Page 1
General Description
The MAX1771 step-up switching controller provides
90% efficiency over a 30mA to 2A load. A unique current-limited pulse-frequency-modulation (PFM) control
scheme gives this device the benefits of pulse-widthmodulation (PWM) converters (high efficiency at heavy
loads), while using less than 110µA of supply current (vs.
2mA to 10mA for PWM converters).
This controller uses miniature external components. Its
high switching frequency (up to 300kHz) allows surface-mount magnetics of 5mm height and 9mm diameter. It accepts input voltages from 2V to 16.5V. The
output voltage is preset at 12V, or can be adjusted
using two resistors.
The MAX1771 optimizes efficiency at low input voltages
and reduces noise by using a single 100mV current-limit
threshold under all load conditions. A family of similar
devices, the MAX770–MAX773, trades some full-load
efficiency for greater current-limit accuracy; they provide
a 200mV current limit at full load, and switch to 100mV
for light loads.
The MAX1771 drives an external N-channel MOSFET
switch, allowing it to power loads up to 24W. If less power
is required, use the MAX756/MAX757 or MAX761/MAX762
step-up switching regulators with on-board MOSFETs. An
evaluation kit is available.
Applications
Positive LCD-Bias Generators
Flash Memory Programmers
High-Power RF Power-Amplifier Supply
Palmtops/Hand-Held Terminals
Battery-Powered Applications
Portable Communicators
Features
♦ 90% Efficiency for 30mA to 2A Load Currents
♦ Up to 24W Output Power
♦ 110µA (max) Supply Current
♦ 5µA (max) Shutdown Current
♦ 2V to 16.5V Input Range
♦ Preset 12V or Adjustable Output Voltage
♦ Current-Limited PFM Control Scheme
♦ Up to 300kHz Switching Frequency
♦ Evaluation Kit Available
Ordering Information
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
________________________________________________________________ Maxim Integrated Products 1
1
2
3
4
8
7
6
5
CS
GND
AGND
REF
SHDN
FB
V+
EXT
TOP VIEW
MAX1771
DIP/SO
Pin Configuration
FB AGND GND
V+
CS
EXT
N
REF
SHDN
ON/OFF
OUTPUT
12V
INPUT
2V TO V
OUT
MAX1771
Typical Operating Circuit
PART TEMP RANGE PIN-PACKAGE
MAX1771CPA 0°C to +70°C 8 Plastic DIP
MAX1771CSA 0°C to +70°C 8 SO
MAX1771C/D 0°C to +70°C Dice*
MAX1771EPA -40°C to +85°C 8 Plastic DIP
MAX1771ESA -40°C to +85°C 8 SO
MAX1771MJA -55°C to +125°C 8 CERDIP**
19-0263; Rev 2; 3/02
EVALUATION KIT MANUAL
AVAILABLE
* Contact factory for dice specifications.
