MAX762ESA+ [MAXIM]
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| 型号： | MAX762ESA+ |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
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19-0201; Rev 0; 11/93
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , Ste p-Up DC-DC Conve rte rs
Q
1/MAX762
_______________Ge n e ra l De s c rip t io n
____________________________Fe a t u re s
♦ High Efficiency for a Wide Range of Load Currents
♦ 12V/150mA Flash Memory Programming Supply
♦ 110µA Max Supply Current
The MAX761/MAX762 s te p -up s witc hing re g ula tors
provide high efficiency over a wide range of load currents,
d e live ring up to 150mA. A uniq ue , c urre nt-limite d
pulse-frequency-modulated (PFM) control scheme gives
the devices the benefits of pulse-width-modulated (PWM)
converters (high efficiency with heavy loads), while using
less than 110µA of supply current (vs. 2mA to 10mA for
PWM converters). The result is high efficiency over a wide
range of loads.
♦ 5µA Max Shutdown Supply Current
♦ 2V to 16.5V Input Voltage Range
♦ 12V (MAX761), 15V (MAX762) or Adjustable Output
♦ Current-Limited PFM Control Scheme
♦ 300kHz Switching Frequency
The MAX761/MAX762 input voltage range is 2V to 16.5V.
Output voltages are preset to 12V (MAX761) and 15V
(MAX762), or they can be set with two external resistors.
With a 5V input, the MAX761 guarantees a 12V, 150mA
output. Its high efficiency, low supply current, fast start-up
time, SHDN controlling capability, and small size make the
MAX761 ideal for powering flash memory.
♦ Internal, 1A, N-Channel Power FET
♦ LBI/LBO Low-Battery Comparator
______________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
The MAX761/MAX762 have an internal 1A power MOS-
FET, making them ideal for minimum-component, low- and
medium-power applications. These devices use tiny exter-
nal components, and their high switching frequencies (up
to 300kHz) allow for small surface-mount magnetics.
MAX761CPA
MAX761CSA
MAX761C/D
MAX761EPA
MAX761ESA
MAX761MJA
MAX762CPA
MAX762CSA
MAX762C/D
MAX762EPA
MAX762ESA
MAX762MJA
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
For increased output drive capability or higher output volt-
ages, use the MAX770–MAX773, which are similar in
design to the MAX761/MAX762, but drive external power
MOSFETs. For stepping up to 5V, see the MAX756/
MAX757 and MAX856-MAX859 data sheets.
8 CERDIP**
8 Plastic DIP
8 SO
0°C to +70°C
0°C to +70°C
Dice*
_________________________Applic a t io n s
Flash Memory Programming
-40°C to +85°C
-40°C to +85°C
-55°C to +125°C
8 Plastic DIP
8 SO
PCMCIA Cards
8 CERDIP**
Battery-Powered Applications
High-Efficiency DC-DC Converters
* Contact factory for dice specifications.
** Contact factory for availability and processing to MIL-STD-883.
__________Typ ic a l Op e ra t in g Circ u it
__________________P in Co n fig u ra t io n
INPUT
4.75V
TO 12V
TOP VIEW
33µF
18µH
OUTPUT
12V
150mA
LX
LBO
LBI
1
2
3
4
V+
8
7
6
5
33µF
MAX761
LX
SHDN
V+
ON/OFF
MAX761
MAX762
FB
GND
REF
LBI
LOW-BATTERY
DETECTOR OUTPUT
SHDN
LOW-BATTERY
LBO
DETECTOR INPUT
REF
FB
GND
DIP/SO
________________________________________________________________ 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 .
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , Ste p-Up DC-DC Conve rte rs
Q
ABSOLUTE MAXIMUM RATINGS
Supply Voltage V+ to GND.......................................-0.3V to 17V
REF, LBO, LBI, SHDN, FB............................-0.3V to (V+ + 0.3V)
LX..............................................................................-0.3V to 17V
LX Peak Current....................................................................1.5A
LBO Current..........................................................................5mA
Operating Temperature Ranges:
MAX76_C_A........................................................0°C to +70°C
MAX76_E_A .....................................................-40°C to +85°C
MAX76_MJA ..................................................-55°C to +125°C
Junction Temperatures:
Continuous Power Dissipation (T = +70°C)
MAX76_C_A/E_A..........................................................+150°C
MAX76_MJA.................................................................+175°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°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
1/MAX762
(V+ = 5V, I
= 0mA, C
= 0.1µF, T = T
A
to T typical values are at T = +25°C, unless otherwise noted.)
