Maxim MAX774CPA -5v/-12v/-15v or adjustable, high-efficiency, low iq inverting dc-dc controller Datasheet

19-0191; Rev 1; 3/94
TION KIT
EVALUA
LE
AVAILAB
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
________________________Applications
LCD-Bias Generators
____________________________Features
♦ 85% Efficiency for 5mA to 1A Load Currents
♦ Up to 5W Output Power
♦ 100µA Max Supply Current
♦ 5µA Max Shutdown Current
♦ 3V to 16.5V Input Range
♦ -5V (MAX774), -12V (MAX775), -15V (MAX776),
or Adjustable Output Voltage
♦ Current-Limited PFM Control Scheme
♦ 300kHz Switching Frequency
______________Ordering Information
TEMP. RANGE
PIN-PACKAGE
MAX774CPA
PART
0°C to +70°C
8 Plastic DIP
MAX774CSA
MAX774C/D
MAX774EPA
MAX774ESA
MAX774MJA
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
Ordering Information continued on last page.
* Contact factory for dice specifications.
High-Efficiency DC-DC Converters
Battery-Powered Applications
Data Communicators
__________Typical Operating Circuit
INPUT
3V TO 16V
__________________Pin Configuration
TOP VIEW
V+
MAX774
ON/OFF
CS
OUT
1
FB
2
SHDN
3
REF
4
8
GND
7
EXT
6
CS
5
V+
SHDN
EXT
P
OUTPUT
-5V
FB
REF
MAX774
MAX775
MAX776
DIP/SO
GND
OUT
________________________________________________________________ Maxim Integrated Products
Call toll free 1-800-998-8800 for free samples or literature.
1
MAX774/MAX775/MAX776
_______________General Description
The MAX774/MAX775/MAX776 inverting switching
regulators deliver high efficiency over three decades of
load current. A unique current-limited, pulsefrequency-modulated (PFM) control scheme provides
the benefits of pulse-width modulation (high efficiency
with heavy loads), while using less than 100µA of supply current (vs. 2mA to 10mA for PWM converters). The
result is high efficiency over a wide range of loads.
These ICs also use tiny external components; their high
switching frequency (up to 300kHz) allows for less than
5mm diameter surface-mount magnetics.
The MAX774/MAX775/MAX776 accept input voltages from
3V to 16.5V, and have preset output voltages of
-5V, -12V, and -15V, respectively. Or, the output voltage
can be user-adjusted with two resistors. Maximum
VIN - VOUT differential voltage is limited only by the breakdown voltage of the chosen external switch transistor.
These inverters use external P-channel MOSFET switches, allowing them to power loads up to 5W. If less
power is required, use the MAX764/MAX765/MAX766
inverting switching regulators with on-board MOSFETs.
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
ABSOLUTE MAXIMUM RATINGS
Supply Voltages
V+ to OUT ...........................................................................21V
V+ to GND ..............................................................-0.3V, +17V
OUT to GND ........................................................-0.3V, to -17V
REF, SHDN, FB, CS...................................-0.3V to (V+ + 0.3V)
EXT ...............................................(VOUT - 0.3V) to (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:
MAX77_C_ _ .........................................................0°C to +70°C
MAX77_E_ _ ......................................................-40°C to +85°C
MAX77_MJA ...................................................-55°C to +125°C
Maximum Junction Temperatures:
MAX77_C_ _/E_ _ ...........................................................+150°C
MAX77_MJA..................................................................+175°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, ILOAD = 0mA, CREF = 0.1µF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
V+ Input Voltage Range
SYMBOL
CONDITIONS
MIN
V+
TYP
3.0