** Contact factory for availability and processing to MIL-STD-883B.
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
Page 2
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
2
_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
Supply Voltage
V+ to GND ...............................................................-0.3V, 17V
EXT, CS, REF, SHDN, FB to GND ...................-0.3V, (V+ + 0.3V)
GND to AGND.............................................................0.1V, -0.1V
Continuous Power Dissipation (T
A
= +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
MAX1771C_A .....................................................0°C to +70°C
MAX1771E_A ..................................................-40°C to +85°C
MAX1771MJA ................................................-55°C to +125°C
Junction Temperatures
MAX1771C_A/E_A.......................................................+150°C
MAX1771MJA ..............................................................+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, I
LOAD
= 0mA, TA= T
MIN
to T
MAX
, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETER
SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Current 85 110 µA
Standby Current
25
µA
4
Output Voltage (Note 1) V
V+ = 2V to 12V, over full load range,
Circuit of Figure 2a
11.52 12.0 12.48
V+ = 5V to 7V, V
OUT
= 12V
I
LOAD
= 700mA, Circuit of Figure 2a
5 mV/V
V+ = 6V, V
OUT
= 12V, I
LOAD
= 0mA to
500mA, Circuit of Figure 2a
20 mV/A
Maximum Switch On-Time tON(max) 12 16 20 µs
Minimum Switch Off-Time t
OFF
(min) 1.8 2.3 2.8 µs
%
Reference Voltage V
REF
I
REF =
0µA
MAX1771C 1.4700 1.5 1.5300
V
MAX1771E 1.4625 1.5 1.5375
MAX1771M 1.4550 1.5 1.5450
Output Voltage Line Regulation
(Note 2)
Output Voltage Load Regulation
(Note 2)
V+ = 5V, V
OUT
= 12V, I
LOAD
= 500mA,
Circuit of Figure 2a
V+ = 10V, SHDN ≥ 1.6V (shutdown)
V+ = 16.5V, SHDN ≥ 1.6V (shutdown)
V+ = 16.5V, SHDN = 0V (normal operation)
Minimum Start-Up Voltage 1.8 2.0 V
MAX1771 (internal feedback resistors) 2.0 12.5
MAX1771C/E (external resistors) 3.0 16.5
MAX1771MJA (external resistors) 3.1 16.5
Input Voltage Range V
Efficiency 92
REF Load Regulation 0µA ≤ I
REF
≤ 100µA
MAX1771C/E 410
mV
MAX1771M 415
3V ≤ V+ ≤ 16.5V 40 100
FB Trip Point Voltage V
FB
MAX1771C 1.4700 1.5 1.5300
V
MAX1771E 1.4625 1.5 1.5375
MAX1771M 1.4550 1.5 1.5450
µV/VREF Line Regulation
Page 3
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
_______________________________________________________________________________________
3
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, I
LOAD
= 0mA, TA= T
MIN
to T
MAX
, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETERS
SYMBOL CONDITIONS MIN TYP MAX UNITS
FB Input Current I
FB
MAX1771C ±20
nAMAX1771E ±40
MAX1771M ±60
SHDN Input High Voltage V
IH
V+ = 2V to 16.5V 1.6 V
SHDN Input Low Voltage V
IL
V+ = 2V to 16.5V 0.4 V
SHDN Input Current ±1 µAV+ = 16.5V, SHDN = 0V or V+
Current-Limit Trip Level V
CS
V+ = 5V to 16V
85 100 115
mV
CS Input Current 0.01 ±1 µA
EXT Rise Time V+ = 5V, 1nF from EXT to ground 55 ns
EXT Fall Time V+ = 5V, 1nF from EXT to ground 55 ns
Note 1: Output voltage guaranteed using preset voltages. See Figures 4a–4d for output current capability versus input voltage.
Note 2: Output voltage line and load regulation depend on external circuit components.
75 100 125
Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
60
65
75
70
80
85
90
95
100
1
10
100
10,000
1000
EFFICIENCY vs. LOAD CURRENT
(BOOTSTRAPED MODE)
LOAD CURRENT (mA)
EFFICIENCY (%)
VIN = 5V
VIN = 3V
VIN = 8V
VIN = 10V
V
OUT
= 12V
CIRCUIT OF
FIGURE 2a
MAX1771–01
60
65
75
70
80
85
90
95
100
1
10
100
10,000
1000
EFFICIENCY vs. LOAD CURRENT
(NON-BOOTSTRAPED MODE)
LOAD CURRENT (mA)
EFFICIENCY (%)
VIN = 5V
VIN =10V
VIN = 8V
V
OUT
= 12V
CIRCUIT OF
FIGURE 2b
MAX1771–02
0
2.00
LOAD CURRENT vs.
MINIMUM START-UP INPUT VOLTAGE
MAX1771-TOC3
MINIMUM START-UP INPUT VOLTAGE (V)
LOAD CURRENT (mA)
100
200
300
400
500
600
700
2.25 2.50 2.75 3.00 3.25 3.50
EXTERNAL FET THRESHOLD
LIMITS FULL-LOAD START-UP
BELOW 3.5V
V
OUT
= 12V, CIRCUIT OF FIGURE 2a
MAX1771C/E
MAX1771M
Page 4
250
0
-60 -20 60 140
REFERENCE OUTPUT RESISTANCE vs.
