Maxim MAX766EPA, MAX766ESA, MAX766MJA, MAX765MJA, MAX766C-D Datasheet

...
19-0176; Rev 0; 6/94
Evaluation Kit
Available
High-Efficiency, Low IQDC-DC Inverters
-5V/-12V/-15V or Adjustable,
_______________General Description
The MAX764/MAX765/MAX766 inverting switching regu­lators are highly efficient over a wide range of load cur­rents, delivering up to 1.5W. A unique, current-limited, pulse-frequency-modulated (PFM) control scheme com­bines the benefits of traditional PFM converters with the benefits of pulse-width-modulated (PWM) converters. Like PWM converters, the MAX764/MAX765/MAX766 are highly efficient at heavy loads. Yet because they are PFM devices, they use less than 120µA of supply current (vs. 2mA to 10mA for a PWM device).
The input voltage range is 3V to 16V. The output volt­age is preset at -5V (MAX764), -12V (MAX765), or -15V (MAX766); it can also be adjusted from -1V to -16V using two external resistors (Dual ModeTM). The maxi­mum operating VIN- V
differential is 20V.
OUT
These devices use miniature external components; their high switching frequencies (up to 300kHz) allow for less than 5mm diameter surface-mount magnetics. A stan­dard 47µH inductor is ideal for most applications, so no magnetics design is necessary.
An internal power MOSFET makes the MAX764/MAX765/ MAX766 ideal for minimum component count, low- and medium-power applications. For increased output drive capability or higher output voltages, use the MAX774/MAX775/MAX776 or MAX1774, which drive an external power P-channel MOSFET for loads up to 5W.
________________________Applications
LCD-Bias Generators Portable Instruments LAN Adapters Remote Data-Acquisition Systems Battery-Powered Applications
__________Typical Operating Circuit
____________________________Features
High Efficiency for a Wide Range of Load Currents250mA Output Current120µA Max Supply Current5µA Max Shutdown Current3V to 16V Input Voltage Range-5V (MAX764), -12V (MAX765), -15V (MAX766),
or Adjustable Output from -1V to -16V
Current-Limited PFM Control Scheme300kHz Switching FrequencyInternal, P-Channel Power MOSFET
______________Ordering Information
PART
MAX764CPA
MAX764CSA MAX764C/D 0°C to +70°C MAX764EPA MAX764ESA -40°C to +85°C MAX764MJA -55°C to +125°C 8 CERDIP** MAX765CPA MAX765CSA MAX765C/D 0°C to +70°C MAX765EPA MAX765ESA -40°C to +85°C MAX765MJA -55°C to +125°C 8 CERDIP**
Ordering Information continued on last page.
* Dice are tested at T **Contact factory for availability and processing to MIL-STD-883.
TEMP. RANGE PIN-PACKAGE
0°C to +70°C 0°C to +70°C
-40°C to +85°C 8 Plastic DIP
0°C to +70°C 8 Plastic DIP 0°C to +70°C
-40°C to +85°C 8 Plastic DIP
= +25°C, DC parameters only.
A
8 Plastic DIP 8 SO Dice*
8 SO
8 SO Dice*
8 SO
__________________Pin Configuration
MAX764/MAX765/MAX766
INPUT
3V TO 15V
ON/OFF
V+
LX
MAX764
SHDN
REF
FB
________________________________________________________________
OUT
GND
47µH
OUTPUT
-5V
TOP VIEW
OUT
SHDN
REF
1
FB
2
MAX764 MAX765
3
MAX766
4
DIP/SO
Maxim Integrated Products
Call toll free 1-800-998-8800 for free samples or literature.
LX
8
V+
7
V+
6
GND
5
1
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQDC-DC Inverters
ABSOLUTE MAXIMUM RATINGS
V+ to GND..............................................................-0.3V to +17V
OUT to GND...........................................................+0.5V to -17V
Maximum Differential (V+ to OUT) ......................................+21V
REF, SHDN, FB to GND ...............................-0.3V to (V+ + 0.3V)
LX to V+..................................................................+0.3V to -21V
LX Peak Current ...................................................................1.5A
Continuous Power Dissipation (T
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.
