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Pobierz
19-1142 Rev 0; 9/96
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
_______________General Description
The MAX887 high-efficiency, step-down DC-DC con-
verter provides an adjustable output from 1.25V to
10.5V. It accepts inputs from 3.5V to 11V and delivers
600mA. Operation to 100% duty cycle minimizes
dropout voltage (300mV typ at 500mA). Synchronous
rectification reduces output rectifier losses, resulting in
efficiency as high as 95%.
Fixed-frequency pulse-width modulation (PWM)
reduces noise in sensitive communications applica-
tions. Using a high-frequency internal oscillator allows
tiny surface-mount components to reduce PC board
area, and eliminates audio-frequency interference. A
SYNC input allows synchronization to an external clock
to avoid interference with sensitive RF and data-
acquisition circuits.
The MAX887 features current-mode operation for supe-
rior load/line-transient response. Cycle-by-cycle current
limiting protects the internal MOSFET and rectifier. A
low-current (2.5µA typ) shutdown mode conserves bat-
tery life.
____________________________Features
©
95% Efficiency
©
600mA Output Current
©
Cycle-by-Cycle Current Limiting
©
Low-Dropout, 100% Duty-Cycle Operation,
300mV at 500mA
©
Internal 0.6
½
(typ) MOSFET
©
Internal Synchronous Rectifier
©
High-Frequency Current-Mode PWM
©
External SYNC or Internal 300kHz Oscillator
©
Guaranteed 260kHz to 340kHz Internal Oscillator
Frequency Limits
©
2.5µA Shutdown Mode
________________________Applications
Portable Instruments
Cellular Phones and Radios
Personal Communicators
Distributed Power Systems
Computer Peripherals
______________Ordering Information
PART
MAX887HC/D
MAX887HESA
TEMP. RANGE
PIN-PACKAGE
Dice*
8 SO
0°C to +70°C
-40°C to +85°C
*
Contact factory for availability. Dice are tested at T
A
= +25°C.
__________Typical Operating Circuit
__________________Pin Configuration
33
m
H
V
OUT
= 3.3V
V
IN
= 3.5V to 11V
TOP VIEW
V+
LX
R1
165k
47
F
m
SHDN
1
8
V+
47
m
F
0.33
m
F
W
C1
100pF
MAX887
2
FB
7
LX
ON
MAX887
SHDN
OFF
SYNC
VL
FB
REF
3
6
SYNC
5
GND
VL
4
REF
R2
100k
GND
W
2.2
m
F
0.047
m
F
SO
V
OUT
= 1.25V (R1/R2 + 1)
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
ABSOLUTE MAXIMUM RATINGS
REF, FB, SYNC, VL to GND..................................... -0.3V to +6V
V+ to GND ............................................................. -0.3V to +12V
SHDN, LX to GND ....................................... -0.3V to (V+ + 0.3V)
PGND to GND ...................................................... -0.3V to +0.3V
Continuous Power Dissipation (T
A
= +70°C)
SO (derate 9.09mW/°C above +70°C) .........................471mW
Operating Temperature Ranges
MAX887HC/D.......................................................0°C to +70°C
MAX887HESA ...................................................-40°C to +85°C