TEMPERATURE
50
MAX1771-07
TEMPERATURE (°C)
REFERENCE OUTPUT RESISTANCE (Ω)
20 100
150
-40 0 8040 120
100
200
100µA
50µA
10µA
1.502
-60 -20 60 140
REFERENCE vs. TEMPERATURE
MAX1771-08
TEMPERATURE (°C)
REFERENCE (V)
20 100-40 0 8040 120
1.500
1.498
1.496
1.494
1.492
1.504
1.506
15.5
16.0
16.5
-60
-30 0 30 60
90
120 150
MAXIMUM SWITCH ON-TIME vs.
TEMPERATURE
TEMPERATURE (°C)
t
ON(MAX) (µs)
MAX1771-09
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
4
_______________________________________________________________________________________
4.0
-60 -20 60 140
SHUTDOWN CURRENT vs. TEMPERATURE
MAX1771-10
TEMPERATURE (°C)
SHUTDOWN CURRENT (µA)
20 100-40 0 8040 120
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
V+ = 15V
V+ = 4V
V+ = 8V
2.20
2.25
2.30
-60
-30 0 30 60
90
120 150
MINIMUM SWITCH OFF-TIME vs.
TEMPERATURE
TEMPERATURE (°C)
t
OFF(MIN) (µs)
MAX1771-11
6.0
7.0
8.0
-60
-30 0 30 60
90
120 150
MAXIMUM SWITCH ON-TIME/
MINIMUM SWITCH OFF-TIME RATIO
vs. TEMPERATURE
TEMPERATURE (°C)
t
ON(MAX)/
t
OFF(MIN) RATIO
MAX1771-12
7.5
6.5
Typical Operating Characteristics (continued)
(TA = +25°C, unless otherwise noted.)
0
1
2
3
4
-75
-50 -25 0
25 50 75 100 125
SUPPLY CURRENT vs. TEMPERATURE
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
V
OUT
= 12V, VIN = 5V
CIRCUIT OF FIGURE 2a
BOOTSTRAPPED MODE
ENTIRE
CIRCUIT
SCHOTTKY DIODE
LEAKAGE EXCLUDED
MAX1771-04
0
0.2
0.4
0.6
0.8
2
4
68
10
12
SUPPLY CURRENT vs. SUPPLY VOLTAGE
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
V
OUT
= 12V
NON-BOOTSTRAPPED
CIRCUIT OF FIGURE 2b
BOOTSTRAPPED
CIRCUIT OF
FIGURE 2a
MAX1771-05
0
100
150
200
250
50
2
4
68
10
12
EXT RISE/FALL TIME vs. SUPPLY VOLTAGE
SUPPLY VOLTAGE (V)
EXT RISE/FALL TIME (ns)
C
EXT
= 2200pF
C
EXT
= 1000pF
C
EXT
= 446pF
C
EXT
= 100pF
MAX1771-06
Page 5
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
_______________________________________________________________________________________
5
2µs/div
V
IN
= 5V, I
OUT
= 900mA, V
OUT
= 12V
A: EXT VOLTAGE, 10V/div
B: INDUCTOR CURRENT, 1A/div
C: V
OUT
RIPPLE, 50mV/div, AC-COUPLED
A
B
C
V
OUT
0V
I
LIM
0A
HEAVY-LOAD SWITCHING WAVEFORMS
Typical Operating Characteristics (continued)
(Circuit of Figure 2a, TA = +25°C, unless otherwise noted.)
10µs/div
V
IN
= 5V, I
OUT
= 500mA, V
OUT
= 12V
A: EXT VOLTAGE, 10V/div
B: INDUCTOR CURRENT, 1A/div
C: V
OUT
RIPPLE, 50mV/div, AC-COUPLED
A
B
C
V
OUT
0V
I
LIM
0A
MEDIUM-LOAD SWITCHING WAVEFORMS
5ms/div
I
OUT
= 700mA, V
OUT
= 12V
A: V
IN
, 5V to 7V, 2V/div
B: V
OUT
RIPPLE, 100mV/div, AC-COUPLED
A
B
5V
7V
0V
LINE-TRANSIENT RESPONSE
5ms/div
V
IN
= 6V, V
OUT
= 12V
A: LOAD CURRENT, 0mA to 500mA, 500mA/div
B: V
OUT
RIPPLE, 100mV/div, AC-COUPLED
A
B
500mA
0A
LOAD-TRANSIENT RESPONSE
Page 6
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
6
_______________________________________________________________________________________
2ms/div
I
OUT
= 500mA, VIN = 5V
A: SHDN, 5V/div
B: V
OUT
, 5V/div
A
B
0V
0V
ENTERING/EXITING SHUTDOWN
5V
Pin Description
Typical Operating Characteristics (continued)
(Circuit of Figure 2a, TA = +25°C, unless otherwise noted.)