= +70°C)
A
ELECTRICAL CHARACTERISTICS
(V+ = 5V, I
V+ Input Voltage Range Supply Current Shutdown Current
MAX764/MAX765/MAX766
FB Input Current
Output Current and Voltage (Note 1)
Reference Voltage
REF Load Regulation
Efficiency (Note 2)
SHDN Input Voltage High SHDN Input Voltage Low
LOAD
= 0mA, C
= 0.1µF, TA= T
REF
to T
MIN
MAX76_C/E
V+ V
MAX76_M V+ = 16V, SHDN < 0.4V
S
I
SHDN
I
V
V+ = 16V, SHDN > 1.6V V+ = 10V, SHDN > 1.6V 3V V+ 16V MAX76_C MAX76_E
I
FB
MAX76_M MAX764, -4.8V V MAX765C/E, -11.52V V
OUT
MAX765M, -11.52V V MAX766, -14.40V V MAX76_C MAX76_E
REF
MAX76_M 0µA I 3V V+ 16V
0mA I 4V V+ 6V
10mA I VIN= 5V
V+ = 16V, SHDN = 0V or V+ 3V V+ 16V
IH
3V V+ 16V V0.4V
IL
, unless otherwise noted. Typical values are at TA= +25°C.)
MAX
100µA
REF
LOAD
LOAD
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
Maximum Junction Temperatures
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
CONDITIONS
3.0 16.0
3.5 90 120I
2 15
OUT
100mA
100mA,
5.2V
OUT
OUT
-15.60V
OUT
MAX76_C/E MAX76_M
V
12.48V
12.48V
= -5V
OUT
= -15V
OUT
150 260
68 120 50 120 35 105
1.4700 1.5 1.5300
1.4625 1.5 1.5375 V
1.4550 1.5 1.5450 410 415
80 82V
±50 ±70 ±90
UNITSMIN TYP MAXSYMBOLPARAMETER
µA
mV-10 10FB Trip Point
nA
mA
mV
µV/V40 100REF Line Regulation
%/mA0.008Load Regulation (Note 2)
%/V0.12Line Regulation (Note 2)
%
µA±1SHDN Leakage Current
V1.6V
2 _______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQDC-DC Inverters
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, I
LX Leakage Current
Peak Current at LX Maximum Switch On-Time Minimum Switch Off-Time
Note 1: See Maximum Output Current vs. Supply Voltage graph in the Note 2: Circuit of Figure 2.
LOAD
= 0mA, C
= 0.1µF, TA= T
REF
MIN
to T
, unless otherwise noted. Typical values are at TA= +25°C.)
MAX
CONDITIONS
MAX76_C
+ (V+) 20V
ILXI
MAX76_E MAX76_M
V
+ (V+) 10V
OUT
I
I
V
PEAK
ON
OFF
OUT
I
+ (V+) 10V
I
Typical Operating Characteristics
correlation to switch on-time, switch off-time, on-resistance, and peak current rating.
UNITSMIN TYP MAXSYMBOLPARAMETER
±5 ±10 µA ±30
1.4 2.5LX On-Resistance
A0.5 0.75I µs12 16 20t µs1.8 2.3 2.8t
. Guarantees are based on
__________________________________________Typical Operating Characteristics
(V+ = 5V, V
100
90 80 70 60 50 40
EFFICIENCY (%)
30 20 10
0
= -5V, TA= +25°C, unless otherwise noted.)
OUT
EFFICIENCY vs. LOAD CURRENT
V+ = 5V
0.1 10 1000
MAX764
V+ = 10V
V+ = 15V
CIRCUIT OF FIGURE 2
= -5V ±4%
V
OUT
1 100
LOAD CURRENT (mA)
MAX764-01
EFFICIENCY vs. LOAD CURRENT
100
V+ = 8V
90 80 70 60 50 40
EFFICIENCY (%)
30 20 10
0
0.1 10 1000
MAX765
V+ = 5V
CIRCUIT OF FIGURE 2
= -12V ±4%
V
OUT
1 100
LOAD CURRENT (mA)
MAX764-02
EFFICIENCY vs. LOAD CURRENT
100
90 80 70 60 50 40
EFFICIENCY (%)
30 20 10
0
0.1 10 1000
MAX766
V+ = 5V
CIRCUIT OF FIGURE 2
= -15V ±4%
V
OUT
1 100
LOAD CURRENT (mA)
MAX764/MAX765/MAX766
MAX764-03
_______________________________________________________________________________________ 3
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQDC-DC Inverters
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, V
= -5V, TA= +25°C, unless otherwise noted.)