V+ = 16.5V, SHDN ≤ 0.4V (operating)
Supply Current
FB Input Current
Output Voltage
Reference Voltage
2
V+ = 16.5V, SHDN ≥ 1.6V (shutdown)
4
VOUT
VREF
-10
5
µA
10
mV
±50
MAX77_E
±70
MAX77_M
±90
MAX774
-4.80
-5
-5.20
MAX775
-11.52
-12
-12.48
MAX776
-14.40
-15
-15.60
IREF = 0µA
0µA ≤ IREF ≤ 100µA
REF Line Regulation
3V ≤ V+ ≤ 16.5V
Output Voltage Load Regulation
(Circuit of Figure 2—
Bootstrapped)
V
MAX77_C
REF Load Regulation
Output Voltage Line Regulation
(Circuit of Figure 2—
Bootstrapped)
2
IFB
UNITS
16.5
100
V+ = 10V, SHDN ≥ 1.6V (shutdown)
3V ≤ V+ ≤ 16.5V
FB Trip Point
MAX
MAX77_C
1.4700
1.5
1.5300
MAX77_E
1.4625
1.5
1.5375
MAX77_M
1.4550
1.5
1.5450
MAX77_C/E
4
10
MAX77_M
4
15
40
100
MAX774, 4V ≤ V+ ≤ 15V, ILOAD = 0.5A
0.035
MAX775, 4V ≤ V+ ≤ 8V, ILOAD = 0.2A
0.088
MAX776, 4V ≤ V+ ≤ 6V, ILOAD = 0.1A
0.137
MAX774, 0A ≤ ILOAD ≤ 1A, V+ = 5V
1.5
MAX775, 0mA ≤ ILOAD ≤ 500mA, V+ = 5V
1.5
MAX776, 0mA ≤ ILOAD ≤ 400mA, V+ = 5V
1.0
_______________________________________________________________________________________
nA
V
V
mV
µV/V
mV/V
mV/A
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
PARAMETER
SYMBOL
Efficiency
(Circuit of Figure 2—
Bootstrapped)
SHDN Input Current
SHDN Input Voltage High
SHDN Input Voltage Low
VIH
VIL
Current-Limit Trip Level
(V+ – CS)
VCS
CONDITIONS
MAX774, V+ = 5V, ILOAD = 1A
MIN
TYP
82
MAX775, V+ = 5V, ILOAD = 500mA
MAX776, V+ = 5V, ILOAD = 400mA
V+ = 16.5V, SHDN = 0V or V+
3V ≤ V+ ≤ 16.5V
3V ≤ V+ ≤ 16.5V
MAX77_C/E
3V ≤ V+ ≤ 16.5V
MAX77_M
180
160
210
210
MAX
UNITS
%
88
87
±1
1.6
0.4
240
260
µA
V
V
mV
±1
µA
Switch Maximum On-Time
tON (max)
V+ = 12V
12
16
20
µs
Switch Minimum Off-Time
tOFF (max)
V+ = 12V
1.8
2.3
2.8
µs
CS Input Current
EXT Rise Time
EXT Fall Time
CEXT = 1nF, V+ = 12V
CEXT = 1nF, V+ = 12V
50
50
MAX774/MAX775/MAX776
ELECTRICAL CHARACTERISTICS (continued)
ns
ns
_______________________________________________________________________________________
3
__________________________________________Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
MAX774
EFFICIENCY vs. LOAD CURRENT
VOUT = -5V (NON-BOOTSTRAPPED)
VIN = 15V
70
ILOAD = 100mA
80
80
EFFICIENCY (%)
VIN = 3V
VIN = 5V
VIN = 4V
EFFICIENCY (%)
80
90
MA774/5/6--1b
VIN = 5V
EFFICIENCY (%)
90
MAX1774/5/6-01a
90
MAX774
EFFICIENCY vs. TEMPERATURE
MAX774/5/6-2
MAX774
EFFICIENCY vs. LOAD CURRENT
VOUT = -5V (BOOTSTRAPPED)
VIN = 15V
70
ILOAD = 600mA
ILOAD = 1A
70
60
60
60
50
50
50
VIN = 5V
BOOTSTRAPPED
100
10
1000
-40
0
20
40
60
80
TEMPERATURE (°C)
MAX776
EFFICIENCY vs. LOAD CURRENT
VOUT = -15V (BOOTSTRAPPED)
MAX776
EFFICIENCY vs. LOAD CURRENT
VOUT = -15V (NON-BOOTSTRAPPED)
MAX775
EFFICIENCY vs. OUTPUT CURRENT
VOUT = -12V (BOOTSTRAPPED)
80
VIN = 3V
70
60
VIN = 15V
VIN = 6V
VIN = 4V
70
60
50
100
70
50
1
1000
VIN = 8V
VIN = 4V
60
50
10
VIN = 5V
80
VIN = 5V
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 4V
100
10
1
1000
100
10
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
OUTPUT CURRENT (mA)
MAX774/MAX775/MAX776
EFFICIENCY vs. LOAD CURRENT
VOUT = -24V (NON-BOOTSTRAPPED)
MAX774/MAX775/MAX776
EFFICIENCY vs. LOAD CURRENT
VOUT = -24V OUTPUT (ZENER CONNECTION)
MAX774
EFFICIENCY vs. INPUT VOLTAGE
VOUT = -5V AT 100mA
VIN = 5V
VIN = 6V
88
86
EFFICIENCY (%)
80
VIN = 4V
70
EFFICIENCY (%)
VIN = 5V
80
90
MA774/5/6--1g
VIN = 6V
MA774/5/6--1f
90
VIN = 4V
70
60
60
50
50
100
90
MA774/5/6-1d
90
VIN = 5V
1
-20
LOAD CURRENT (mA)
MA774/5/6--1e
1
1000
VIN = 6V
80
EFFICIENCY (%)
100
MA774/5/6-1c
90
10
LOAD CURRENT (mA)
BOOTSTRAPPED
MAX774/5/6-3
1
EFFICIENCY (%)
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
84
82
80
NON-BOOTSTRAPPED
78
76
VOUT = -5V AT 100mA
1
10
100
LOAD CURRENT (mA)
4
1000
74
1
10
100
LOAD CURRENT (mA)
1000
2
4
6
8
10
12
INPUT VOLTAGE (V)