PIN NAME FUNCTION
1 EXT Gate Drive for External N-Channel Power Transistor
2 V+
3 FB
4 SHDN
5 REF
6 AGND Analog Ground
7 GND High-Current Ground Return for the Output Driver
8 CS
Power-Supply Input. Also acts as a voltage-sense point when in bootstrapped mode.
Feedback Input for Adjustable-Output Operation. Connect to ground for fixed-output operation.
Use a resistor divider network to adjust the output voltage. See Setting the Output Voltage section.
Active-High TTL/CMOS Logic-Level Shutdown Input. In shutdown mode, V
OUT
is a diode drop
below V+ (due to the DC path from V+ to the output) and the supply current drops to 5µA
maximum. Connect to ground for normal operation.
1.5V Reference Output that can source 100µA for external loads. Bypass to GND with 0.1µF.
The reference is disabled in shutdown.
Positive Input to the Current-Sense Amplifier. Connect the current-sense resistor between CS
and GND.
Page 7
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
_______________________________________________________________________________________
7
Detailed Description
The MAX1771 is a BiCMOS, step-up, switch-mode
power-supply controller that provides a preset 12V output, in addition to adjustable-output operation. Its
unique control scheme combines the advantages of
pulse-frequency modulation (low supply current) and
pulse-width modulation (high efficiency with heavy
loads), providing high efficiency over a wide output
current range, as well as increased output current
capability over previous PFM devices. In addition, the
external sense resistor and power transistor allow the
user to tailor the output current capability for each application. Figure 1 shows the MAX1771 functional diagram.
The MAX1771 offers three main improvements over
prior pulse-skipping control solutions: 1) the converter
operates with miniature (5mm height and less than
9mm diameter) surface-mount inductors due to its
300kHz switching frequency; 2) the current-limited PFM
control scheme allows 90% efficiencies over a wide
range of load currents; and 3) the maximum supply
current is only 110µA.
The device has a shutdown mode that reduces the
supply current to 5µA max.
Bootstrapped/Non-Bootstrapped Modes
Figure 2 shows the standard application circuits for
bootstrapped and non-bootstrapped modes. In bootstrapped mode, the IC is powered from the output
(V
OUT
, which is connected to V+) and the input voltage
range is 2V to V
OUT
. The voltage applied to the gate of
the external power transistor is switched from V
OUT
to
ground, providing more switch gate drive and thus
reducing the transistor’s on-resistance.
In non-bootstrapped mode, the IC is powered from the
input voltage (V+) and operates with minimum supply
current. In this mode, FB is the output voltage sense
point. Since the voltage swing applied to the gate of the
external power transistor is reduced (the gate swings
from V+ to ground), the power transistor’s on-resistance
1.5V
REFERENCE
Q TRIG
QS
F/F
R
QTRIG
LOW-VOLTAGE
OSCILLATOR
2.5V
0.1V
MAX ON-TIME
ONE-SHOT
MIN OFF-TIME
ONE-SHOT
CURRENT-SENSE
AMPLIFIER
DUAL-MODE
COMPARATOR
FB
REF
50mV
ERROR
COMPARATOR
SHDN
V+
EXT
CS
BIAS
CIRCUITRY
N
MAX1771
2.3µs
16µs
Figure 1. Functional Diagram
Page 8
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
8
_______________________________________________________________________________________
increases at low input voltages. However, the supply
current is also reduced because V+ is at a lower voltage, and because less energy is consumed while
charging and discharging the external MOSFET’s gate
capacitance. The minimum input voltage is 3V when
using external feedback resistors. With supply voltages
below 5V, bootstrapped mode is recommended.
