFET
**SUMIDA CDRH125-470
Figure 1. Test Circuit
_______________De t a ile d De s c rip t io n
The MAX1649/MAX1651 a re BiCMOS, s te p -d own,
s witc h-mod e p owe r-s up p ly c ontrolle rs tha t p rovid e
adjustable and fixed outputs of 5V and 3.3V, respec-
tive ly. The ir uniq ue c ontrol s c he me c omb ine s 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.
The MAX1649/MAX1651 also limit the peak inductor cur-
rent, which allows them to run in continuous-conduction
mode and maintain high efficiency with heavy loads
(Figure 3). 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 (32µs later), or 2) the current limit is reached.
EXT swings from V+ to GND and provides the drive out-
put for an external P-channel power MOSFET.
6
_______________________________________________________________________________________
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
49/MAX651
V+
FB
DUAL-MODE™
COMPARATOR
MAX1649
MAX1651
50mV
OUT
SHDN
REF
ERROR
COMPARATOR
1.5V
REFERENCE
N
MINIMUM
Q
OFF-TIME TRIG
ONE-SHOT
FROM V+
EXT
S
Q
F/F
MAXIMUM
TRIG ON-TIME
ONE-SHOT
Q
R
CURRENT
COMPARATOR
CS
110mV
FROM V+
GND
™ Dual-Mode is a trademark of Maxim Integrated Products
Figure 2. Block Diagram
S h u t d o w n Mo d e
When SHDN is high, the MAX1649/MAX1651 enter shut-
down mode. In this mode, the internal biasing circuitry is
turned off (including the reference) and the supply cur-
rent drops to less than 5µA. EXT goes high, turning off the
external MOSFET. SHDN is a 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 device's typical quiescent cur-
rent is 78µA. In an actual application, 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 Figure 1, with V+ at 5V and
V
OUT
at 3.3V, typical no-load supply current for the
entire circuit is 90µA.
_______________________________________________________________________________________
7
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
V
IN
C4
C1
0.1µF 100µF
5
V+
R1
0.05Ω
MAX1649
MAX1651
6
CS
1.5A
1A
P1
Si9430
L1
47µH
3
4
OUTPUT
@ 1.5A
7
SHDN
EXT
1
2
0A
REF
OUT
FB
R2
GND
8
C2
330µF
C3
0.1µF
2µs/div
CIRCUIT OF FIGURE 1, R1 = 75mΩ
D1
1N5820
V+ = 10V, I
= 1.3A
R3
150k
LOAD
V
OUT
49/MAX651
R2 = R3
– 1
(
)
V
REF
V
REF
= 1.5V
Figure 3. MAX1649 Continuous-Conduction Mode, Heavy
Load-Current Waveform (500mA/div)
Figure 4. Adjustable-Output Operation
Mo d e s o f Op e ra t io n
When delivering high output currents, the MAX1649/
MAX1651 operate in continuous-conduction mode. In
this mode, current always flows in the inductor, and
the control circuit adjusts the switch duty cycle to main-
tain regulation without exceeding the switch current
capability (Figure 3). This provides excellent load-tran-
sient response and high efficiency.
__________________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 MAX1649/MAX1651 are preset for 5V and 3.3V out-
put 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, using external
resistors R2 and R3 configured as shown in Figure 4. For
adjustable-output operation, 150kΩ is recommended for
resistor R3—high enough to avoid wasting energy, yet
low enough to avoid RC delays caused by parasitic
capacitance at FB. R2 is given by:
In discontinuous-conduction mode, current through the
ind uc tor s ta rts a t ze ro, ris e s to a p e a k va lue , the n
ramps down to zero. Although efficiency is still excel-
lent, the output ripple increases slightly, and the switch
waveform exhibits ringing (at the inductor's self-reso-
nant frequency). This ringing is to be expected and
poses no operational problems.
V
OUT
——— -1
R2 = R3 x
(
)
V
REF
where V
= 1.5V.