_______________________________________________________________________________________
14
16
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
4.5
3.5
VOUT = -5V
3.0
4.0
VOUT = -24V
3.5
3.0
2.5
1000
100
10
LOAD CURRENT (mA)
EXT RISE AND FALL TIMES
vs. TEMPERATURE
EXT RISE AND FALL TIMES
vs. TEMPERATURE
500
110
80
5V FALL
70
60
12V RISE
50
CEXT = 5nF
4
6
8
10
12
14
16
INPUT VOLTAGE (V)
SWITCH ON-TIME vs. TEMPERATURE
17
V+ = 5V
5V RISE
350
300
250
16
5V FALL
200
12V RISE
150
40
12V FALL
30
100
12V FALL
50
20
-60 -40 -20
0
20
40 60
80 100 120 140
15
-60 -40 -20
0
20
40 60
-60
80 100 120 140
60
0
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
SWITCH OFF-TIME vs. TEMPERATURE
SWITCH ON-TIME/OFF-TIME RATIO
SHUTDOWN CURRENT
vs. TEMPERATURE
7.8
V+ = 5V
3.5
3.0
7.4
7.2
ICC (µA)
tON/tOFF RATIO (µs/µs)
7.6
2.0
4.0
120
MAX774/5/6-7
V+ = 5V
MAX774/5/6-6
8.0
MAX761-13
2.5
tOFF (µs)
2
ton (µs)
90
tRISE & tFALL (ns)
5V RISE
1200
1000
450
400
100
1400
1000
MAX774/5/6-10
120
1
LOAD CURRENT (mA)
CEXT = 1nF
BOOTSTRAPPED
1600
800
0.1
MAX774/5/6-9
130
100
10
1800
NON-BOOTSTRAPPED
VOUT = -5V
2.5
1
tRISE & tFALL (ns)
VOUT = -15V
VOUT = -5V
2000
LOAD CURRENT (mA)
VOUT = -12V
MAX774/5/6-16
VOUT = -12V
START-UP VOLTAGE (V)
START-UP VOLTAGE (V)
4.5
2200
MA744/5/6-15
VOUT = -15V
4.0
5.0
MA744/5/6-14
5.0
MAX774
MAXIMUM LOAD vs. INPUT VOLTAGE
START-UP VOLTAGE
vs. LOAD CURRENT (NON-BOOTSTRAPPED)
MAX761-13
START-UP VOLTAGE
vs. LOAD CURRENT (BOOTSTRAPPED)
7.0
6.8
2.5
V+ = 15V
2.0
1.5
6.6
V+ = 8V
1.0
6.4
1.5
-60
0
60
TEMPERATURE (°C)
120
6.2
0.5
6.0
0
V+ = 4V
-60 -40 -20
0
20
40 60
80 100 120 140
TEMPERATURE (°C)
-60 -40 -20
0
20
40 60
80 100 120 140
TEMPERATURE (°C)
_______________________________________________________________________________________
5
MAX774/MAX775/MAX776
_____________________________Typical Operating Characteristics (continued)
(TA = +25°C, unless otherwise noted.)
_____________________________Typical Operating Characteristics (continued)
(TA = +25°C, unless otherwise noted.)
OPERATING SUPPLY CURRENT
vs. TEMPERATURE
REFERENCE
TEMPERATURE COEFFICIENT
76
V+ = 10V
74
72
V+ = 3V
68
MAX774/5/6-12
1.500
1.498
1.496
1.494
1.492
66
0
20
40 60
80 100 120 140
-60 -40 -20
0
20
40 60
80 100 120 140
TEMPERATURE (°C)
TEMPERATURE (°C)
CS TRIP LEVEL
REFERENCE
OUTPUT RESISTANCE
MAX774/5/6-11
235
230
225
220
215
210
205
200
195
190
250
REFERENCE OUTPUT RESISTANCE (Ω)
-60 -40 -20
200
IREF = 10µA
150
IREF = 50µA
100
50
IREF = 100µA
0
185
-60 -40 -20
0
20
40 60
80 100 120 140
TEMPERATURE (°C)
6
1.502
MAX774/5/6-13
ICC (µA)
1.504
REFERENCE OUTPUT (V)
V+ = 16.5V
78
70
1.506
MAX774/5/6-8
80
CS TRIP LEVEL (mV)
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
-60 -40 -20
0
20
40 60
80 100 120 140
TEMPERATURE (°C)
_______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
INDUCTOR CURRENT NEAR FULL LOAD
OPERATING WAVEFORMS
A
1A/div
B
0A
C
20µs/div
10µs/div
CIRCUIT OF FIGURE 2
VOUT = -5V, V+ = 4.7V
ILOAD = 1.05A (1A/div)
CIRCUIT OF FIGURE 2
V+ = 6.5V, ILOAD = 1A, VOUT = -5V
A: OUTPUT RIPPLE, 200mV/div
B: EXT WAVEFORM, 10V/div
C: INDUCTOR CURRENT, 2A/div
CONTINUOUS CONDUCTION
AT ONE-HALF CURRENT LIMIT
ENTRY/EXIT FROM SHUTDOWN
A
1A/div
B
0A
20µs/div
CIRCUIT OF FIGURE 2
ILOAD = 300mA, VOUT = -5V
V+ = 8V, L = 22µH
2ms/div
CIRCUIT OF FIGURE 2
V+ = 6V, ILOAD = 1A, VOUT = -5V
A: SHUTDOWN PULSE, 0V TO V+, 5V/div
B: VOUT, 2V/div
_______________________________________________________________________________________
7
MAX774/MAX775/MAX776
__________________________________________Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
____________________________Typical Operating Characteristics (continued)
(TA = +25°C, unless otherwise noted.)