Note: When using the MAX1771 in non-bootstrapped mode, there is no preset output operation
because V+ is also the output voltage sense point
for fixed-output operation. External resistors must
be used to set the output voltage. Use 1% external
feedback resistors when operating in adjustable-output
mode (Figures 2b, 2c) to achieve an overall output voltage accuracy of ±5%. To achieve highest efficiency,
operate in bootstrapped mode whenever possible.
External Power-Transistor
Control Circuitry
PFM Control Scheme
The MAX1771 uses a proprietary current-limited PFM
control scheme to provide high efficiency over a wide
range of load currents. This control scheme combines the
ultra-low supply current of PFM converters (or pulse skippers) with the high full-load efficiency of PWM converters.
Unlike traditional PFM converters, the MAX1771 uses a
sense resistor to control the peak inductor current. The
device also operates with high switching frequencies
(up to 300kHz), allowing the use of miniature external
components.
As with traditional PFM converters, the power transistor
is not turned on until the voltage comparator senses
the output is out of regulation. However, unlike traditional PFM converters, the MAX1771 switch uses the combination of a peak current limit and a pair of one-shots
that set the maximum on-time (16µs) and minimum offtime (2.3µs); there is no oscillator. 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.
Figure 2a. 12V Preset Output, Bootstrapped
Figure 2b. 12V Output, Non-Bootstrapped
Figure 2c. 9V Output, Bootstrapped
MAX1771
VIN = 5V
REF
SHDN
AGND
GND
N
MTD20N03HDL
7
EXT
CS
FB
L1
22µH
D1
1N5817-22
R1
18k
Ω
C4
300µF
C5
100pF
C3
0.1µF
5
4
6
1
8
3
2
V+
C1
68µF
V
OUT
= 12V
AT 0.5A
R2
127k
Ω
R
SENSE
40m
Ω
C2
0.1µF
V
OUT
V
REF
R2 = (R1) ( -1)
V
REF
= 1.5V
C3
0.1µF
VIN = 5V
C2
5
4
3
6
0.1µF
REF
SHDN
FB
AGND
2
V+
MAX1771
GND
7
EXT
L1
22µH
1
8
CS
D1
1N5817-22
N
Si9410DY/
MTD20N03HDL
R
SENSE
Ω
40m
= 4V
V
IN
C2
0.1µF
5
REF
C3
0.1µF
4
SHDN
6
AGND
V
= 1.5V
OUT
V
REF
R2 = (R1) ( -1)
V
REF
2
V+
MAX1771
GND
7
EXT
CS
22µH
1
8
3
FB
D1
1N5817-22
N
Si9410DY/
MTD20N03HDL
R
SENSE
Ω
40m
R1
Ω
28k
47µF
L1
C1
68µF
C1
C4
200µF
R2
140k
C5
100pF
Ω
V
C4
300µF
V
= 12V
OUT
AT 0.5A
OUT
= 9V
Page 9
The control circuitry allows the IC to operate in continuous-conduction mode (CCM) while maintaining high
efficiency with heavy loads. When the power switch is
turned on, it stays on until either 1) the maximum ontime one-shot turns it off (typically 16µs later), or 2) the
switch current reaches the peak current limit set by the
current-sense resistor.
The MAX1771 switching frequency is variable (depending on load current and input voltage), causing variable
switching noise. However, the subharmonic noise generated does not exceed the peak current limit times the
filter capacitor equivalent series resistance (ESR). For
example, when generating a 12V output at 500mA from
a 5V input, only 100mV of output ripple occurs using
the circuit of Figure 2a.
Low-Voltage Start-Up Oscillator
The MAX1771 features a low input voltage start-up oscillator that guarantees start-up with no load down to 2V
when operating in bootstrapped mode and using internal feedback resistors. At these low voltages, the supply
voltage is not large enough for proper error-comparator
operation and internal biasing. The start-up oscillator
has a fixed 50% duty cycle and the MAX1771 disregards the error-comparator output when the supply voltage is less than 2.5V. Above 2.5V, the error-comparator
and normal one-shot timing circuitry are used. The lowvoltage start-up circuitry is disabled if non-bootstrapped
mode is selected (FB is not tied to ground).