OUT
MAXIMUM OUTPUT CURRENT
600
500
400
300
200
100
MAXIMUM OUTPUT CURRENT (mA)
0
4.0
3.5
MAX764/MAX765/MAX766
3.0
2.5
2.0
1.5
1.0
SHUTDOWN CURRENT (µA)
0.5 0
7.2
7.1
7.0
6.9
6.8
6.7
6.6
6.5
6.4
SWITCH ON/OFF-TIME RATIO (µs/µs)
6.3
6.2
vs. SUPPLY VOLTAGE
CIRCUIT OF FIGURE 2
V
= -5V
OUT
V
= -12V
OUT
V
= -15V
OUT
3
4 5 6 7 8 9 10 11 12 13 14 15 16
SUPPLY VOLTAGE (V)
SHUTDOWN CURRENT
vs. TEMPERATURE
V+ = 15V
V+ = 8V
V+ = 4V
-60
-40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
SWITCH ON/OFF-TIME RATIO
vs. TEMPERATURE
V+ = 5V
-60
-40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
MAX764 -04
NO-LOAD SUPPLY CURRENT (µA)
MAX764 -07
MAXIMUM SWITCH ON-TIME (µs)
MAX764 -10
START-UP SUPPLY VOLTAGE (V)
NO-LOAD SUPPLY CURRENT
100
95 90 85 80 75 70 65 60
17.0
16.8
16.6
16.4
16.2
16.0
15.8
15.6
15.4
15.2
15.0
vs. SUPPLY VOLTAGE
3
4 5 6 7 8 9 10 11 12 13 14 15 16
SUPPLY VOLTAGE (V)
MAXIMUM SWITCH ON-TIME
vs. TEMPERATURE
V+ = 5V
-60
-40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
START-UP SUPPLY VOLTAGE
8 7 6 5 4 3 2 1 0
vs. OUTPUT CURRENT
CIRCUIT OF FIGURE 2
0
50 100 150 200 250 300
OUTPUT CURRENT (mA)
V+ = 15V
110 105
MAX764 -05
100
95 90 85 80 75 70 65 60
NO-LOAD SUPPLY CURRENT (µA)
55 50
-60
-40 -20 0 20 40 60 80 100 120 140
2.60
2.55
MAX764 -08
2.50
2.45
2.40
2.35
2.30
MINIMUM SWITCH OFF-TIME (µs)
2.25
2.20
-60
10,000
MAX764 -11
1000
100
10
LX LEAKAGE CURRENT (nA)
1
20
NO-LOAD SUPPLY CURRENT
vs. TEMPERATURE
V+ = 15V
V+ = 5V
TEMPERATURE (°C)
MINIMUM SWITCH OFF-TIME
vs. TEMPERATURE
V+ = 15V
V+ = 5V
-40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
LX LEAKAGE CURRENT
vs. TEMPERATURE
IV
I + (V+) = 20V
OUT
30 40 50 60 70 80 90 100 110 120 130
TEMPERATURE (°C)
MAX764 -06
MAX764 -09
MAX764-12
4 _______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
MAX764
14
0
High-Efficiency, Low IQDC-DC Inverters
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, V
2.2
2.0
1.8
1.6
1.4
1.2
LX ON-RESISTANCE ()
1.0
0.8
= -5V, TA= +25°C, unless otherwise noted.)