LOAD-TRANSIENT RESPONSE
LINE-TRANSIENT RESPONSE
A
A
B
B
100µs/div
2ms/div
CIRCUIT OF FIGURE 2
V+ = 6V, VOUT = -5V
A: ILOAD, 30mA TO 1A, 1A/div
B: VOUT, 100mV/div, AC-COUPLED
CIRCUIT OF FIGURE 2
VOUT = -5V, ILOAD = 1A
A: V+, 3V TO 8V, 5V/div
B: VOUT, 100mV/div, AC-COUPLED
______________________________________________________________Pin Description
8
PIN
NAME
FUNCTION
1
OUT
The sense input for fixed-output operation (VFB = VREF). OUT is connected to the internal voltage divider,
and it is the negative supply input for the EXT driver.
2
FB
3
SHDN
4
REF
1.5V reference output that can source 100µA. Bypass to ground with 0.1µF.
5
V+
Positive power-supply input
6
CS
Noninverting input to the current-sense comparator. Typical trip level is 210mV (relative to V+).
7
EXT
The gate-drive output for an external P-channel power MOSFET. EXT swings from OUT to V+.
8
GND
Ground
Feedback input. When VFB = VREF, the output will be the factory preset value. For adjustable operation,
use an external voltage divider, as described in the Adjustable Output section.
Active-high shutdown input. With SHDN high, the part is in shutdown mode and the supply current is less
than 5µA. Connect to GND for normal operation.
_______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
MAX774/MAX775/MAX776
V+
FB
MODE
COMPARATOR
REF
MAX774
MAX775
MAX776
50mV
SHDN
ERROR
COMPARATOR
OUT
N
1.5V
REFERENCE
Q
TRIG
ONE-SHOT
FROM V+
S
TRIG
Q
EXT
Q
R
ONE-SHOT
FROM OUT
CURRENT
COMPARATOR
CS
0.2V
(FULL
CURRENT)
CURRENT
CONTROL CIRCUITS
0.1V
(HALF
CURRENT)
FROM
V+
GND
Figure 1. Block Diagram
_______________Detailed Description
The MAX774/MAX775/MAX776 are negative-output,
inverting power controllers that can be configured to drive
an external P-channel MOSFET. The output voltages are
preset to -5V (MAX774), -12V (MAX775), or -15V
(MAX776). Additionally, all three parts can be set to any
desired output voltage using an external resistor divider.
The MAX774/MAX775/MAX776 have a unique control
scheme (Figure 1) that combines the advantage of
pulse-skipping, pulse-frequency-modulation (PFM)
converters (ultra-low supply current) with the advantage of pulse-width-modulation (PWM) converters (high
efficiency with heavy loads). This control scheme
allows the devices to achieve 85% efficiency with loads
from 5mA to 1A.
As with traditional PFM converters, the external
P-channel MOSFET power transistor is turned on when
the voltage comparator senses that the output is below
the reference voltage. However, unlike traditional PFM
converters, switching is controlled by the combination
of a switch current limit (210mV/R SENSE ) and
on-time/off-time limits set by one-shots. Once turned
on, the MOSFET stays on until:
1) the 16µs maximum on-time limit is reached
or
2) the switch current reaches its limit (as set by the
current-sense resistor).
Once off, the switch is typically held off for a minimum of
2.3µs. It will stay off until the output drops below the level
determined by VREF and the feedback divider network.
_______________________________________________________________________________________
9
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
VIN
VIN
C2
0.1µF
C1
150µF
1 OUT
3
2
4
V+
1
5
R1
0.07Ω
MAX774
SHDN MAX775 CS 6
MAX776
FB
R2
EXT
C1
150µF
Q1
Si9435
P
7
VOUT
GND
8
1N5822/
MBR340
L1
22µH
C4*
MAX775, MAX776 = 120µF, 20V
OUTPUT
VOLTAGE (V)
INPUT
VOLTAGE (V)
MAX774
-5
3 to 15
1
MAX775
-12
3 to 8
0.5
MAX776
-15
3 to 5
0.4
NOTE: Si9435 HAS VGS OF 20V MAX
Figure 2. Bootstrapped Connection Using Fixed Output
Voltages
VIN
1 OUT
C2
0.1µF
V+ 5
R3
0.07Ω
R1
C3
0.1µF
3 SHDN MAX774
2
MAX775 CS 6
FB
MAX776
EXT
4
REF
GND
8
7
L1
22µH
Q1
Si9435
P
VOUT
1N5822/
MBR340
C4*
* MAX774 = 330µF, 10V
MAX775, MAX776 = 120µF, 20V
Figure 3. Bootstrapped Connection Using External Feedback
Resistors
With light loads, the MOSFET switches on for one or
more cycles and then switches off, much like in traditional PFM converters. To increase light-load efficiency,
the current limit for the first two pulses is set to one-half
the peak current limit. If those pulses bring the output
voltage into regulation, the voltage comparator keeps
10
5
R3
0.07Ω
SHDN MAX774
FB
MAX775 CS 6
MAX776
7
EXT
R1
4
C3
0.1µF
REF
GND
8
L1
22µH
Q1
Si9435
P
VOUT
1N5822/
MBR340
C4*
MAX775, MAX776 = 120µF, 20V
Figure 4. Non-Bootstrapped Operation (VIN > 4.5V)
OUTPUT
CURRENT (A)
R2
V+
* MAX774 = 330µF, 10V
* MAX774 = 330µF, 10V
C1
150µF
2
C2
0.1µF
REF
C3
0.1µF
PRODUCT
3
OUT
the MOSFET off, and the current limit remains at one-half
the peak current limit. If the output voltage is out of
regulation after two consecutive pulses, the current limit
for the next pulse will equal the full current limit.