Shutdown Mode
When SHDN is high, the MAX1771 enters shutdown
mode. In this mode, the internal biasing circuitry is
turned off (including the reference) and V
OUT
falls to a
diode drop below VIN(due to the DC path from the
input to the output). In shutdown mode, the supply
current drops to less than 5µA. SHDN is a TTL/CMOS
logic-level input. Connect SHDN to GND for normal
operation.
Design Procedure
Setting the Output Voltage
To set the output voltage, first determine the mode of
operation, either bootstrapped or non-bootstrapped.
Bootstrapped mode provides more output current capability, while non-bootstrapped mode reduces the supply
current (see the Typical Operating Characteristics). If a
decaying voltage source (such as a battery) is used,
see the additional notes in the Low Input Voltage
Operation section.
The MAX1771’s output voltage can be adjusted from
very high voltages down to 3V, using external resistors
R1 and R2 configured as shown in Figure 3. For
adjustable-output operation, select feedback resistor
R1 in the 10kΩ to 500kΩ range. R2 is given by:
V
OUT
R2 = (R1) (––––– -1
)
V
REF
where V
REF
equals 1.5V.
For preset-output operation, tie FB to GND (this
forces bootstrapped-mode operation.
Figure 2 shows various circuit configurations for bootstrapped/non-bootstrapped, preset/adjustable operation.
Determining R
SENSE
Use the theoretical output current curves shown in
Figures 4a–4d to select R
SENSE
. They were derived
using the minimum (worst-case) current-limit comparator threshold value over the extended temperature
range (-40°C to +85°C). No tolerance was included for
R
SENSE
. The voltage drop across the diode was
assumed to be 0.5V, and the drop across the power
switch r
DS(ON)
and coil resistance was assumed to be
0.3V.
Determining the Inductor (L)
Practical inductor values range from 10µH to 300µH.
22µH is a good choice for most applications. In applications with large input/output differentials, the IC’s
output current capability will be much less when the
inductance value is too low, because the IC will always
operate in discontinuous mode. If the inductor value
is too low, the current will ramp up to a high level before
the current-limit comparator can turn off the switch.
The minimum on-time for the switch (tON(min)) is
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
_______________________________________________________________________________________
9
MAX1771
R1
R2
C5*
GND
FB
V
OUT
R1 = 10kΩ TO 500k
Ω
* SEE TEXT FOR VALUE
V
OUT
V
REF
R2 = R1 ( -1
)
V
REF
= 1.5V
Figure 3. Adjustable Output Circuit
Page 10
MAX1771
approximately 2µs; select an inductor that allows the current to ramp up to I
LIM
.
The standard operating circuits use a 22µH inductor.
If a different inductance value is desired, select L such
that:
VIN(max) x 2µs
L ≥ —————----—--
I
LIM
Larger inductance values tend to increase the start-up
time slightly, while smaller inductance values allow the
coil current to ramp up to higher levels before the
switch turns off, increasing the ripple at light loads.
Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for
use with high switching frequencies. Make sure the
inductor’s saturation current rating (the current at which
the core begins to saturate and the inductance starts to
fall) exceeds the peak current rating set by R
SENSE
.
However, it is generally acceptable to bias the inductor
into saturation by approximately 20% (the point where
the inductance is 20% below the nominal value). For
highest efficiency, use a coil with low DC resistance,
preferably under 20mΩ. To minimize radiated noise,
use a toroid, a pot core, or a shielded coil.
Table 1 lists inductor suppliers and specific recommended inductors.