OUT
LX ON-RESISTANCE vs. TEMPERATURE
IV
I + (V+) = 10V
OUT
IV
I + (V+) = 15V
OUT
IV
I + (V+) = 20V
OUT
-60
-40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
REFERENCE OUTPUT
1.506
1.504
1.502
1.500
1.498
1.496
REFERENCE OUTPUT (V)
1.494
1.492
-60
vs. TEMPERATURE
-40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
0.95
MAX764 -13
0.90
0.85
0.80
0.75
CURRENT AT LX (A)
0.70
0.65
PEAK CURRENT AT LX
vs. TEMPERATURE
IV
I + (V+) = 20V
OUT
IV
I + (V+) = 15V
OUT
IV
I + (V+) = 10V
OUT
-40 -20 0 20 40 60 80 100 120 14
-60 TEMPERATURE (°C)
MAX764 -16
1000
100
10
1
SUPPLY CURRENT (mA)
0.1
0.01
REFERENCE OUTPUT RESISTANCE
250
-
200
150
100
50
REFERENCE OUTPUT RESISTANCE ()
0
-60
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
0 2 4 6 10 12 14816
SUPPLY VOLTAGE (V)
vs. TEMPERATURE
I
= 10µA
REF
I
= 50µA
REF
I
= 100µA
REF
-40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C)
I
= 100mA
LOAD
I
= 0mA
LOAD
CIRCUIT OF FIGURE 2
MAX764-17
MAX764/MAX765/MAX766
MAX764 -15
_______________________________________________________________________________________
5
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQDC-DC Inverters
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, V
= -5V, TA= +25°C, unless otherwise noted.)
OUT
TIME TO ENTER/EXIT SHUTDOWN
0V
A
0V
CIRCUIT OF FIGURE 2, V+ = 5V, I A: V
, 2V/div
OUT
B: SHUTDOWN PULSE, 0V TO 5V, 5V/div
2ms/div
LOAD
= 100mA, V
OUT
B
= -5V
MAX764/MAX765/MAX766
LINE-TRANSIENT RESPONSE
A
B
0V
LOAD-TRANSIENT RESPONSE
0mA
CIRCUIT OF FIGURE 2, V+ = 5V, V A: V
, 50mV/div, AC-COUPLED
OUT
B: I
, 0mA TO 100mA, 100mA/div
LOAD
DISCONTINUOUS CONDUCTION AT
HALF AND FULL CURRENT LIMIT
0A
0V
5ms/div
A
B
= -5V
OUT
A
B
C
CIRCUIT OF FIGURE 2, V A: V
, 50mV/div, AC-COUPLED
OUT
B: V+, 5V TO 10V, 5V/div
OUT
5ms/div
= -5V, I
LOAD
= 100mA
CIRCUIT OF FIGURE 2, V+ = 5V, V A: OUTPUT RIPPLE, 100mV/div B: INDUCTOR CURRENT, 500mA/div C: LX WAVEFORM, 10V/div
5µs/div
OUT
= -5V, I
6 _______________________________________________________________________________________
LOAD
= 140mA
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQDC-DC Inverters
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, V
0A
0V
= -5V, TA= +25°C, unless otherwise noted.)
OUT
DISCONTINUOUS CONDUCTION AT
HALF CURRENT LIMIT
CIRCUIT OF FIGURE 2, V+ = 5V, V A: OUTPUT RIPPLE, 100mV/div B: INDUCTOR CURRENT, 500mA/div C: LX WAVEFORM, 10V/div
5µs/div
OUT
= -5V, I
LOAD
= 80mA
CONTINUOUS CONDUCTION AT
FULL CURRENT LIMIT
A
B
C
0A
0V
CIRCUIT OF FIGURE 2, V+ = 5V, V A: OUTPUT RIPPLE, 100mV/div B: INDUCTOR CURRENT, 500mA/div C: LX WAVEFORM, 10V/div
5µs/div
= -5V, I
LOAD
= 240mA
OUT
A
B
C
______________________________________________________________Pin Description
PIN
Sense Input for Fixed-Output Operation (VFB= V
FB2
SHDN3
Feedback Input. Connect FB to REF to use the internal voltage divider for a preset output. For adjustable­output operation, use an external voltage divider, as described in the section
Active-High Shutdown Input. With SHDN high, the part is in shutdown mode and the supply current is less than 5µA. Connect to ground for normal operation.
1.5V Reference Output that can source 100µA for external loads. Bypass to ground with a 0.1µF capacitor.REF4 GroundGND5
V+6, 7
Positive Power-Supply Input. Must be tied together. Place a 0.1µF input bypass capacitor as close to
the V+ and GND pins as possible.
