Maxim MAX651ESA 5v/3.3v/3v or adjustable, high-efficiency, Datasheet

19-0225; Rev 3; 9/97
IT
K
ATION
EVALU
BLE
AVAILA
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
________________________Applications
5V-to-3.3V Green PC Applications
High-Efficiency Step-Down Regulation
Minimum-Component DC-DC Converters
____________________________Features
♦
♦
♦
♦
♦
♦
♦
More than 90% Efficiency (10mA to 1.5A Loads)
More than 12.5W Output Power
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
♦ Current-Limited Control Scheme
♦ Up to 300kHz Switching Frequency
______________Ordering Information
PART
MAX649CPA
TEMP. RANGE
0°C to +70°C
MAX649CSA
MAX649C/D
MAX649EPA
MAX649ESA
MAX649MJA
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
-55°C to +125°C
PIN-PACKAGE
8 Plastic DIP
8 SO
Dice*
8 Plastic DIP
8 SO
8 CERDIP**
Ordering Information continued at end of data sheet.
* Dice are tested at TA = +25°C.
**Contact factory for availability and processing to MIL-STD-883.
Battery-Powered Applications
__________Typical Operating Circuit
INPUT
4V TO 16.5V
__________________Pin Configuration
TOP VIEW
V+
MAX651
ON/OFF
SHDN
CS
EXT
OUT
REF
FB
OUT
1
8
GND
FB
2
7
EXT
6
CS
5
V+
SHDN 3
P
OUTPUT
3.3V
REF 4
MAX649
MAX651
MAX652
DIP/SO
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.
MAX649/MAX651/MAX652
_______________General Description
The MAX649/MAX651/MAX652 BiCMOS, step-down DCDC 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.
These devices use miniature external components.
Their high switching frequency (up to 300kHz) allows
for less than 9mm diameter surface-mount inductors.
The MAX649/MAX651/MAX652 have dropout voltages
less than 1V and accept input voltages up to 16.5V.
Output voltages are preset at 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.
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
converters with on-chip FETs, which allow up to a
225mA load current.
MAX649/MAX651/MAX652
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, V+ to GND.......................................-0.3V, +17V
REF, SHDN, FB, CS, EXT, OUT .......................-0.3V, (V+ + 0.3V)
Continuous Power Dissipation (TA = +70°C)
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
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
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, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
V+ Input Voltage Range
V+
Supply Current
IQ
FB Trip Point
FB Input Current
Output Voltage
Reference Voltage
IFB
VOUT
VREF
CONDITIONS
V+ = 16.5V, SHDN ≤ 0.4V (operating, switch off)
80
V+ = 16.5V, SHDN ≥ 1.6V (shutdown)
4
V+ = 10V, SHDN ≥ 1.6V (shutdown)
2
MAX
UNITS
16.5
V
100
µA
5
MAX649C, MAX65_C
1.470
1.5
1.530
MAX649E, MAX65_E
1.4625
1.5
1.5375
MAX649M, MAX65_M
1.455
1.5
1.545
MAX649C, MAX65_C
±50
MAX649E, MAX65_E
±70
MAX649M, MAX65_M
±90
MAX649, V+ = 6V to 16.5V
4.80
5.0
5.20
MAX651, V+ = 4V to 16.5V
3.17
3.3
3.43
MAX652, V+ = 4V to 16.5V
2.88
3.0
3.12
MAX649C, MAX65_C, IREF = 0
1.470
1.5
1.530
MAX649E, MAX65_E, IREF = 0
1.4625
1.5
1.5375
MAX649M, MAX65_M, IREF = 0
1.455
1.5
1.545
MAX649C/E, MAX65_C/E
4
10
MAX649M, MAX65_M
4
15
40
100
Circuit of
Figure 1
0 ≤ IREF ≤ 100µA,
sourcing only
REF Line Regulation
4V ≤ V+ ≤ 16.5V
2
TYP
4.0
REF Load Regulation
Output Voltage
Line Regulation