With heavy loads, the MOSFET first switches twice at
one-half the peak current value. Subsequently, it stays
on until the switch current reaches the full current limit,
and then turns off. After it is off for 2.3µs, the MOSFET
switches on once more, and remains on until the switch
current again reaches its limit. This cycle repeats until
the output is in regulation.
A benefit of this control scheme is that it is highly efficient over a wide range of input/output ratios and load
currents. Additionally, PFM converters do not operate
with constant-frequency switching, and have relaxed
stability criterion (unlike PWM converters). As a result,
their external components require smaller values.
With PFM converters, the output voltage ripple is not
concentrated at the oscillator frequency (as it is with
PWM converters). So for applications where the ripple
frequency is important, the PWM control scheme must
be used. However, for many other applications, the
smaller capacitors and lower supply current of the PFM
control scheme make it the better choice. The output
voltage ripple with the MAX774/MAX775/MAX776 can
be held quite low. For example, using the circuit of
Figure 2, only 100mV of output ripple is produced when
generating a -5V at 1A output from a +5V input.
Bootstrapped vs.
Non-Bootstrapped Operation
Figures 2 and 3 are the standard application circuits for
bootstrapped mode, and Figure 4 is the circuit for nonbootstrapped mode. Since EXT is powered by OUT,
______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
RZ
tions, you should see a short current pulse at half the
peak current approximately every 100ms (the exact
period depends on actual circuit leakages).
EXT Drive Voltages
1
OUT
GND
8
R2
2
FB
R1
4
MAX774
MAX775
MAX776
REF
0.1µF
6V ≤ VZ + VIN ≤ 10V
VOUT – VZ
> IZ
RZ
IZ = ZENER BREAKDOWN CURRENT
VZ = ZENER BREAKDOWN VOLTAGE
VIN = INPUT SUPPLY VOLTAGE
Figure 5. Connection Using Zener Diode to Boost Base Drive
EXT swings from OUT to V+ and provides the drive output for an external power MOSFET. When using the onchip feedback resistors for the preset output voltages,
the voltage at OUT equals the output voltage. When
using external feedback resistors, OUT may be tied to
GND or some other potential between VOUT and GND.
Always observe the V+ to OUT absolute maximum rating of 21V. For V+ to output differentials greater than
21V, OUT must be tied to a potential more positive than
the output and, therefore, the output voltage must be
set with an external resistor divider.
In non-bootstrapped operation with low input voltages
(<4V), tie OUT to a negative voltage to fully enhance the
external MOSFET. Accomplish this by creating an intermediate voltage for VOUT with a zener diode (Figure 5).
__________________Design Procedure
Setting the Output Voltage
using bootstrapped or non-bootstrapped mode will
directly affect the gate drive to the FET. EXT swings
from V+ to VOUT. In bootstrapped operation, OUT is
connected to the output voltage (-5V, -12V, -15V). In
non-bootstrapped operation, OUT is connected to
ground, and EXT now swings from V+ to ground.
At high input-to-output differentials, it may be necessary to use non-bootstrapped mode to avoid the 21V
V+ to VOUT maximum rating. Also, observe the V GS
maximum rating of the external transistor. At intermediate voltages and currents, the advantages of bootstrapped vs. non-bootstrapped operation are slight.
When input voltages are less than about 4V, always use
the bootstrapped circuit.
Shutdown and Quiescent Current
The MAX774/MAX775/MAX776 are designed to save
power in battery-powered applications. A TTL/CMOS
logic-level shutdown input (SHDN) has been provided
for the lowest-power applications. When shut down
(SHDN = V+), most internal bias current sources and
the reference are turned off so that less than 5µA of
current is drawn.
In normal operation, the quiescent current will be less
than 100µA. However, this current is measured by
forcing the external switch transistor off. Even with no
load, in an actual application, additional current will be
drawn to supply the feedback resistors’ and the diode’s
and capacitor’s leakage current. Under no-load condi-
The MAX774/MAX775/MAX776 are preset for -5V, -12V,
and -15V output voltages, respectively; however, they
may also be adjusted to other values with an external
voltage divider. For the preset output voltage, connect
FB to REF and connect OUT to the output (Figure 3). In
this case, the output voltage is sensed by OUT.