Drain of the Internal P-Channel Power MOSFET. LX has a peak current limit of 0.75A.LX8
FUNCTIONNAME
). OUT must be connected to V
REF
.OUT1
OUT
Setting the Output Voltage.
MAX764/MAX765/MAX766
_______________________________________________________________________________________ 7
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQDC-DC Inverters
FB
COMPARATOR
REF
SHDN
MAX764/MAX765/MAX766
Figure 1. Block Diagram
ERROR
COMPARATOR
TRIG Q
ONE-SHOT
ONE-SHOT
TRIGQ
SRQ
CURRENT
CONTROL CIRCUITS
_______________Detailed Description
The MAX764/MAX765/MAX766 are BiCMOS, inverting, switch-mode power supplies that provide fixed outputs of -5V, -12V, and -15V, respectively; they can also be set to any desired output voltage using an external resistor divider. Their unique control scheme combines the advantages of pulse-frequency modulation (pulse skipping) and pulse-width modulation (continuous puls­ing). The internal P-channel power MOSFET allows peak currents of 0.75A, increasing the output current capability over previous pulse-frequency-modulation (PFM) devices. Figure 1 shows the MAX764/MAX765/ MAX766 block diagram.
The MAX764/MAX765/MAX766 offer three main improvements over prior solutions:
Operating Principle
MAX764 MAX765 MAX766
N
1.5V
REFERENCE
FROM V+
P
COMPARATOR
GND
CURRENT
FROM OUT
(FULL
CURRENT)
0.2V
0.1V (HALF CURRENT)
FROM V+
1) They can operate with miniature (less than 5mm diameter) surface-mount inductors, because of their 300kHz switching frequency.
2) The current-limited PFM control scheme allows efficien­cies exceeding 80% over a wide range of load currents.
3) Maximum quiescent supply current is only 120µA.
Figures 2 and 3 show the standard application circuits for these devices. In these configurations, the IC is powered from the total differential voltage between the input (V+) and output (V
). The principal benefit of
OUT
this arrangement is that it applies the largest available signal to the gate of the internal P-channel power MOS­FET. This increased gate drive lowers switch on-resis­tance and increases DC-DC converter efficiency.
Since the voltage on the LX pin swings from V+ (when the switch is ON) to IV
plus a diode drop (when the
OUT
I
OUT
V+
V+
LX
8 _______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQDC-DC Inverters
switch is OFF), the range of input and output voltages is limited to a 21V absolute maximum differential voltage.
When output voltages more negative than -16V are required, substitute the MAX764/MAX765/MAX766 with Maxim’s MAX774/MAX775/MAX776 or MAX1774, which use an external switch.
V
IN
-12
-15
5
-5
GND
V+
V+
LX
47µH
7
6
8
L1
INPUT
VOLTAGE (V)
3 to 15
3 to 8 3 to 5
7
V+
6
V+
8
LX
L1
47µH
D1
1N5817
1N5817
V
OUT
C4
68µF
20V
V
OUT
D1
-1V to
-16V
C4
68µF
20V
1
OUT
C2
C1
120µF
20V
0.1µF 3
MAX764
SHDN
MAX765
MAX766
2
FB
4
REF
GND
5
OUTPUT
VOLTAGE (V)
0.1µF
C3
PRODUCT
MAX764 MAX765
MAX766
Figure 2. Fixed Output Voltage Operation
V
IN
C1
120µF
20V
C2
0.1µF
R2
1
OUT
3
MAX764
SHDN
MAX765
MAX766
2
FB
R1
4
REF
C3
0.1µF
Figure 3. Adjustable Output Voltage Operation
_______________________________________________________________________________________ 9
The MAX764/MAX765/MAX766 use a proprietary, cur-
PFM Control Scheme
rent-limited PFM control scheme that blends the best features of PFM and PWM devices. It combines the ultra-low supply currents of traditional pulse-skipping PFM converters with the high full-load efficiencies of current-mode pulse-width modulation (PWM) convert­ers. This control scheme allows the devices to achieve high efficiencies over a wide range of loads, while the current-sense function and high operating frequency allow the use of miniature external components.