REF
Dro p o u t
The MAX1649/MAX1651 are in dropout when the input
voltage (V+) is low enough that the output drops below
the minimum outp ut volta g e s p e c ific a tion (s e e
Electrical Characteristics). The dropout voltage is the
difference between the input and output voltage when
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 Drop out Volta g e vs . Loa d
Current and Dropout Voltage vs. Temperature graphs.
When using external resistors, it does no harm to con-
nect OUT and the output together, or to leave OUT
unconnected.
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 110mV/R
, where R
is the value of
SENSE
SENSE
the current-sense resistor, and 110mV is the current-
limit trip level (see Electrical Characteristics).
8
_______________________________________________________________________________________
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
49/MAX651
To maximize efficiency and reduce the size and cost
of e xte rna l c omp one nts, minimize the pe a k c urre nt.
However, since the available output current is a func -
tion of the peak current, the peak current must not be
too low.
trollers’ high switching frequency. With a high inductor
value, the MAX1649/MAX1651 will begin continuous-cur-
rent operation (see Detailed Description) at a lower frac-
tion of full-load current. In general, smaller values pro-
duce higher ripple (see below) while larger values require
larger size for a given current rating.
To choose the proper current-sense resistor for a par-
ticular output voltage, determine the minimum input
voltage and the maximum load current. Next, refer-
ring to Figures 5a or 5b, 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 a c c ount the s e ns e -re s is tor (± 5%) a nd ind uc tor
(47µH ± 10%) va lue s , the d iod e d rop (0.4), a nd the
IC’s current-sense trip level (85mV); an external MOS-
In both the continuous and discontinuous modes, the
lower limit of the inductor is important. With a too-small
inductor value, the current rises faster and overshoots the
desired peak current limit because the current-limit com-
parator has a finite response time (300ns). This reduces
efficiency and, more importantly, could cause the current
rating of the external components to be exceeded.
Calculate the minimum inductor value as follows:
(V+(max) - V
) x 0.3µs
OUT
FET on-resistance of 0.07Ω is assumed for V = -5V.
GS
L(min) = ——————————––——
∆I x I
LIM
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 inductance
and are well suited for use as current-sense resistors.
A U-shaped 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).
whe re ∆I is the ind uc tor-c urre nt ove rs hoot fa c tor,
= V /R , and 0.3µs is the time it takes the com-
I
LIM
CS SENSE
parator to switch. Set ∆I = 0.1 for an overshoot of 10%.
For highest efficiency, use a coil with low DC resis-
tance; a value smaller than 0.1V/I
works best. To
LIM
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-
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).
In d u c t o r S e le c t io n
The MAX1649/MAX1651 operate with a wide range of
inductor values, although for most applications coils
between 10µH and 68µH take best advantage of the con-
3.0
3.0
V
OUT
= 5V
V
OUT
= 3.3V
r
r
r
= 0.030
= 0.040
= 0.050
r
r
r
= 0.030
= 0.040
= 0.050
s
s
s
s
s
s
2.5
2.0
1.5
1.0
0.5
0
2.5
2.0
1.5
1.0
0.5
0
r
r
= 0.060
= 0.080
r
r
= 0.060
= 0.080
s
s
s
s
r
s
= 0.100
r
s
= 0.100
5.0
5.4
5.8
6.2
6.6
16.0
3.0
3.4
3.8
4.2
4.6
16.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Figure 5a. MAX1649 Current-Sense Resistor Graph
Figure 5b. MAX1651 Current-Sense Resistor Graph
_______________________________________________________________________________________
9
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
Table 1. Component Selection Guide
PRODUCTION
METHOD
CURRENT-SENSE
RESISTORS
INDUCTORS
CAPACITORS
DIODES
MOSFETS
Sumida
CDRH125-470 (1.8A) AVX
CDRH125-220 (2.2A) TPS series
Siliconix
Little Foot series
Motorola
MBRS340T3
Dale
WSL Series
Surface Mount
Motorola
medium-power
surface-mount products
CoilCraft
DO3316-473 (1.6A)
DO3340-473 (3.8A)
Sprague
595D series
Nihon
NSQ series
IRC
LRC series
Sanyo
Miniature
Through-Hole
Sumida
OS-CON series
IRC
OAR series
Motorola
RCH875-470M (1.3A) low-ESR organic
semiconductor
Nichicon
PL series
low-ESR electrolytics
Motorola
1N5817 to
1N5823
Low-Cost
Through-Hole
CoilCraft
PCH-45-473 (3.4A)
Motorola
TMOS power MOSFETs
49/MAX651
United Chemi-Con
LXF series
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 2.7A. Using a slightly underrated inductor
can sometimes reduce size and cost, with only a minor
impact on efficiency.