For an adjustable output (Figures 3 and 4), connect an
external resistor divider from the output voltage to FB,
and from FB to REF. In this case, the divided-down
output voltage is sensed via the FB pin.
There are three reasons to use the external resistor divider:
1) You desire an output voltage other than a preset
value
2) The input-to-output differential exceeds 21V
or
3) The output voltage (VOUT to GND) exceeds -15V.
For adjustable operation, refer to Figures 3 and 4. The
impedance of the feedback network should be low
enough that the input bias current of FB is not a factor.
For best efficiency and precision, allow 10µA to flow
through the network. Calculate (V REF - VFB) / R1 =
10µA. Since VREF = 1.5V and VFB = 0V, R1 becomes
150kΩ. Then calculate R2 as follows:
R2 _______
VOUT
___
=
R1
VREF
(or, ______
VOUT = 10µA)
R2
______________________________________________________________________________________
11
MAX774/MAX775/MAX776
VOUT
0.1µF
RSENSE = 0.06Ω
2000
1500
1000
RSENSE = 0.07Ω
RSENSE = 0.08Ω
500
0
RSENSE = 0.09Ω
VOUT = -5V
3 4
5 6
VOUT = -12V
800
RSENSE = 0.05Ω
RSENSE = 0.06Ω
RSENSE = 0.07Ω
600
400
RSENSE = 0.08Ω
RSENSE = 0.09Ω
200
0
7 8 9 10 11 12 13 14 15
INPUT VOLTAGE (V)
Figure 6. MAX774 Maximum Output Current vs. Input Voltage
(VOUT = -5V)
Choosing an Inductor
Practical inductor values range from 10µH to 50µH.
The maximum inductor value is not particularly critical.
For highest current at high VOUT  to V+ ratios, the
inductor should not be so large that the peak current
never reaches the current limit. That is:
[V+(min) - VSW(max)] x 12µs
L(max) ≤ _______________________________
ILIM(max)
This is only important if
VIN
1
t OFF(min)
= ___________
VOUT
6
t ON(max)
More important is that the inductor not be so small that the
current rises much faster than the current-limit comparator
can respond. This would be wasteful and reduce efficiency. Calculate the minimum inductor value as follows:
_______ < —
[V+(max) - VSW(min)] x 0.3µs
L(min) ≥ _______________________________
δ(I) x ILIM(min)
Where L is in µH, 0.3µs is an ample time for the comparator response, ILIM is the current limit (see CurrentSense Resistor section), and δ(I) is the allowable percentage of overshoot. As an example, Figure 2's circuit
uses a 3A peak current. If we allow a 15% overshoot
and 15V is the maximum input voltage, then L(min) is
16µH. The actual value of L above this limit has minimal
effect on this circuit's operation.
For highest efficiency, use a coil with low DC resistance.
Coils with 30mΩ or lower resistance are available. To
12
1000
MAX775-FIG07
RSENSE = 0.05Ω
MAXIMUM OUTPUT CURRENT (mA)
MAX775-fig6
2500
MAXIMUM OUTPUT CURRENT (mA)
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
3
4
5
6
7
INPUT VOLTAGE (V)
8
9
Figure 7. MAX775 Maximum Output Current vs. Input Voltage
(VOUT = -12V)
minimize radiated noise, use a torroid, pot-core, or shielded-bobbin inductor. Inductors with a ferrite core or equivalent are recommended. Make sure that the inductor’s
saturation current rating is greater than ILIM(max).
Diode Selection
The ICs’ high switching frequencies demand a highspeed rectifier. Schottky diodes such as the 1N5817 to
1N5822 families are recommended. Choose a diode
with an average current rating approximately equal to
or greater than ILIM (max) and a voltage rating higher
than VIN(max) + VOUT. For high-temperature applications, Schottky diodes may be inadequate due to their
high leakage currents; instead, high-speed silicon
diodes may be used. At heavy loads and high temperature, the benefits of a Schottky diode’s low forward
voltage may outweigh the disadvantages of its high
leakage current.
Current-Sense Resistor
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 currentsense comparator threshold (see Current-Limit Trip
Level in the Electrical Characteristics).
To maximize efficiency and reduce the size and cost of
external components, minimize the peak current.
However, since the output current is a function of the
peak current, do not set the limit too low. Refer to
Figures 6–9 to determine the sense resistor (and, therefore, peak current) for the required load current.