MAX, A
LOAD
REF
MIN
PARAMETER
Supply Voltage
SYMBOL
CONDITIONS
MIN
2
TYP
MAX
16.5
16.5
16.5
UNITS
Figure 2, bootstrapped
V+
MAX76_C/E
MAX76_M
3
V
Figure 3 or 5 with
external resistors.
3.1
Minimum Operating Voltage
Minimum Start-Up Voltage
Figure 2, bootstrapped
Figure 2, bootstrapped
1.7
1.7
V
V
2.0
V+ = 16.5V, normal operation, SHDN = 0V,
non-bootstrapped
88
110
Supply Current
µA
µA
Figure 2, MAX761, V = 5V, SHDN = 0V,
IN
normal operation
300
Shutdown Current
V+ = 10.0V, shutdown mode, SHDN = V+
1
5
0mA ≤ I
3V ≤ V+ ≤ 12V
≤ 75mA,
LOAD
11.52
11.52
14.4
12.0
12.48
Figure 2,
MAX761,
bootstrapped
0mA ≤ I ≤ 150mA,
LOAD
12.0
15.0
15.0
12.48
15.6
15.6
4.75V ≤ V+ ≤ 12V
Output Voltage
(Note 1)
V
OUT
V
0mA ≤ I ≤ 50mA,
LOAD
Figure 2,
MAX762,
bootstrapped
3V ≤ V+ ≤ 15V
0mA ≤ I ≤ 100mA,
LOAD
14.4
4.75V ≤ V+ ≤ 15V
Peak Current at LX
Maximum Switch-On Time
Minimum Switch-Off Time
Load Regulation
I
See Figure 4b
0.75
6
1.0
8
1.25
10
A
µs
PEAK
t
ON
t
1.0
1.3
1.6
µs
OFF
Figure 2, 0mA ≤ I
≤ 200mA, bootstrapped
0.0042
0.08
%/mA
%/V
LOAD
Line Regulation
Figure 2, 4V ≤ V ≤ 6V, bootstrapped
IN
Figure 2, bootstrapped, V
= 12V,
OUT
Efficiency
86
%
60mA ≤ I
≤ 120mA
LOAD
MAX76_C
MAX76_E
MAX76_M
1.4700
1.4625
1.4550
1.50
1.50
1.50
1.5300
1.5375
1.5450
Reference Voltage
V
REF
V
2
_______________________________________________________________________________________
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , Ste p-Up DC-DC Conve rte rs
Q
1/MAX762
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, I
= 0mA, C
= 0.1µF, T = T
A
to T , typical values are at T = +25°C, unless otherwise noted.)