MIN
Circuit of
Figure 1
MAX649, 6V ≤ V+ ≤ 16V,
ILOAD = 1A
2.6
MAX651, 4.5V ≤ V+ ≤ 16V,
ILOAD = 1A
1.7
MAX652, 4V ≤ V+ ≤ 16V,
ILOAD = 1A
1.9
_______________________________________________________________________________________
V
nA
V
V
mV
µV/V
mV/V
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
(V+ = 5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
Output Voltage
Load Regulation
CONDITIONS
Circuit of
Figure 1
Circuit of
Figure 1
Efficiency
SHDN Input Current
TYP
-47
MAX651, 0 ≤ ILOAD ≤ 1.5A,
VIN = 5V
-45
MAX652, 0 ≤ ILOAD ≤ 1.5A,
VIN = 5V
-45
MAX649, V+ = 10V,
ILOAD = 1A
92
MAX651, V+ = 5V,
ILOAD = 1A
89
MAX652, V+ = 5V,
ILOAD = 1A
88
V+ = 16.5V, SHDN = 0V or V+
SHDN Input Voltage High
VIH
4V ≤ V+ ≤ 16.5V
SHDN Input Voltage Low
VIL
4V ≤ V+ ≤ 16.5V
Current-Limit Trip
Level (V+ to CS)
MIN
MAX649, 0 ≤ ILOAD ≤ 1.5A,
VIN = 10V
VCS
4V ≤ V+ ≤ 16.5V
%
1
MAX649C/E, MAX65_C/E
180
210
240
MAX649M, MAX65_M
160
210
260
tON
(max)
V+ = 12V
12
Switch Minimum
Off-Time
tOFF
(min)
V+ = 12V
1.8
µA
V
0.4
Switch Maximum
On-Time
UNITS
mV/A
1.6
4V ≤ V+ ≤ 16.5V
CS Input Current
MAX
V
mV
±1
µA
16
20
µs
2.3
2.8
µs
EXT Rise Time
CEXT = 0.001µF, V+ = 12V
50
ns
EXT Fall Time
CEXT = 0.001µF, V+ = 12V
50
ns
_______________________________________________________________________________________
3
MAX649/MAX651/MAX652
ELECTRICAL CHARACTERISTICS (continued)
__________________________________________Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
SHUTDOWN CURRENT
vs. TEMPERATURE
3.5
3.0
V+ = 16.5V
2.5
74
I+ (mA)
I+ (mA)
76
V+ = 10V
72
2.0
1.5
V+ = 8V
70
1.0
V+ = 4V
68
0.5
2500
MAX649-A03
V+ = 16.5V
MAX649-A02
78
4.0
MAX649-A01
80
MAX649 MAXIMUM LOAD CURRENT
vs. SUPPLY VOLTAGE
MAXIMUM LOAD CURRENT (mA)
SUPPLY CURRENT vs. TEMPERATURE
2000
1500
1000
500
VOUT = 5V
CIRCUIT OF FIGURE 1
V+ = 4V
0
20 40 60 80 100 120 140
0
-60 -40 -20 0
TEMPERATURE (°C)
MAX649
EFFICIENCY vs. LOAD CURRENT
INPUT VOLTAGE (V)
MAX652
EFFICIENCY vs. LOAD CURRENT
90
80
100
MAX649-A05
100
MAX649-A04
90
90
80
80
50
40
30
60
50
40
30
20
VOUT = 5V
0
100µ
1m
10m
100m
20
VOUT = 3.3V
VOUT = 3V
0
100µ
1m
10m
100m
SWITCH ON-TIME
vs. TEMPERATURE
SWITCH OFF-TIME
vs. TEMPERATURE
2.5
MAX649-A07
40
10
LOAD CURRENT (A)
V+ = 5V
50
20
LOAD CURRENT (A)
17
60
10
0
1
TOP TO BOTTOM:
VIN = 4.3V
VIN = 5V
VIN = 8V
VIN = 10V
VIN = 12V
VIN = 15V
70
30
V+ = 5V
100µ
1
1m
10m
100m
1
LOAD CURRENT (A)
SWITCH ON-TIME/OFF-TIME RATIO
vs. TEMPERATURE
8.0
MAX649-A08
60
TOP TO BOTTOM:
VIN = 4.3V
VIN = 5V
VIN = 8V
VIN = 10V
VIN = 12V
VIN = 15V
70
EFFICIENCY (%)
TOP TO BOTTOM:
VIN = 6V
VIN = 8V
VIN = 10V
VIN = 12V
VIN = 15V
70
EFFICIENCY (%)
EFFICIENCY (%)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 1415
MAX651
EFFICIENCY vs. LOAD CURRENT
100
10
20 40 60 80 100 120 140
TEMPERATURE (°C)
MAX649-A06
-60 -40 -20 0
MAX649-A9
66
V+ = 5V
7.8
7.6
16
tON/tOFF RATIO
tOFF (ms)
7.4
tON (ms)
MAX649/MAX651/MAX652
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
2.0
7.2
7.0
6.8
6.6
6.4
6.2
15
-60 -40 -20
0
20
40
60
TEMPERATURE (°C)
4
6.0
1.5
80 100 120
-60 -40 -20 0 20 40 60
TEMPERATURE (°C)
80 100 120
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
_______________________________________________________________________________________
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
450
110