______________________________________________________________________________________
500
400
300
200
RSENSE = 0.08Ω
RSENSE = 0.09Ω
600
MAX776-FIG09
RSENSE = 0.05Ω
RSENSE = 0.06Ω
RSENSE = 0.07Ω
600
800
MAXIMUM OUTPUT CURRENT (mA)
MAXIMUM OUTPUT CURRENT (mA)
VOUT = -15V
VOUT = -24V
RSENSE = 0.05Ω
RSENSE = 0.06Ω
RSENSE = 0.07Ω
400
200
RSENSE = 0.08Ω
RSENSE = 0.09Ω
0
100
3
4
5
6
INPUT VOLTAGE (V)
7
3 4
5 6
7 8 9 10 11 12 13 14 15
INPUT VOLTAGE (V)
Figure 8. MAX776 Maximum Output Current vs. Input Voltage
(VOUT = -15V)
Figure 9. MAX774/MAX775/MAX776 Maximum Output Current
vs. Input Voltage (VOUT = -24V)
To choose the proper current-sense resistor, simply follow the two-step procedure outlined below.
1) Determine:
• Input voltage range, V+
• Maximum (absolute) output voltage, VOUT
• Maximum output current, ILOAD
resistors. If you want to use through-hole resistors, IRC
has a wire resistor that is simply a band of metal
shaped as a “U” so that inductance is less than 10nH
(an order of magnitude less than metal-film resistors).
These are available in resistance values between
5mΩ and 0.1Ω.
For example, let V+ range from 4V to 6V, and choose
VOUT = -24V and IOUT = 150mA.
2) Next, referring to Figure 9, find the curve with the
lowest current limit whose output current (with the
lowest input voltage) meets your requirements.
In our example, we want a curve where IOUT is >150mA
with a 4V input and a -24V output.
The RSENSE = 80mΩ (shown in Figure 9) shows only
approximately 125mA of output current with a 4V input,
so we look next at the RSENSE = 70mΩ line. It shows
IOUT >150mA for V+ = 4V and VOUT = -24V. The current limit will be 0.210V / 0.070Ω = 3A. These curves
take into account worst-case inductor (±10%) and
current-sense trip levels, but not sense-resistor tolerance. The switch on resistance is 70mΩ.
Standard wire-wound and metal-film resistors have
an inductance high enough to degrade performance.
Metal-film resistors are usually deposited on a ceramic
rod in a spiral, making their inductances relatively high.
Surface-mount (or chip) resistors have very little inductance and are well suited for use as current-sense
The MAX774/MAX775/MAX776 are capable of driving
P-channel enhancement-mode MOSFET transistors only.
The choice of power transistor is dictated by input and
output voltage, peak current rating, on resistance, gatesource threshold, and gate capacitance. The drain-tosource rating must be greater than the V+ - V OUT
input-to-output voltage differential. The gate-to-source
rating must be greater than V+ (the source voltage) plus
the absolute value of the most negative swing of EXT.
For bootstrapped operation, the most negative swing of
EXT is VOUT. In non-bootstrapped operation, this may
be ground or some other negative voltage. Gate capacitance is not normally a limiting factor, but values should
be less than 1nF for best efficiency. For maximum efficiency, the MOSFET should have a very low on resistance at the peak current and be capable of handling
that current. The transistor chosen for the typical operating circuit has a 30V drain-source voltage limit and a
0.07Ω drain-source on resistance at VGS = -10V.
Table 1 lists suppliers of switching transistors suitable
for use with the MAX774/MAX775/MAX776.
External Switching Transistor
______________________________________________________________________________________
13
MAX774/MAX775/MAX776
700
MAX776-FIG08
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
Table 1. Component Suppliers
SUPPLIER
PHONE
FAX
Coiltronics
(407) 241-7876
(407) 241-9339
Gowanda
(716) 532-2234
(716) 532-2702
Sumida USA
(708) 956-0666
(708) 956-0702
Sumida Japan
81-3-3607-5111
81-3-3607-5144
Kemet
(803) 963-6300
(803) 963-6322
Matsuo
(714) 969-2491
(714) 960-6492
Nichicon
(708) 843-7500
(708) 843-2798
INDUCTORS
CAPACITORS
Sanyo USA
(619) 661-6835
(619) 661-1055
Sanyo Japan
81-7-2070-6306
81-7-2070-1174
Sprague
(603) 224-1961
(603) 224-1430
United Chemi-Con
(714) 255-9500
(714) 255-9400
Motorola
(800) 521-6274
(602) 952-4190
Nihon USA
(805) 867-2555
(805) 867-2556
Nihon Japan
81-3-3494-7411
81-3-3494-7414
Harris
(407) 724-3729
(407) 724-3937
International Rectifier
(310) 322-3331
(310) 322-3332
Siliconix
(408) 988-8000
(408) 970-3950
DIODES
POWER MOSFETS
CURRENT-SENSE RESISTORS
IRC
(704) 264-8861
When evaluating this equation, be sure to use the
capacitance value at the switching frequency. At
200kHz, the 330µF tantalum capacitor of Figures 2, 3,
or 4 may degrade by a factor of ten, which will significantly alter the ripple voltage calculation.
The ESR of both the bypass and filter capacitors also
affects efficiency. Best performance is obtained by
doubling up on the filter capacitors or using low-ESR
capacitors. Capacitors must have a ripple current rating equal to the peak current.
The smallest low-ESR SMT capacitors currently available are the Sprague 595D series. Sanyo OS-CON
organic semiconductor through-hole capacitors also
exhibit low ESR and are especially effective at low temperatures. Table 1 lists the phone numbers of these
and other manufacturers.