MAX A
LOAD
REF
MIN
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
10
UNITS
mV
MAX76_C/E
MAX76_M
Reference Load Regulation
Reference Line Regulation
0µA ≤ I
≤ 100µA
LOAD
15
3.0V ≤ V+ ≤ 16.5V
30
100
5
µV/V
MAX76_C
MAX76_E
MAX76_M
-5
-10
V+ = 16.5V,
LX = 17V
LX Leakage Current
FB Leakage Current
Voltage Trip Point
10
µA
nA
V
-30
30
MAX76_C
-20
20
I
FB
MAX76_E
-40
40
MAX76_M
-60
60
MAX76_C
1.4700
1.4625
1.4550
1.50
1.50
1.50
1.0
1.5300
1.5375
1.5450
2.2
V
FB
MAX76_E
MAX76_M
LX On Resistance
V+ > 5.0V
Ω
V
SHDN Input High Voltage
SHDN Input Low Voltage
SHDN Leakage Current
V
IH
2.0V ≤ V+ ≤ 16.5V
2.0V ≤ V+ ≤ 16.5V
1.6
V
IL
0.4
1
V
V+ = 16.5V, SHDN = 0V or V+
MAX76_C
-1
µA
1.4700
1.4625
1.4550
1.50
1.50
1.50
20
1.5300
1.5375
1.5450
LBI Threshold Voltage
LBI falling
MAX76_E
MAX76_M
V
LBI Hysteresis
mV
nA
µA
V
LBI Leakage Current
LBO Leakage Current
LBO Voltage
V+ = 16.5V, V = 1.5V
-20
-1
20
1
LBI
V+ = 16.5V, V
= 16.5V
LBO
V
OL
V+ = 5.0V, I
= 1mA
0.4
SINK
LBI to LBO Delay
Overdrive = 5mV
2.5
µs
Note 1: See Typical Operating Characteristics for output current capability versus input voltage. Guarantees based on correlation
to switching on and off times, on-resistance, and peak-current ratings.
_______________________________________________________________________________________
3
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , Ste p-Up DC-DC Conve rte rs
Q
__________________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s
(Circuit of Figure 2, T = +25°C, unless otherwise noted.)
A
EFFICIENCY vs. OUTPUT CURRENT
BOOTSTRAPPED
QUIESCENT CURRENT vs.
INPUT VOLTAGE
EFFICIENCY vs. OUTPUT CURRENT
NON-BOOTSTRAPPED
100
90
100
90
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
V
= 12V
V
= 10V
OUT
IN
V
IN
= 10V
BOOTSTRAPPED
(INTERNAL RESISTORS)
V
= 5V
80
70
IN
80
70
V
IN
= 5V
60
50
40
30
20
10
0
60
50
40
30
20
10
0
V
= 2V
IN
BOOTSTRAPPED
(EXTERNAL RESISTORS)
V
OUT
= 12V
V
OUT
= 12V
1/MAX762
NON-BOOTSTRAPPED
0
0.1
1
10
100
1000
0.1
1
10
100
1000
0
0.5
1
1.5 2 2.5
3
3.5
4
4.5
5
5.5
6
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT vs.
INPUT VOLTAGE
REFERENCE vs.TEMPERATURE
COEFFICIENT
REFERENCE OUTPUT RESISTANCE vs.
TEMPERATURE
400
250
200
1.506
1.504
1.502
350
300
250
200
150
100
50
BOOTSTRAPPED
10µA
150
100
50
1.500
1.498
1.496
1.494
1.492
50µA
NON-BOOTSTRAPPED
= 12V
100µA
V
OUT
0
0
3.0
3.5
4.0
4.5
5.0
5.5
6.0
-60 -40 -20
0
20 40 60 80 100 120 140
-60 -40 -20
0 20 40 60 80 100 120 140
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
TEMPERATURE (°C)
NO-LOAD START-UP VOLTAGE
LX ON-RESISTANCE vs.
TEMPERATURE
MAX761
START-UP VOLTAGE vs. R
LOAD
3.5
3.0
2.5
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.6
1.4
1.2
V
= 12V
OUT
BOOTSTRAPPED
(EXTERNAL RESISTORS)
V
= 12V
OUT
BOOTSTRAPPED
INTERNAL RESISTORS
V+ = 5V
NON-BOOTSTRAPPED
(EXTERNAL RESISTORS)
2.0
1.5
1.0
0.5
1.0
0.8
0.6
0.4
V+ = 12V
BOOTSTRAPPED
(INTERNAL RESISTORS)
-60 -40 -20
0
20 40 60 80 100 120 140
0.1
1
10
(kΩ)
100
1000
-60 -40 -20
0
20 40 60 80 100 120 140
TEMPERATURE (°C)
R
TEMPERATURE (°C)
LOAD
4
_______________________________________________________________________________________
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , Ste p-Up DC-DC Conve rte rs
Q
1/MAX762
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(Circuit of Figure 2, T = +25°C, unless otherwise noted.)