tRISE & tFALL (ns)
90
80
V+ = 5V, tFALL
70
60
300
V+ = 12V, tRISE
V+ = 5V, tFALL
200
V+ = 12V, tRISE
50
350
250
1000
150
40
30
V+ = 12V, tFALL
20 40 60 80 100 120 140
DROPOUT VOLTAGE
vs. TEMPERATURE
CS TRIP LEVEL
vs. TEMPERATURE
MAX652
800
MAX651
0.2
0.4
0.6
0.8
1.0
1.2
1.4 1.6
LOAD CURRENT (A)
REFERENCE OUTPUT VOLTAGE
vs. TEMPERATURE
1.506
1.504
225
700
220
215
210
205
200
195
ILOAD = 1A
CIRCUIT OF FIGURE 1
1.502
1.500
1.498
1.496
1.494
190
600
1.492
185
-60 -40 -20 0
0
MAX649-A14
230
CS TRIP LEVEL (mV)
900
235
MAX649-A13
MAX649
400
300
20 40 60 80 100 120 140
TEMPERATURE (°C)
1000
500
0
-60 -40 -20 0
TEMPERATURE (°C)
1100
MAX651, VOUT = 3.3V
600
100
V+ = 12V, tFALL
50
-60 -40 -20 0
800
700
200
100
20
MAX649, VOUT = 5V
MAX652, VOUT = 3V
900
REFRENCE OUTPUT (V)
tRISE & tFALL (ns)
V+ = 5V, t RISE
400
100
20 40 60 80 100 120 140
-60 -40 -20 0
TEMPERATURE (°C)
20 40 60 80 100 120 140
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
TEMPERATURE (°C)
REFERENCE OUTPUT RESISTANCE
vs. TEMPERATURE
250
REFRENCE OUTPUT RESISTANCE (Ω)
DROPOUT VOLTAGE (mV)
CEXT = 5nF
MAX649-A12
500
MAX649-A15
V+ = 5V, tRISE
MAX649-A11
CEXT = 1nF
MAX649-A16
120
MAX649-A10
130
DROPOUT VOLTAGE
vs. LOAD CURRENT
EXT RISE AND FALL TIMES
vs. TEMPERATURE (5nF)
DROPOUT VOLTAGE (mV)
EXT RISE AND FALL TIMES
vs. TEMPERATURE (1nF)
IREF = 10µA
200
150
IREF = 50µA
100
50
IREF = 100µA
0
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
_______________________________________________________________________________________
5
MAX649/MAX651/MAX652
____________________________Typical Operating Characteristics (continued)
(TA = +25°C, unless otherwise noted.)
MAX649/MAX651/MAX652
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
_____________________________Typical Operating Characteristics (continued)
MAX649
LOAD-TRANSIENT RESPONSE
MAX649
LINE-TRANSIENT RESPONSE
A
A
B
B
250µs/div
250µs/div
ILOAD = 1A
A: INPUT VOLTAGE (7V & 12V), 5V/div
B: 5V OUT, AC COUPLED, 100mV/div
A: LOAD CURRENT (100mA & 1A), 500mA/div
B: 5V OUTPUT VOLTAGE, AC COUPLED, 50mV/div
MAX649
SHUTDOWN RESPONSE TIME
A
B
1ms/div
ILOAD = 1A
A: SHDN INPUT VOLTAGE (0V & 5V), 2V/div
B: 5V OUTPUT VOLTAGE, 2V/div
______________________________________________________________Pin Description
PIN
6
NAME
FUNCTION
1
OUT
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.
2
FB
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.
3
SHDN
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.
4
REF
1.5V reference output that can source 100µA. Bypass with 0.1µF.
5
V+
Positive power-supply input
6
CS
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.
7
EXT
Gate drive for external P-channel MOSFET. EXT swings between V+ and GND.
8
GND
Ground
_______________________________________________________________________________________
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
V+
4
CS
SHDN
EXT
REF
OUT
FB
C3
0.1µF
C1
100µF
R1
0.1Ω
MAX649
MAX651
MAX652
3
C4
0.1µF
5
6
P1
Si9430*
7
OUTPUT
@ 1.5A
L1
22µH**
1
GND
2
8
D1
NSQ03A02L
C2
330µF
*SILICONIX SURFACE-MOUNT MOSFET
**SUMIDA CDR125-220
Figure 1. Test Circuit
_______________Detailed Description
The MAX649/MAX651/MAX652 are BiCMOS, stepdown, switch-mode power-supply controllers that provide 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.