As with traditional PFM converters, the internal power MOSFET is turned on when the voltage comparator senses that the output is out of regulation (Figure 1). 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 on-time (16µs) and minimum off-time (2.3µs) for the switch. 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 MAX764/MAX765/MAX766 limit the peak inductor
current, which allows them to run in continuous-con­duction mode and maintain high efficiency with heavy loads. (See the photo Continuous Conduction at Full Current Limit in the
Typical Operating Characteristics
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 is set to half the peak current limit for the first two pulses. 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 is raised to its 0.75A peak for the next pulse. (See the photo Discontinuous Conduction at Half and Full Current Limit in the
Typical Operating Characteristics
.)
Shutdown Mode
When SHDN is high, the MAX764/MAX765/MAX766 enter a shutdown mode in which the supply current drops to less than 5µA. In this mode, the internal biasing circuitry (including the reference) is turned off and OUT discharges to ground. SHDN is a TTL/CMOS-logic level input. Connect SHDN to GND for normal operation. With a current-limited supply, power-up the device while unloaded or in shutdown mode (hold SHDN high until V+ exceeds 3.0V) to save power and reduce power-up cur­rent surges. (See the Supply Current vs. Supply Voltage graph in the
Typical Operating Characteristics
.)
MAX764/MAX765/MAX766
.)
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQDC-DC Inverters
When delivering high output currents, the MAX764/ MAX765/MAX766 operate in continuous-conduction mode. In this mode, current always flows in the induc­tor, and the control circuit adjusts the duty-cycle of the switch on a cycle-by-cycle basis to maintain regulation without exceeding the switch-current capability. This provides excellent load-transient response and high efficiency.
In discontinuous-conduction mode, current through the inductor starts at zero, rises to a peak value, then ramps down to zero on each cycle. Although efficiency is still excellent, the output ripple may increase slightly.
__________________Design Procedure
Modes of Operation
The MAX764/MAX765/MAX766’s output voltage can be adjusted from -1.0V to -16V using external resistors R1 and R2, configured as shown in Figure 3. For adjustable-output operation, select feedback resistor R1 = 150k. R2 is given by:
MAX764/MAX765/MAX766
where V For fixed-output operation, tie FB to REF.
In both continuous- and discontinuous-conduction modes, practical inductor values range from 22µH to 68µH. 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, wasting power and reducing efficiency. The maximum inductor value is not critical. A 47µH inductor is ideal for most applications.
For highest efficiency, use a coil with low DC resis­tance, preferably under 100m. To minimize radiated noise, use a toroid, pot core, or shielded coil. Inductors with a ferrite core or equivalent are recom­mended. The inductor’s incremental saturation-current rating should be greater than the 0.75A peak current limit. It is generally acceptable to bias the inductor into saturation by approximately 20% (the point where the inductance is 20% below the nominal value).
Table 1 lists inductor types and suppliers for various applications. The listed surface-mount inductors’ effi­ciencies are nearly equivalent to those of the larger­size through-hole inductors.
REF
= 1.5V.
Setting the Output Voltage
V
R2 = (R1) I———
V
OUT REF
I
Inductor Selection
The MAX764/MAX765/MAX766’s high switching fre-
Diode Selection
quency demands a high-speed rectifier. Use a Schottky diode with a 0.75A average current rating, such as the 1N5817 or 1N5818. High leakage currents may make Schottky diodes inadequate for high-temper­ature and light-load applications. In these cases you can use high-speed silicon diodes, such as the MUR105 or the EC11FS1. At heavy loads and high temperatures, the benefits of a Schottky diode’s low for­ward voltage may outweigh the disadvantages of its high leakage current.
Capacitor Selection
Output Filter Capacitor
The primary criterion for selecting the output filter capacitor (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 68µF, 20V Sanyo OS-CON capacitor with ESR = 45m(SA series) typically provides 50mV ripple when converting from 5V to -5V at 150mA.
Output filter capacitor ESR also affects efficiency. To obtain optimum performance, use a 68µF or larger, low-ESR capacitor with a voltage rating of at least 20V. The smallest low-ESR surface-mount tantalum capacitors currently available are from the Sprague 595D series. Sanyo OS-CON series organic semi­conductors and AVX TPS series tantalum capacitors also exhibit very low ESR. OS-CON capacitors are particularly useful at low temperatures. Table 1 lists some suppliers of low-ESR capacitors.