A
PEAK CURRENT AT LX vs. TEMPERATURE
SHUTDOWN CURRENT
vs. TEMPERATURE
LX LEAKAGE vs. TEMPERATURE
1000
100
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
4.0
3.5
3.0
2.5
V+ = 15V
V+ = 12V
10
1
V+ = 5V
2.0
1.5
1.0
0.5
0
V+ = 15V
V+ = 8V
0.1
V
= 16.5V
LX
V+ = 4V
-60 -40 -20
0.01
20
40
60
80
100 120 140
-60 -40 -20
0
20 40 60 80 100 120 140
0
20 40 60 80 100 120 140
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
SWITCH-ON TIME vs. TEMPERATURE
SWITCH-OFF TIME vs. TEMPERATURE
POWER-SUPPLY CURRENT
vs. TEMPERATURE
8.5
8.0
7.5
2.0
1.5
1.0
100
V+ = 16.5V
V+ = 3V
V+ = 5V
V+ = 5V
90
80
-60
0
60
120
-60
0
60
120
-60
0
60
120
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
SWITCH-ON/SWITCH-OFF TIME RATIO
vs.TEMPERATURE
SHDN RESPONSE TIME
7
6
5
12V
V+ = 5V
5V
4V
0V
2ms/div
-60
0
60
120
TEMPERATURE (°C)
I
= 100mA, V = 5V
IN
LOAD
A: V , 2V/div
OUT
B: SHDN (0V to 4V)
_______________________________________________________________________________________
5
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , Ste p-Up DC-DC Conve rte 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 )
(Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
LOAD–TRANSIENT RESPONSE
LINE–TRANSIENT RESPONSE
200mA
A
6V
A
0mA
4V
B
B
1/MAX762
5µs/div
5ms/div
A: I , (0mA to 200mA)
LOAD
A: V (4V to 6V)
IN
B: V , AC COUPLED, 100mV/div
OUT
B: V , AC COUPLED, 20mV/div
OUT
V
IN
= 5V, V = 12V
I
= 50mA, V = 12V
OUT
OUT OUT
______________________________________________________________P in De s c rip t io n
PIN
NAME
FUNCTION
Low-battery output is an open-drain output that goes low when LBI is less than 1.5V.
Connect to V+ through a pull-up resistor. Leave LBO floating if not used.
LBO
LBI
1
2
Input to the internal low-battery comparator. Tie to GND or V+ if not used.
Feedback input. For fixed-output bootstrapped operation, connect FB to GND. For
adjustable-output bootstrapped operation, connect a resistor divider between V+, FB and
GND. For non-bootstrapped operation, there is no fixed-output option. Connect a resistor
3
4
FB
divider network between V , FB and GND. See Bootstrapped/Non-Bootstrapped
OUT
Modes section.
Active-high TTL/CMOS logic-level input. In shutdown mode (SHDN = V+), the internal
switch is turned off and the output voltage equals V+ minus a diode drop (due to the DC
path from the input to the output). Tie to GND for normal operation.
SHDN
1.5V reference output that can source 100µA for external loads. Bypass with 0.1µF
or larger capacitor.
5
6
7
8
REF
GND
LX
Ground
Drain of the internal N-channel FET. LX has an output resistance of 1Ω and a peak current
limit of 1A.
V+
Power-supply input. In bootstrapped mode, V+ is also the output voltage sense input.
6
_______________________________________________________________________________________
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , Ste p-Up DC-DC Conve rte rs
Q
1/MAX762
LBO
V+
FB
DUAL-MODE
COMPARATOR
MAX761
MAX762
LBI
N
LBI
100mV
ERROR
COMPARATOR
REF
V+
1.5V
REFERENCE
Q
TRIG
ONE-SHOT
N
Q
S
R
LOW INPUT
VOLTAGE
OSCILLATOR
UNDER VOLTAGE
COMPARATOR
2.5V
Q
TRIG
ONE-SHOT
LX
CURRENT
COMPARATOR
N
0.2V
0.1V
CURRENT CONTROL
CIRCUITRY
GND
Figure 1. Simple Block Diagram
because of their 300kHz switching frequency, (2) the
current-limited PFM control scheme allows 86% efficien-
cies over a wide range of load currents, and (3) the max-
imum supply current is only 110µA.