The MAX649/MAX651/MAX652 offer three main
improvements over prior solutions:
1) The converters operate with tiny (less than 9mm
diameter) surface-mount inductors, due to their
300kHz switching frequency.
2) The current-limited PFM control scheme allows
greater than 90% efficiencies over a wide range of
load currents (1.0mA to 1.5A).
3) The maximum supply current is only 100µA.
PFM Control Scheme
The MAX649/MAX651/MAX652 use a proprietary, current-limited PFM control scheme. As with traditional
PFM converters, 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 oneshots 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.
The MAX649/MAX651/MAX652 also limit the peak inductor current, which allows them to run in continuous-conduction mode and maintain high efficiency with heavy loads
(Figure 3a). This current-limiting feature is a key component 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.
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 voltage is still out of regulation after two pulses, 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.
Shutdown Mode
When SHDN is high, the MAX649/MAX651/MAX652 enter
shutdown mode. In this mode, the internal biasing circuitry 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.
Quiescent Current
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 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 VOUT at 3.3V, the typical
quiescent current is 90µA.
EXT Drive Voltage Range
EXT swings from V+ to GND and provides the drive output for an external P-channel power MOSFET.
Modes of Operation
When delivering high output currents, the MAX649/
MAX651/MAX652 operate in continuous-conduction
mode (CCM). In this mode, current always flows in the
_______________________________________________________________________________________
7
MAX649/MAX651/MAX652
VIN
MAX649/MAX651/MAX652
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
V+
FB
DUAL-MODE™
COMPARATOR
SHDN
MAX649
MAX651
MAX652
ERROR
COMPARATOR
OUT
REF
1.5V
REFERENCE
N
Q
MINIMUM
OFF-TIME TRIG
ONE-SHOT
FROM V+
S
MAXIMUM
TRIG ON-TIME Q
ONE-SHOT
EXT
Q
R
CURRENT
COMPARATOR
CS
CURRENT
CONTROL CIRCUITS
0.2V
(FULL CURRENT)
0.1V
(HALF CURRENT)
FROM V+
GND
Figure 2. Block Diagram
8
_______________________________________________________________________________________
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
MAX649/MAX651/MAX652
2.5A
1.5A
2.0A
1A
1.5A
1.0A
0A
0.5A
0A
5µs/div
2µs/div
V+ = 10V, ILOAD = 1.3A
CIRCUIT OF FIGURE 1, R1 = 150mΩ
V+ = 10V, ILOAD = 1.4A
CIRCUIT OF FIGURE 1, R1 = 100mΩ
Figure 3a. MAX649 Continuous-Conduction Mode, Heavy
Load-Current Waveform (500mA/div)
inductor, and the control circuit adjusts the switch duty
cycle to maintain regulation without exceeding the
switch current capability (Figure 3a). This provides
excellent load-transient response and high efficiency.
In discontinuous-conduction mode (DCM), current
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 expected and poses no operational problems.
Figure 3b. MAX649 Light/Medium Load-Current Waveform
(500mA/div)
VIN
V+
MAX649
MAX651
MAX652
3
4
The MAX649/MAX651/MAX652 are said to be in
dropout when the input voltage (V+) is low enough
that the output drops below the minimum output
voltage specification (see Electrical Characteristics ).
The dropout voltage is the difference between the
input and output voltage when dropout occurs.
See the Typical Operating Characteristics for the
Dropout Voltage vs. Load Current and Dropout Voltage
vs. Temperature graphs.