Storage Temperature Range ........................... -65°C to +165°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+ = +7V, PGND = GND = 0V, SHDN = V+,
(T
A
= 0°C to T
MAX
)
, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Supply Range
Quiescent Supply Current
(PWM Mode)
V+
3.5
11.0
V
I
V+, PWM
I
OUT
= 0mA, SYNC = 3.0V
2.7
4.0
mA
Quiescent Supply Current
(PFM Mode)
I
V+, PFM
I
OUT
= 0mA, SYNC = GND
0.2
0.5
mA
Shutdown Supply Current
I
V+, SHDN
SHDN = GND
Circuit of Figure 2
I
OUT
= 0mA to 500mA
V
IN
= 4V to 11V, PWM mode
2.5
5
µA
Output Voltage Range
V
OUT, RANGE
1.25
10.50
V
Load Regulation
0.005
%/mA
Line Regulation
0.2
%/V
PWM FB Feedback Threshold
V
FB
SYNC = 3.0V, PWM duty cycle = 50%
1.225
1.250
1.275
V
FB Input Current
I
FB
FB = 1.30V
±0.10
µA
SYNC Frequency
f
SYNC
25
440
kHz
SYNC Pulse Width High or Low
SYNC, PW
500
ns
PWM Maximum Duty Cycle
PWM, DUTY
SYNC = 3.0V, FB = 1.18V
100
%
PWM Switching Frequency
f
OSC
SYNC = 3.0V
260
300
340
kHz
High-Side Current Limit
I
LIM+
0.75
1.0
1.40
A
LX On-Resistance
R
ON, LX
I
LX
= ±100mA
0.6
½
LX Leakage Current
I
LXLKG
V+ = 12V, LX = GND to 12V
-10
1.0
10
µA
LX Reverse Leakage Current,
Regulator Off
I
LXLKGR
V+ = floating, LX = 5V, SHDN = GND
1.0
20
µA
Undervoltage Lockout
V
+, UVLO
V+ falling
3.0
3.3
V
Startup Voltage
V
+, START
V+ rising
3.1
3.5
V
SYNC Input High Voltage
V
IH, SYNC
2.5
V
SYNC Input Low Voltage
V
IL, SYNC
0.5
V
SYNC Input Current
I
IN, SYNC
SYNC = GND or 3V
±1
µA
SHDN Input High Voltage
V
IH, SHDN
2.4
V
SHDN Input Low Voltage
V
IL, SHDN
0.8
V
I
IN-, SHDN
±1
µA
SHDN Input Current, Sinking
SHDN = GND or V+
C
IN, SHDN
(Note 1)
10
pF
SHDN Input Capacitance
VL Output Voltage
V
L
I
VL
= 0mA to 1mA
3.3
V
REF Output Voltage
V
REF
0µA to 30µA
1.25
V
Note 1:
Guaranteed by design and not production tested.
2
_______________________________________________________________________________________
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
ELECTRICAL CHARACTERISTICS
(V+ = +7V, PGND = GND = 0V, SHDN = V+,
(T
A
= -40°C to +85°C)
, unless otherwise noted.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Supply Range
Quiescent Supply Current
(PWM Mode)
V+
3.5
11.0
V
I
V+, PWM
I
OUT
= 0mA, SYNC = 3.0V
2.7
4.0
mA
Quiescent Supply Current
(PFM Mode)
I
V+, PFM
I
OUT
= 0mA, SYNC = GND
0.2
0.6
mA
Shutdown Supply Current
I
V+, SHDN
2.5
5
µA
SHDN = GND
Circuit of Figure 2
Output Voltage Range
V
OUT, RANGE
1.25
10.50
V
PWM FB Feedback Threshold
V
FB
SYNC = 3.0V, PWM duty cycle = 50%
1.222
1.250
1.278
V
FB Input Current
I
FB
FB = 1.30V
±0.10
µA
PWM Switching Frequency
f
OSC
SYNC = 3.0V
250
300
350
kHz
High-Side Current Limit
I
LIM+
0.75
1.00
1.50
A
Undervoltage Lockout
V
+, UVLO
V+ falling
3.0
3.3
V
Startup Voltage
V
+, START
V+ rising
3.1
3.5
V
Note 2:
Specifications from 0°C to -40°C are guaranteed by design and not production tested.
__________________________________________Typical Operating Characteristics
(Circuit of Figure 2, T
A
= +25°C, unless otherwise noted.)