________________De t a ile d De s c rip t io n
Op e ra t in g P rin c ip le
The MAX761/MAX762 BiCMOS step-up switch-mode
power supplies provide fixed outputs of 12V and 15V,
respectively. They have a unique control scheme that
combines the advantages of pulse-frequency modulation
(low supply current) and pulse-width modulation (high
efficiency at high loads). The internal N-channel power
MOSFET allows 1A peak currents, increasing the output
current capability over previous pulse-frequency-modu-
la tion (PFM) d e vic e s . Fig ure 1 s hows the MAX761/
MAX762 block diagram.
Bo o t s t ra p p e d /No n -Bo o t s t ra p p e d Mo d e s
Figures 2 and 3 show the standard application circuits
for bootstrapped and non-bootstrapped modes. In boot-
stra p p e d mode , the IC is p owe re d from the outp ut
(V ). In other words, the current needed to power the
OUT
bootstrapped circuit is different from the V+ current the
chip consumes. The voltage applied to the gate of the
internal N-channel FET is switched from V
to ground,
OUT
providing more switch-gate drive and increasing the effi-
ciency of the DC-DC converter compared with non-boot-
strapped operation.
The MAX761/MAX762 offer three main improvements
over prior solutions: (1) the converters operate with tiny
s urfa c e -mount ind uc tors (le s s tha n 5mm d ia me te r)
_______________________________________________________________________________________
7
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , Ste p-Up DC-DC Conve rte rs
Q
L1
18µH
D1
1N5817
ADJUSTABLE
OUTPUT (V
L1
18µH
D1
1N5817
V
IN
V
IN
=
+12V at
150mA
)
OUT
+5V
C4
V
OUT
C1
33µF
C4
33µF
R2 = R1
(
-1)
V
REF
7
7
LX
LX
5
4
2
3
8
REF
SHDN
LBI
V+
R2
R1
C1
C2
C3
0.1µF
R4
MAX761
MAX761
MAX762
8
3
1
100k
V+
FB
R4
R3
2
5
C2
0.1µF
LBI
100k
REF
R3
C3
1
4
FB
LBO
SHDN
LBO
LOW-BATTERY
OUTPUT
LOW-BATTERY
DETECT OUTPUT
GND
6
GND
6
1/MAX762
LOW-BATTERY
DETECT
V
- V
TRIP REF
R4 = R3
(
)
V
REF
V
= 1.5V NOMINAL
REF
C1 = 33µF
C2 = 0.1µF
C3 = 0.1µF
C4 = 33µF
Figure 2. Bootstrapped Operating Circuit
In non-bootstrapped mode, the IC is powered from the
supply voltage, V , and operates with minimum supply
IN
Figure 3. Non-Bootstrapped Operating Circuit
current. Since the voltage applied to the gate of the inter-
nal FET is reduced, efficiency declines with low input
voltages. Note: In non-bootstrapped mode, there is no
fixed-output operation; external resistors must be
used to set the output voltage. Use 1% external feed-
back resistors when operating in non-bootstrapped
mode (Figure 3).
rent limit and a pair of one-shots that set the maximum
on-time (8µs) a nd minimum off-time (1.3µs) for the
switch. Once off, the minimum off-time one-shot holds
the switch off for 1.3µs. 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.
Use bootstrapped mode when V is below approxi-
IN
mately 4V. For V between 4V and 6V, the trade-off is
IN
The MAX761/MAX762 also limit the peak inductor cur-
rent, allowing the devices to run in continuous-conduc-
tion mode (CCM) and maintain high efficiency with
heavy loads (Figure 4a). 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 (8µs later), or (2) the current
limit is reached.
lower supply current in non-bootstrapped mode versus
hig he r outp ut c urre nt in b oots tra p p e d mod e (s e e
Typical Operating Characteristics).