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
10
______________________________________________________________________________________
MAXIMUM OUTPUT CURRENT (A)
0
INPUT VOLTAGE (V)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
2345
R
SENSE
= 20m
Ω
R
SENSE
= 25m
Ω
R
SENSE
= 35m
Ω
R
SENSE
= 100m
Ω
R
SENSE
= 50m
Ω
V
OUT
= 5V
L = 22µH
Figure 4a. Maximum Output Current vs. Input Voltage
(V
OUT
= 5V)
Figure 4b. Maximum Output Current vs. Input Voltage
(V
OUT
= 12V)
Figure 4c. Maximum Output Current vs. Input Voltage
(V
OUT
= 15V)
Figure 4d. Maximum Output Current vs. Input Voltage
(V
OUT
= 24V)
MAXIMUM OUTPUT CURRENT (A)
0
INPUT VOLTAGE (V)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
2 4 6 8 10 12 14 16
R
SENSE
= 100m
Ω
R
SENSE
= 50m
Ω
V
OUT
= 15V
L = 22µH
R
SENSE
= 20m
Ω
R
SENSE
= 25m
Ω
R
SENSE
= 35m
Ω
3.5
V
= 12V
OUT
L = 22µH
3.0
2.5
2.0
1.5
1.0
MAXIMUM OUTPUT CURRENT (A)
0.5
R
SENSE
R
SENSE
R
= 35m
SENSE
0
2 4 6 8 10 12
Ω
= 20m
Ω
= 25m
Ω
INPUT VOLTAGE (V)
R
R
SENSE
SENSE
= 50m
= 100m
Ω
Ω
0.8
V
= 24V
OUT
L =150µH
0.6
= 50m
Ω
R
SENSE
= 100m
Ω
R
SENSE
0.4
0.2
MAXIMUM OUTPUT CURRENT (A)
0
2
= 200m
Ω
R
SENSE
61014
INPUT VOLTAGE (V)
Page 11
Power Transistor Selection
Use an N-channel MOSFET power transistor with the
MAX1771.
To ensure the external N-channel MOSFET (N-FET) is
turned on hard, use logic-level or low-threshold
N-FETs when the input drive voltage is less than 8V. This
applies even in bootstrapped mode, to ensure start-up.
N-FETs provide the highest efficiency because they do
not draw any DC gate-drive current.
When selecting an N-FET, three important parameters
are the total gate charge (Qg), on-resistance (r
DS(ON)
),
and reverse transfer capacitance (C
RSS
).
Qgtakes into account all capacitances associated with
charging the gate. Use the typical Qgvalue for best
results; the maximum value is usually grossly overspecified since it is a guaranteed limit and not the measured value. The typical total gate charge should be
50nC or less. With larger numbers, the EXT pins may
not be able to adequately drive the gate. The EXT
rise/fall time varies with different capacitive loads as
shown in the Typical Operating Characteristics.
The two most significant losses contributing to the
N-FET’s power dissipation are I2R losses and switching
losses. Select a transistor with low r
DS(ON)
and low
C
RSS
to minimize these losses.
Determine the maximum required gate-drive current
from the Qgspecification in the N-FET data sheet.
The MAX1771’s maximum allowed switching frequency
during normal operation is 300kHz; but at start-up, the
maximum frequency can be 500kHz, so the maximum
current required to charge the N-FET’s gate is
f(max) x Qg(typ). Use the typical Qgnumber from the
transistor data sheet. For example, the Si9410DY has a
Qg(typ) of 17nC (at VGS= 5V), therefore the current
required to charge the gate is:
I
GATE
(max)
= (500kHz) (17nC) = 8.5mA.
The bypass capacitor on V+ (C2) must instantaneously
furnish the gate charge without excessive droop (e.g.,
less than 200mV):
Q
g
∆V+ = ——
C2
Continuing with the example, ∆V+ = 17nC/0.1µF = 170mV.
Figure 2a’s application circuit uses an 8-pin Si9410DY
surface-mount N-FET that has 50mΩ on-resistance with
4.5V V
GS
, and a guaranteed VTHof less than 3V. Figure
2b’s application circuit uses an MTD20N03HDL logiclevel N-FET with a guaranteed threshold voltage (V
TH
)
of 2V.