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 the minimum available V
,
GS
which equals V+(min). Select a transistor with an on-
re s is ta nc e b e twe e n 50% a nd 100% of the c urre nt-
sense resistor. The Si9430 transistor chosen for the
Typical Operating Circuit has a drain-to-source rating
of -20V and a typical on-resistance of 0.070Ω at 2A with
VGS = -4.5V. Tables 1 and 2 list suppliers of switching
transistors suitable for use with these devices.
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.
Dio d e S e le c t io n
The MAX1649/MAX1651’s high switching frequency
demands a high-speed rectifier. Schottky diodes, such
as the 1N5817 through 1N5823 (and their surface-
mount e q uiva le nts ), a re re c omme nd e d . Choos e a
diode with an average current rating equal to or greater
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-
re nt va ria tion a nd the outp ut filte r c a p a c itor’s ESR
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 = 0.15Ω is used, 40mV of output ripple is typically
observed when stepping down from 10V to 5V at 1A.
The output filter capacitor's ESR also affects efficiency.
Again, low-ESR capacitors perform best. Table 1 lists
some suppliers of low-ESR capacitors.
tha n I
(ma x) a nd a volta g e ra ting hig he r tha n
LIM
V+(max).
Ex t e rn a l S w it c h in g Tra n s is t o r
The MAX1649/MAX1651 drive P-channel enhancement-
mode MOSFET transistors only. The choice of power
transistor is primarily dictated by the input voltage and
the peak current. The transistor’s on-resistance, gate-
source threshold, and gate charge must also be appro-
p ria te ly c hos e n. The d ra in-to-s ourc e a nd g a te -to-
source breakdown voltage ratings must be greater than
V+. The total gate-charge specification is normally not
10 ______________________________________________________________________________________
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
49/MAX651
La yo u t Co n s id e ra t io n s
Table 2. Component Suppliers
Proper PC board layout is essential because of high
current levels and fast switching waveforms that radi-
ate noise. Minimize ground noise by connecting the
a nod e of the re c tifie r, the inp ut b yp a s s c a p a c itor
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 radiated noise. In particular, the traces connected
to FB (if an external resistor divider is used) and EXT
must be short. Place the 0.1µF ceramic bypass capac-
itor as close as possible to the V+ and GND pins.
COMPANY
PHONE
FAX
(207) 282-5111
or
(800) 282-4975
AVX
USA
(207) 283-1941
Coiltronics
CoilCraft
Dale
USA
USA
USA
(407) 241-7876
(708) 639-6400
(402) 564-3131
(407) 241-9339
(708) 639-1469
(402) 563-1841
International
Rectifier
USA
USA
(310) 322-3331
(310) 322-3332
(512) 992-3377
IRC
(512) 992-7900
(602) 244-3576
or
(602) 244-5303
Motorola
USA
(602) 244-4015
MAX1 6 4 9 /MAX1 6 5 1 vs . MAX6 4 9 /MAX6 5 1
The MAX1649 and MAX1651 are pin compatible with
the MAX649 and MAX651, but have been optimized for
improved dropout performance and efficiency—partic-
ularly with low input voltages. The MAX1649/MAX1651
feature increased maximum switch duty cycle (96.5%)
a nd re d uc e d c urre nt-limit s e ns e volta g e (110mV).