For best results when using capacitors other than those suggested in Table 1 (or their equivalents), increase the output filter capacitor’s size or use capacitators in parallel to reduce ESR.
Input Bypass Capacitor
The input bypass capacitor, C1, reduces peak currents drawn from the voltage source and reduces the amount of noise at the voltage source caused by the switching action of the MAX764–MAX766. 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 highly recommended. For output currents up to 250mA, a 100µF to 120µF capacitor with a voltage rating of at least 20V (C1) in parallel with a 0.1µF capacitor (C2) is adequate in most applications. C2 must be placed as close as possi-
ble to the V+ and GND pins.
10 ______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQDC-DC Inverters
Bypass REF with a 0.1µF capacitor (C3). The REF out-
Reference Capacitor
put can source up to 100µA for external loads.
Layout Considerations
Proper PC board layout is essential to reduce noise generated by high current levels and fast switching waveforms. 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 configuration). Also minimize lead lengths to reduce stray capacitance, trace resistance, and radiat­ed noise. In particular, keep the traces connected to FB and LX short. C2 must be placed as close as pos- sible to the V+ and GND pins. If an external resistor divider is used (Figure 3), the trace from FB to the resis­tors must be extremely short.
Table 1. Component Suppliers
PRODUCTION METHOD INDUCTORS CAPACITORS DIODES
Sumida CD75/105 series
Surface Mount
Miniature Through-Hole
Low-Cost Through-Hole
SUPPLIER PHONE FAX
AVX USA: (803) 448-9411 (803) 448-1943 Coilcraft USA: (708) 639-6400 (708) 639-1469 Coiltronics USA: (407) 241-7876 (407) 241-9339
Matsuo Motorola USA: (800) 521-6274 (602) 952-4190 Nichicon
Nihon Renco USA: (516) 586-5566 (516) 586-5562 Sanyo OS-CON Sprague Electric Co. USA: (603) 224-1961 (603) 224-1430 Sumida
Coiltronics CTX series
Coilcraft DT/D03316 series
Sumida RCH895 series
Renco RL1284 series
USA: (714) 969-2491 Japan: 81-6-337-6450
USA: (708) 843-7500 Japan: 81-7-5231-8461
USA: (805) 867-2555 Japan: 81-3-3494-7411
USA: (619) 661-6835 Japan: 81-7-2070-1005
USA: (708) 956-0666 Japan: 81-3-3607-5111
Matsuo 267 series
Sprague 595D/293D series
AVX TPS series
Sanyo OS-CON series (very low ESR)
Nichicon PL series
(714) 960-6492 81-6-337-6456
(708) 843-2798 81-7-5256-4158
(805) 867-2556 81-3-3494-7414
(619) 661-1055 81-7-2070-1174
(708) 956-0702 81-3-3607-5144
Nihon EC10QS02L (Schottky)
EC11FS1 (high-speed silicon)
Motorola 1N5817, 1N5818, (Schottky) MUR105 (high-speed silicon)
MAX764/MAX765/MAX766
______________________________________________________________________________________ 11
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQDC-DC Inverters
_Ordering Information (continued) ___________________Chip Topography
PART
MAX766CPA
MAX766CSA MAX766C/D 0°C to +70°C MAX766EPA MAX766ESA -40°C to +85°C MAX766MJA -55°C to +125°C 8 CERDIP**
* Dice are tested at TA= +25°C, DC parameters only. **Contact factory for availability and processing to MIL-STD-883.
TEMP. RANGE PIN-PACKAGE
0°C to +70°C 0°C to +70°C
-40°C to +85°C 8 Plastic DIP
8 Plastic DIP 8 SO Dice*
8 SO
OUT
LX
0.145"
FB
(3683µm)
V+
MAX764/MAX765/MAX766
SHDN
REF
0.080"
(2032µm)
TRANSISTOR COUNT: 443 SUBSTRATE CONNECTED TO V+
V+
GND
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
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© 1994 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
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