P u ls e -Fre q u e n c y Mo d u la t io n
(P FM) Co n t ro l S c h e m e
The MAX761/MAX762 use a proprietary current-limited
PFM control scheme. This control scheme combines
the ultra-low supply current of pulse-skipping PFM con-
verters with the high full-load efficiency characteristic of
current-mode pulse-width-modulation (PWM) convert-
ers. It allows the devices to achieve high efficiency over
a wide range of loads, while the current-sense function
and high operating frequency allow the use of tiny
external components.
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 the peak current limit. If the
output voltage is still out of regulation after two pulses,
the current limit for the next pulse is raised to the full
current limit of 1A (Figure 4b).
As with traditional PFM converters, the internal power
MOSFET is turned on when the voltage comparator
s e ns e s the outp ut is out of re g ula tion (Fig ure 1).
However, unlike traditional PFM converters, switching is
accomplished through the combination of a peak cur-
In t e rn a l vs . Ex t e rn a l Re s is t o rs
When external feedback resistors are used, an internal
undervoltage lockout system prevents start-up until V+
rises to about 2.7V. When external feedback resistors are
8
_______________________________________________________________________________________
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , Ste p-Up DC-DC Conve rte rs
Q
1/MAX762
1A
1A
500mA
500mA
0A
Figure 4a. CCM, Heavy Load Current Waveform (500mA/div)
Figure 4b. Light/Medium Load Current Waveform (500mA/div)
used in a bootstrapped circuit (Figure 5), undervoltage
loc kout p re ve nts s ta rt-up a t low inp ut volta g e s ; b ut
once started, operation can continue down to a lower
voltage that depends on the load.
ringing (the inductor's self-resonant frequency). This
ringing is normal and poses no operational problems.
Lo w -Ba t t e ry De t e c t o r
The MAX761/MAX762 provide a low-battery comparator
that compares the voltage on LBI to the 1.5V reference
There is no undervoltage lockout when the internal feed-
back resistors are used (Figure 2), and special circuitry
guarantees start-up at 2.0V. The start-up circuitry fixes
the duty cycle at 50% until V+ is driven to 2.5V, above
which the normal control system takes over.
voltage. When the LBI voltage is below V , LBO (an
REF
open-drain output) goes low. The low-battery compara-
tor’s 20mV of hysteresis adds noise immunity, prevent-
ing repeated triggering of LBO. Use a resistor-divider
network between V+, LBI, and GND to set the desired
S h u t d o w n Mo d e
The MAX761/MAX762 e nte r s hutd own mod e whe n
SHDN is high. In this mode, the internal biasing circuitry
trip voltage V
(Figure 3). When SHDN is high, LBI is
TRIP
ig nore d a nd LBO is hig h imp e d a nc e . The va lue of
resistor R3 should be no larger than 500kΩ to ensure
the LBI leakage current does not cause inaccuracies in
is turned off (including the reference) and V
equals
OUT
V+ minus a diode drop (due to the DC path from the
input to the output). In shutdown mode, the supply cur-
rent drops to less than 5µA. SHDN is a TTL/CMOS logic
level input. Connect SHDN to GND for normal operation.
LBO is high impedance during shutdown.
V
TRIP
.
__________________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 MAX761/MAX762’s output voltage can be adjusted
from 5V to 16.5V using external resistors R1 and R2
configured as shown in Figures 3 and 5. For adjustable-
output operation, select feedback resistor R1 in the
10kΩ to 250kΩ range. Higher R1 values within this
range give lowest supply current and best light-load
efficiency. R2 is given by:
Mo d e s o f Op e ra t io n
When delivering high output currents, the MAX761/
MAX762 operate in CCM. In this mode, current always
flows in the inductor, and the control circuit adjusts the
switch’s duty cycle on a cycle-by-cycle basis to maintain
regulation without exceeding the switch-current capabili-
ty. This provides excellent load-transient response and
high efficiency.
V
OUT
R2 = (R1)(
- 1)
V
REF
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 on each cycle. Although effi-
ciency is still excellent, the switch waveforms contain
where V
= 1.5V.