CS
EXT
REF
OUT
GND
FB
6
7
(
VOUT
R2 = R3
–1
VREF
)
P1
Si9430
L1
22µH
OUTPUT
@ 1.5A
1
2
R2
8
C3
0.1µF
C1
100µF
R1
0.1Ω
SHDN
Dropout
C4
0.1µF
5
C2
330µF
D1
1N5820
R3
150k
VREF = 1.5V
Figure 4. Adjustable-Output Operation
_______________________________________________________________________________________
9
RS = 0.07Ω
2.0
RS = 0.08Ω
1.5
RS = 0.10Ω
RS = 0.12Ω
1.0
RS = 0.14Ω
0.5
RS = 0.06Ω
MAX649
VOUT = 5V
5 6 7
RS = 0.07Ω
2.0
RS = 0.08Ω
1.5
RS = 0.10Ω
1.0
RS = 0.12Ω
RS = 0.14Ω
0.5
MAX651
VOUT = 3.3V
0
0
3 4
2.5
MAX649-A26
3.0
MAXIMUM OUTPUT CURRENT (A)
RS = 0.06Ω
2.5
MAX649-A25
3.0
MAXIMUM OUTPUT CURRENT (A)
8 9 10 11 12 13 14 15 16
3 4 5
INPUT VOLTAGE (V)
6 7 8 9 10 11 12 13 14 15 16
INPUT VOLTAGE (V)
Figure 5a. MAX649 Current-Sense Resistor Graph
Figure 5b. MAX651 Current-Sense Resistor Graph
Setting the Output Voltage
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,
using external resistors R2 and R3 configured as
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:
VOUT
R2 = R3 x ——— -1
VREF
[
]
where VREF = 1.5V.
When using external resistors, it does no harm to connect OUT and the output together, or to leave OUT
unconnected.
Current-Sense Resistor Selection
The current-sense resistor limits the peak switch current to 210mV/RSENSE, where RSENSE is the value of
the current-sense resistor, and 210mV is the currentlimit 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 function of the peak current, the peak current must not
be too low.
10
3.0
RS = 0.06Ω
2.5
MAX649-A27
__________________Design Procedure
MAXIMUM OUTPUT CURRENT (A)
MAX649/MAX651/MAX652
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
RS = 0.07Ω
2.0
RS = 0.08Ω
RS = 0.10Ω
1.5
RS = 0.12Ω
1.0
RS = 0.14Ω
0.5
MAX652
VOUT = 3.0V
0
3 4 5
6 7 8 9 10 11 12 13 14 15 16
INPUT VOLTAGE (V)
Figure 5c. MAX652 Current-Sense Resistor Graph
To choose the proper current-sense resistor for a particular output voltage, determine the minimum input
voltage and the maximum load current. Next, referring
to Figures 5a, 5b, or 5c, using the minimum input voltage, 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
on-resistance of 0.13Ω is assumed for VGS = -4.5V.
______________________________________________________________________________________
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
Inductor Selection
Practical inductor values range from 10µH to 50µH or more.
The circuit operates in discontinuous-conduction mode if:
VOUT x (R + 1)
VD
V + ≤ ———————— + —— + VSW
R
R
R, the switch on-time/off-time ratio, equals 6.7. VD is the
diode’s drop, and VSW is the voltage drop across the
P-channel FET. To get the full output capability in
discontinuous-conduction mode, choose an inductor
value no larger than:
RSENSE x 12µs x (V+ - VSW - VOUT)
L(max) = —————————————————
VCS
where VCS is the current-sense voltage.
In both the continuous and discontinuous modes, the
lower limit of the inductor is more important. With a
small inductor value, the current rises faster and overshoots the desired peak current limit because the current-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:
(V+(max) - VSW - VOUT) x 0.3µs
L(min) = ————————————––——
∆I x ILIM(min)
where ∆I is the percentage of inductor-current overshoot, where ILIM = VCS/RSENSE and 0.3µs is the time
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 exceeded. Smaller inductance values cause
higher output ripple because of overshoot. Larger values tend to produce physically larger coils.
For highest efficiency, use a coil with low DC resistance; a value smaller than 0.1V/I LIM works best. To
minimize radiated noise, use a toroid, pot core, 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 ILIM(max).
However, it is generally acceptable to bias the inductor
into saturation by about 20% (the point where 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.
Table 1 lists inductor types and suppliers for various
applications. The efficiencies of the listed surfacemount inductors are nearly equivalent to those of the
larger size through-hole versions.
Diode Selection
The MAX649/MAX651/MAX652’s high switching frequency 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
current rating equal to or greater than ILIM(max) and a
voltage rating higher than V+(max). For high-temperature applications, where Schottky diodes can be
inadequate because of high leakage currents, 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.
External Switching Transistor
The MAX649/MAX651/MAX652 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
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
the minimum available VGS , which equals V+(min).
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
MAX649/MAX651/MAX652
Standard wire-wound and metal-film resistors have an
inductance high enough to degrade performance.
Surface-mount (chip) resistors have very little inductance 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).
MAX649/MAX651/MAX652
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
a drain-to-source rating of -20V and a typical on-resistance of 0.115Ω at 2A with VGS = -4.5V. Tables 1 and 2
list suppliers of switching transistors suitable for use
with these devices.