Their predecessors, the MAX649/MAX651, use a high-
er two-step (210mV/110mV) current-limit sense voltage
to provide tighter current-sense accuracy and reduced
inductor peak current at light loads.
USA
Japan
(708) 843-7500
81-7-5231-8461
(708) 843-2798
81-7-5256-4158
Nichicon
Nihon
USA
Japan
(805) 867-2555
81-3-3494-7411
(805) 867-2556
81-3-3494-7414
USA
Japan
(619) 661-6835
81-7-2070-6306
(619) 661-1055
81-7-2070-1174
Sanyo
(408) 988-8000
or
(800) 554-5565
Siliconix
USA
USA
(408) 970-3950
Sprague
Sumida
(603) 224-1961
(603) 224-1430
USA
Japan
(708) 956-0666
81-3-3607-5111
(708) 956-0702
81-3-3607-5144
___________________Ch ip To p o g ra p h y
United
Chemi-Con
USA
(714) 255-9500
(714) 255-9400
GND
OUT
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
amount of noise at the voltage source caused by the
switching action of the MAX1649/MAX1651. The input
voltage source impedance determines the size of the
capacitor required at the V+ input. As with the output fil-
ter capacitor, a low-ESR capacitor is recommended.
Bypass the IC separately with a 0.1µF ceramic capac-
itor placed close to the V+ and GND pins.
EXT
FB
0.106"
(2.692mm)
CS
SHDN
REF
Reference Capacitor
Bypass REF with a 0.1µF or larger capacitor.
V+
0. 081"
(2. 057mm)
TRANSISTOR COUNT: 428
SUBSTRATE CONNECTED TO V+
______________________________________________________________________________________ 11
5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
________________________________________________________P a c k a g e In fo rm a t io n
INCHES
MILLIMETERS
DIM
MIN
MAX
0.200
–
MIN
–
MAX
5.08
–
E
A
–
A2
A3
E1
A1 0.015
A2 0.125
A3 0.055
0.38
3.18
1.40
0.41
1.14
0.20
1.27
15.24
13.34
2.54
15.24
–
D
A
0.175
0.080
0.020
0.065
0.012
0.090
0.625
0.575
–
4.45
2.03
0.51
1.65
0.30
2.29
15.88
14.61
–
B
0.016
B1 0.045
0.008
D1 0.050
0.600
E1 0.525
0.100
eA 0.600
0°-15°
C
A1
e
C
L
E
eA
eB
B1
B
e
–
–
D1
eB
L
–
0.700
0.150
17.78
3.81
0.120
3.05
Plastic DIP
PLASTIC
49/MAX651
INCHES
MILLIMETERS
PKG. DIM PINS
MIN
MAX MIN MAX
DUAL-IN-LINE
PACKAGE
(0.600 in.)
P
P
P
D
D
D
24
28
40
1.230 1.270 31.24 32.26
1.430 1.470 36.32 37.34
2.025 2.075 51.44 52.71
21-0044A
INCHES
MILLIMETERS
DIM
MIN
0.053
MAX
0.069
0.010
0.019
0.010
0.157
MIN
1.35
0.10
0.35
0.19
3.80
MAX
1.75
0.25
0.49
0.25
4.00
A
D
A1 0.004
B
C
E
e
0.014
0.007
0.150
0°-8°
A
0.101mm
0.004in.
0.050
1.27
e
H
L
0.228
0.016
0.244
0.050
5.80
0.40
6.20
1.27
A1
C
B
L
INCHES
MILLIMETERS
DIM PINS
Narrow SO
SMALL-OUTLINE
PACKAGE
MIN MAX
MIN
MAX
5.00
8.75
8
0.189 0.197 4.80
D
D
D
E
H
14 0.337 0.344 8.55
16 0.386 0.394 9.80 10.00
21-0041A
(0.150 in.)
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.
12 __________________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
© 1995 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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