REF
Note: Tie FB to GND for fixed-output operation
(bootstrapped mode only).
_______________________________________________________________________________________
9
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , Ste p-Up DC-DC Conve rte rs
Q
Inductors with a ferrite core or equivalent are recom-
D1
1N5817
L1
18µH
mended. The inductor’s incremental saturation-current
rating should be greater than the 1A peak current limit. It
is generally acceptable to bias the inductor into satura-
tion by approximately 20% (the point where the induc-
tance is 20% below the nominal value). For highest effi-
ciency, use a coil with low DC resistance, preferably
under 100mΩ. To minimize radiated noise, use a toroid,
a pot core, or a shielded coil.
V
V
IN
OUT
C1
C4
C2
7
LX
8
MAX761
MAX762
5
V+
REF
Table 1 lists inductor types and suppliers for various
applications. The listed surface-mount inductors’ efficien-
cies are nearly equivalent to those of the larger through-
hole inductors.
C3
2
4
R2
LBI
3
FB
SHDN
Dio d e S e le c t io n
The MAX761/MAX762’s hig h s witc hing fre q ue nc y
demands a high-speed rectifier. Use a Schottky diode
with a 1A average current rating, such as a 1N5817. For
high-temperature applications, use a high-speed silicon
diode, such as the MUR105 or the EC11FS1. These
d iod e s ha ve lowe r hig h-te mp e ra ture le a ka g e tha n
Schottky diodes (Table 1).
R1
GND
6
7
C1 = 33µF
C2 = 0.1µF
C3 = 0.1µF
C4 = 33µF
V
V
OUT
R2 = R1
(
-1)
V
= 1.5V NOMINAL
REF
REF
Figure 5. Bootstrapped Operation with Adjustable Output
S e le c t in g t h e In d u c t o r (L)
In both CCM and DCM, practical inductor values range
from 10µH to 50µH. If the inductor value is too low, the
current in the coil will ramp up to a high level before the
current-limit comparator can turn off the switch. The mini-
Ca p a c it o r S e le c t io n
Output Filter Capacitor
The primary criterion for selecting the output filter capac-
itor (C4) is low effective series resistance (ESR). The
product of the inductor current variation and the output
filter capacitor’s ESR determines the amplitude of the
high-frequency ripple seen on the output voltage. A
33µF, 16V Sanyo OS-CON capacitor with 100mΩ ESR
typically provides 100mV ripple when stepping up from
5V to 12V at 150mA.
mum on-time for the switch (t ) is approximately
ON(min)
2.5µs, so select an inductance that allows the current to
ramp up to I in no less than 2.5µs. Choosing a value
/
LIM 2
of I
/ allows the half-size pulses to occur, giving high-
LIM 2
er light-load efficiency and minimizing ripple. Hence, cal-
culate the minimum inductance value as:
Because the output filter capacitor’s ESR affects efficien-
cy, use low-ESR capacitors for best performance. The
smallest low-ESR SMT tantalum capacitors currently
available are the Sprague 595D series. Sanyo OS-CON
organic semiconductor through-hole capacitors and
Nichicon PL series also exhibit very low ESR. Table 1
lists some suppliers of low-ESR capacitors.
(V
)(t
)
IN(max) ON(min)
L
≥
I
LIM/2
OR
L
≥
(V
IN(max)
)(5)
where V
is in volts and L is in microhenries.
IN(max)
The coil’s inductance need not satisfy this criterion
exactly, as the circuit can tolerate a wide range of val-
ues. Larger inductance values tend to produce physical-
ly larger coils and increase the start-up time, but are oth-
erwise acceptable. Smaller inductance values allow the
coil current to ramp up to higher levels before the switch
can turn off, producing higher ripple at light loads. In
general, an 18µH inductor is sufficient for most applica-
Input Bypass Capacitors
The input bypass capacitor, C1, reduces peak currents
drawn from the voltage source, and also reduces noise
at the voltage source caused by the MAX761/MAX762’s
switching action. 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. For output currents up to
250mA, 33µF (C1) is adequate, although smaller bypass
capacitors may also be acceptable. Bypass the IC sepa-
rately with a 0.1µF ceramic capacitor, C2, placed close
to the V+ and GND pins.
tions (V ≤ 5V). An 18µH inductor is appropriate for
IN
input voltages up to 3.6V, as calculated above. However,
the same 18µH coil can be used with input voltages up
to 5V with only small increases in peak current, as shown
in Figures 4a and 4b.