Capacitor Selection
Output Filter Capacitor
The primary criterion for selecting the output filter
capacitor is low equivalent series resistance (ESR),
rather than high capacitance. An electrolytic capacitor
with low enough ESR will automatically have high
enough capacitance. The product of the inductor-current variation and the ESR of the output filter capacitor
determines the amplitude of the high-frequency ripple
seen on the output voltage. When 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.
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 OSCON organic semiconductor through-hole capacitors
and the Nichicon PL series also exhibit very low ESR.
Table 1 lists some suppliers of low-ESR capacitors.
amount of noise at the voltage source caused by the
switching action of the MAX649/MAX651/MAX652.
The input voltage source impedance determines the
size of the capacitor required at the V+ input. As
with the output filter capacitor, a low-ESR capacitor
is recommended. Bypass the IC separately with a
0.1µF ceramic capacitor placed close to the V+ and
GND pins.
Reference Capacitor
Bypass REF with a 0.1µF or larger capacitor. REF can
source at least 100µA.
Layout Considerations
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 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 capacitor as
close as possible to V+ and GND.
Input Bypass Capacitor
The input bypass capacitor reduces peak currents
drawn from the voltage source, and also reduces the
Table 1. Component Selection Guide
PRODUCTION
METHOD
INDUCTORS
CAPACITORS
Sumida
Matsuo
CDR125-220 (22µH) 267 series
Surface Mount
Miniature
Through-Hole
Low-Cost
Through-Hole
12
DIODES
CURRENT-SENSE
RESISTORS
MOSFETS
Siliconix
Little Foot series
Nihon
NSQ series
Coiltronics
CTX 100 series
Sprague
595D series
Sumida
RCH855-220M
Sanyo
OS-CON series
low-ESR organic
semiconductor
Renco
RL 1284-22
Nichicon
PL series
Motorola
low-ESR electrolytics
1N5820,
1N5823
United Chemi-Con
LXF series
IRC
LRC series
IRC
OAR series
Motorola
medium-power
surface-mount products
Motorola
Motorola
TMOS power MOSFETs
______________________________________________________________________________________
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
MAX649/MAX651/MAX652
Table 2. Component Suppliers
COMPANY
PHONE
FAX
Coiltronics
USA
(407) 241-7876
(407) 241-9339
Harris
USA
(800) 442-7747
(407) 724-3937
International Rectifier
USA
(310) 322-3331
(310) 322-3332
IRC
USA
(704) 264-8861
(704) 264-8866
Matsuo
USA
Japan
(714) 969-2491
81-6-337-6450
(714) 960-6492
81-6-337-6456
Motorola
USA
(800) 521-6274
(602) 244-4015
Nichicon
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
USA
(516) 586-5566
(516) 586-5562
Sanyo
USA
Japan
(619) 661-6835
81-7-2070-6306
(619) 661-1055
81-7-2070-1174
Siliconix
USA
(408) 988-8000
(408) 970-3950
Sprague
USA
(603) 224-1961
(603) 224-1430
Sumida
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
__Ordering Information (continued)
TEMP. RANGE
PIN-PACKAGE
MAX651CPA
PART
0°C to +70°C
8 Plastic DIP
MAX651CSA
MAX651C/D
MAX651EPA
MAX651ESA
MAX651MJA
MAX652CPA
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
-55°C to +125°C
0°C to +70°C
8 SO
Dice*
8 Plastic DIP
8 SO
8 CERDIP**
8 Plastic DIP
MAX652CSA
MAX652C/D
MAX652EPA
MAX652ESA
MAX652MJA
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
-55°C to +125°C
8 SO
Dice*
8 Plastic DIP
8 SO
8 CERDIP**
* Dice are tested at TA = +25°C.
**Contact factory for availability and processing to MIL-STD-883.
___________________Chip Topography
GND
OUT
EXT
FB
0.109"
(2.769mm)
CS
SHDN
REF
V+
0.080"
(2.032mm)
TRANSISTOR COUNT: 442;
SUBSTRATE CONNECTED TO V+.
______________________________________________________________________________________
13
PDIPN.EPS
________________________________________________________Package Information
SOICN.EPS
MAX649/MAX651/MAX652
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
14
______________________________________________________________________________________
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
CDIPS.EPS
______________________________________________________________________________________
15
MAX649/MAX651/MAX652
___________________________________________Package Information (continued)
MAX649/MAX651/MAX652
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
NOTES
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 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 1997 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.