PC Layout and Grounding
(704) 264-8866
Capacitors
Choose the output capacitor (C4 of Figures 2, 3, and 4)
to be consistent with your size, ripple, and output voltage requirements. Place capacitors in parallel if the
size you want is unobtainable. You will not only
increase the capacitance, but also decrease the
capacitor’s ESR (a major contributor of ripple). A
330µF tantalum output filter capacitor with 0.07Ω ESR
14
typically maintains 120mV p-p output ripple when
generating -5V at 1A from a 5V input. Smaller capacitors are acceptable for lighter loads or in applications
that can tolerate higher output ripple.
The value of C4 is chosen such that it acquires as
small a charge as possible during the switch on-time.
The amount of ripple as a function of capacitance is
give by:
VOUT x IOUT x ESR
IOUT x tOFF(min)
∆Vp-p = _____________________ + _________________
C
VIN
Due to high current levels and fast switching waveforms, proper PC board layout is essential. Use a star
ground configuration; connect the ground lead of the
input bypass capacitor, the output capacitor, the inductor, and the GND pin of the MAX774/MAX775/MAX776
at a common point very close to the device. Additionally, input capacitor C2 (Figures 3 and 4) should be
placed extremely close to the device.
If an external resistor divider is used (Figures 3 and 4),
the trace from FB to the resistors must be extremely short.
______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
TEMP. RANGE
PIN-PACKAGE
MAX775CPA
PART
0°C to +70°C
8 Plastic DIP
MAX775CSA
MAX775C/D
MAX775EPA
MAX775ESA
MAX775MJA
MAX776CPA
MAX776CSA
MAX776C/D
MAX776EPA
MAX776ESA
MAX776MJA
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
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
8 Plastic DIP
8 SO
Dice*
8 Plastic DIP
8 SO
8 CERDIP
___________________Chip Topography
OUT
GND
EXT
FB
.109"
(2.769mm)
CS
SHDN
* Contact factory for dice specifications.
REF
V+
0.080
(2.032mm)
TRANSISTOR COUNT: 442;
SUBSTRATE CONNECTED TO V+.
________________________________________________________Package Information
DIM
E
A
A1
B
C
D
E
e
H
h
L
α
H
INCHES
MAX
MIN
0.069
0.053
0.010
0.004
0.019
0.014
0.010
0.007
0.197
0.189
0.157
0.150
0.050 BSC
0.244
0.228
0.020
0.010
0.050
0.016
8˚
0˚
MILLIMETERS
MIN
MAX
1.35
1.75
0.10
0.25
0.35
0.49
0.19
0.25
4.80
5.00
3.80
4.00
1.27 BSC
5.80
6.20
0.25
0.50
0.40
1.27
0˚
8˚
21-325A
h x 45˚
D
α
A
0.127mm
0.004in.
e
A1
C
L
8-PIN PLASTIC
SMALL-OUTLINE
PACKAGE
B
______________________________________________________________________________________
15
MAX774/MAX775/MAX776
_Ordering Information (continued)
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
________________________________________________________Package Information
DIM
D1
A
A1
A2
A3
B
B1
C
D
D1
E
E1
e
eA
eB
L
α
E
E1
D
A3
A
A2
L
A1
INCHES
MAX
MIN
0.200
–
–
0.015
0.175
0.125
0.080
0.055
0.022
0.016
0.065
0.050
0.012
0.008
0.390
0.348
0.035
0.005
0.325
0.300
0.280
0.240
0.100 BSC
0.300 BSC
0.400
–
0.150
0.115
15˚
0˚
MILLIMETERS
MIN
MAX
–
5.08
0.38
–
3.18
4.45
1.40
2.03
0.41
0.56
1.27
1.65
0.20
0.30
8.84
9.91
0.13
0.89
7.62
8.26
6.10
7.11
2.54 BSC
7.62 BSC
–
10.16
2.92
3.81
0˚
15˚
21-324A
α
8-PIN PLASTIC
DUAL-IN-LINE
PACKAGE
C
e
B1
eA
B
eB
DIM
S
S1
E1
D
E
B2
A
MILLIMETERS
MIN
MAX
–
5.08
0.36
0.58
0.97
1.65
0.58
1.14
0.20
0.38
–
10.29
5.59
7.87
7.37
8.13
2.54 BSC
3.18
5.08
3.81
–
0.38
1.52
–
1.40
0.13
–
0˚
15˚
21-326D
α
Q
L
A
B
B1
B2
C
D
E
E1
e
L
L1
Q
S
S1
α
INCHES
MAX
MIN
0.200
–
0.023
0.014
0.065
0.038
0.045
0.023
0.015
0.008
0.405
–
0.310
0.220
0.320
0.290
0.100 BSC
0.200
0.125
–
0.150
0.060
0.015
0.055
–
–
0.005
15˚
0˚
e
L1
B1
B
C
8-PIN CERAMIC
DUAL-IN-LINE
PACKAGE
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
© 1994 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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