10 ______________________________________________________________________________________
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
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Q
1/MAX762
Reference Capacitor
Bypass REF with a 0.1µF capacitor. REF can source up
to 100µA.
Connect a pull-up resistor (e.g., 100kΩ) between LBO
and V . Tie LBI to GND or V+ and leave LBO floating
if the low-battery detector is not used.
OUT
___________Ap p lic a t io n s In fo rm a t io n
S e t t in g t h e Lo w -Ba t t e ry De t e c t o r Vo lt a g e
To set the low-battery detector’s falling trip voltage
La yo u t Co n s id e ra t io n s
Proper PC board layout is essential because of high cur-
rent levels and fast switching waveforms that radiate
noise. Minimize ground noise by connecting GND, the
input bypass-capacitor ground lead, and the output filter-
capacitor ground lead to a single point (star ground con-
figuration). Also minimize lead lengths to reduce stray
capacitance, trace resistance, and radiated noise. The
traces connected to FB and LX, in particular, must be
short. Place bypass capacitor C2 as close as possible to
V+ and GND.
(V
), select R3 between 10kΩ and 500kΩ (Figures 2
TRIP
and 3), and calculate R4 as follows:
(V
- V
)
TRIP
REF
R4 = R3 [
]
V
REF
where VREF = 1.5V.
The rising trip voltage is higher because of the compara-
tor’s hysteresis of approximately 20mV, and can be cal-
culated by:
V
TRIP
(rising) = (V
+ 20mV)(1 + R4/R3).
REF
Connect a high-value resistor (larger than R3 + R4)
between LBI and LBO if additional hysteresis is required.
Table 1. Component Suppliers
CAPACITORS
PRODUCTION METHOD
INDUCTORS
DIODES
Sumida
CD54-180 (22µH)
Matsuo
267 series
Nihon
EC10 series
Surface Mount
Coiltronics
CTX 100-series
Sanyo
Sumida
OS-CON series
Low-ESR organic
semiconductor
Miniature Through-Hole
Low-Cost Through-Hole
RCH855-180M
Motorola
1N5817,
MUR105
Nichicon
PL series
Low-ESR electrolytics
Renco
RL 1284-18
United Chemi-Con
LXF series
Coiltronics
Matsuo
Matsuo
Nichicon
Nihon
Renco
Sanyo
Sanyo
Sumida
Sumida
(USA)
(407) 241-7876
(714) 969-2491
81-6-337-6450
(708) 843-7500
(805) 867-2555
(516) 586-5566
(619) 661-6835
(0720) 70-1005
(708) 956-0666
81-3-607-5111
(714) 255-9500
FAX (407) 241-9339
FAX (714) 960-6492
FAX 81-6-337-6456
FAX (708) 843-2798
FAX (805) 867-2556
FAX (516) 586-5562
FAX (619) 661-1055
FAX (0720) 70-1174
(USA)
(Japan)
(USA)
(USA)
(USA)
(USA)
(Japan)
(USA)
(Japan)
FAX 81-3-607-5144
FAX (714) 255-9400
United Chem-Con (USA)
______________________________________________________________________________________ 11
1 2 V/1 5 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w I , Ste p-Up DC-DC Conve rte rs
Q
___________________Ch ip To p o g ra p h y
LBO
V+
LX
LBI
0. 142"
(3. 607mm)
FB
1/MAX762
GND
REF
SHDN
0. 080"
(2. 030mm)
TRANSISTOR COUNT: 492;
SUBSTRATE CONNECTED TO V+.
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
© 1993 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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