DROPOUT VOLTAGE
vs. LOAD CURRENT
EFFICIENCY vs. OUTPUT CURRENT
EFFICIENCY vs. OUTPUT CURRENT
0.7
100
100
PWM MODE
(SYNC = VL)
V
OUT
= 3.3V
PWM MODE
(SYNC = VL)
V
OUT
= 5V
90
90
0.6
80
80
3.3V SETTING
V
OUT
= 3.135V
0.5
70
70
V
IN
= 9V
V
IN
= 4V
V
IN
= 9V
60
60
V
IN
= 5V
V
IN
= 7V
0.4
50
50
0.3
40
40
V
IN
= 11V
V
IN
= 5.5V
V
IN
= 7V
V
IN
= 11V
30
0.2
30
5V SETTING
V
OUT
= 4.75V
20
20
0.1
10
10
0
0
0
0
200
400
600
800
1000
0.0001
0.001
0.01
0.1
0.6
0.0001
0.001
0.01
0.1
0.6
LOAD CURRENT (mA)
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
_______________________________________________________________________________________
3
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, T
A
= +25°C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT
EFFICIENCY vs. OUTPUT CURRENT
MAXIMUM OUTPUT CURRENT
vs. SUPPLY VOLTAGE
100
100
1200
V
IN
= 5.5V
V
IN
= 4V
3.3V SETTING, V
OUT
= 3.135V
90
90
1000
80
V
IN
= 5V
80
V
IN
= 11V
70
70
5V SETTING, V
OUT
= 4.75V
V
IN
= 11V
800
60
60
50
50
600
V
IN
= 9V
40
40
V
IN
= 9V
400
30
30
V
IN
= 7V
IDLE MODE
(SYNC = GND)
V
OUT
= 3.3V
20
20
IDLE MODE
(SYNC = GND)
V
OUT
= 5V
200
GUARANTEED OUTPUT CURRENT
OF FIGURE 2 IS 600mA
V
IN
= 7V
10
10
0
0
0
0.0001
0.001
0.01
0.1
0.6
0.0001
0.001
0.01
0.1
0.6
3
5
7
9
11
13
15
17
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
SUPPLY VOLTAGE (V)
QUIESCENT CURRENT
vs.TEMPERATURE
MAXIMUM OUTPUT CURRENT
vs. SYNC FREQUENCY
QUIESCENT SUPPLY CURRENT
vs. SUPPLY VOLTAGE
1200
3.5
3.0
V
IN
= 5.3V
V
OUT
= 3.3V
A
3.0
2.5
1000
A
PWM MODE
2.5
800
2.0
2.0
1.5
A: V
OUT
= 3.3V, PWM MODE
B: V
OUT
= 3.3V, PFM MODE
600
1.5
1.0
400
C2, C3 = 47µF
L1 = 33µH
V
IN
= 5V
V
OUT
= 3.3V
V
OUT
= -5% at I
OUT(MAX)
1.0
C
0.5
200
0.5
PFM MODE
B
0
0
0
-60 -40 -20
0
20
40
60
80 100 120 140
0
100
200
300
400
500
600
02468 0 2 4 6 8
TEMPERATURE (°C)
SYNC FREQUENCY (kHz)
SUPPLY VOLTAGE (V)
SWITCHING FREQUENCY
vs. SUPPLY VOLTAGE
SWITCHING FREQUENCY
vs. TEMPERATURE
OUTPUT RIPPLE AND HARMONICS
350
350
4
V
OUT
= 3.3V
V
OUT
= 3.3V
V
IN
= 5V
V
OUT
= 3.3V
I
OUT
= 500mA
PWM MODE
340
340
330
330
3
320
320
310
310
2
300
300
290
290
1
280
280
270
270
0
260
260
250
250
-1
02468 0 2 4 6 8
-60 -40 -20
0
20
40
60
80 100 120 140
10k
100k
1M
10M
TEMPERATURE (°C)
SUPPLY VOLTAGE (V)
FREQUENCY (Hz)
4
_______________________________________________________________________________________
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, T
A
= +25°C, unless otherwise noted.)
LIGHT-LOAD, PWM-MODE
SWITCHING WAVEFORMS
HEAVY-LOAD, PWM-MODE
SWITCHING WAVEFORMS
A
A
B
B
C
0mA
C
1
m
s/div
1
m
s/div
V
IN
= 5V, V
OUT
= 3.3V, LOAD = 500mA
A: LX, 5V/div
B: V
OUT
, 20mV/div, AC COUPLED
C: INDUCTOR CURRENT, 500mA/div
V
IN
= 5V, V
OUT
= 3.3V, LOAD = 0mA
A: LX, 5V/div
B: V
OUT
, 20mV/div, AC COUPLED
C: INDUCTOR CURRENT, 500mA/div
LIGHT-LOAD, PFM-MODE
SWITCHING WAVEFORMS
MEDIUM-LOAD, PFM-MODE
SWITCHING WAVEFORMS
A
A
B
B
C
C
1
m
s/div
10
m
s/div
V
IN
= 5V, V
OUT
= 3.3V, LOAD = 0mA
A: LX, 5V/div
B: V
OUT
, 20mV/div, AC COUPLED
C: INDUCTOR CURRENT, 200mA/div
V
IN
= 5V, V
OUT
= 3.3V, LOAD = 70mA
A: LX, 5V/div
B: V
OUT
, 20mV/div, AC COUPLED
C: INDUCTOR CURRENT, 200mA/div
_______________________________________________________________________________________
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