Diode Selection
The MAX1771’s high switching frequency demands a
high-speed rectifier. Schottky diodes such as the
1N5817–1N5822 are recommended. Make sure the
Schottky diode’s average current rating exceeds the
peak current limit set by R
SENSE
, and that its break-
down voltage exceeds V
OUT
. For high-temperature
applications, Schottky diodes may be inadequate due
to their high leakage currents; high-speed silicon
diodes such as the MUR105 or EC11FS1 can be used
instead. 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 peak inductor current and the output filter
capacitor’s ESR determines the amplitude of the ripple
seen on the output voltage. Two OS-CON 150µF, 16V
output filter capacitors in parallel with 35mΩ of ESR each
typically provide 75mV ripple when stepping up from 5V
to 12V at 500mA (Figure 2a). Smaller-value and/or higher-ESR capacitors are acceptable for light loads or in
applications that can tolerate higher output ripple.
Since the output filter capacitor’s ESR affects efficiency, use low-ESR capacitors for best performance. See
Table 1 for component selection.
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 switching action of
the MAX1771. 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
1A, 68µF (C1) is adequate, although smaller bypass
capacitors may also be acceptable.
Bypass the IC with a 0.1µF ceramic capacitor (C2)
placed as close to the V+ and GND pins as possible.
Reference Capacitor
Bypass REF with a 0.1µF capacitor (C3). REF can
source up to 100µA of current for external loads.
Feed-Forward Capacitor
In adjustable output voltage and non-bootstrapped
modes, parallel a 47pF to 220pF capacitor across R2,
as shown in Figures 2 and 3. Choose the lowest capacitor value that insures stability; high capacitance values
may degrade line regulation.
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
______________________________________________________________________________________
11
Page 12
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
12
______________________________________________________________________________________
Table 1. Component Suppliers
PRODUCTION INDUCTORS CAPACITORS TRANSISTORS
Surface Mount
Sumida
CD54 series
CDR125 series
Coiltronics
CTX20 series
Coilcraft
DO3316 series
DO3340 series
Matsuo
267 series
Sprague
595D series
AVX
TPS series
Siliconix
Si9410DY
Si9420DY (high voltage)
Motorola
MTP3055EL
MTD20N03HDL
MMFT3055ELT1
MTD6N1O
MMBT8099LT1
MMBT8599LT1
Through Hole
Sumida
RCH855 series
RCH110 series
Sanyo
OS-CON series
Nichicon
PL series
Motorola
1N5817–1N5822
MUR115 (high voltage)
MUR105 (high-speed
silicon)
Central Semiconductor
CMPSH-3
CMPZ5240
Nihon
EC11 FS1 series (highspeed silicon)
Motorola
MBRS1100T3
MMBZ5240BL
DIODES
AVX USA: (803) 448-9411 (803) 448-1943
SUPPLIER PHONE FAX
Coiltronics USA: (516) 241-7876 (516) 241-9339
Sumida
USA: (708) 956-0666 (708) 956-0702
Japan: 81-3-3607-5111 81-3-3607-5144
Matsuo
USA: (714) 969-2491 (714) 960-6492
Japan: 81-6-337-6450 81-6-337-6456
Coilcraft USA: (708) 639-6400 (708) 639-1469
Motorola USA: (800) 521-6274 (602) 952-4190
Central
Semiconductor
USA: (516) 435-1110 (516) 435-1824
Nihon USA: (805) 867-2555 (805) 867-2556
Sanyo
USA: (619) 661-6835 (619) 661-1055
Japan: 81-7-2070-1005 81-7-2070-1174
Siliconix USA: (800) 554-5565 (408) 970-3950
Sprague USA: (603) 224-1961 (603) 224-1430
Nichicon USA: (708) 843-7500 (708) 843-2798
Page 13
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
______________________________________________________________________________________
13
Applications Information
Low Input Voltage Operation
When using a power supply that decays with time
(such as a battery), the N-FET transistor will operate in
its linear region when the voltage at EXT approaches
the threshold voltage of the FET, dissipating excessive
power. Prolonged operation in this mode may damage
the FET. This effect is much more significant in nonbootstrapped mode than in bootstrapped mode, since
bootstrapped mode typically provides much higher
V
GS
voltages. To avoid this condition, make sure V
EXT
is above the VTHof the FET, or use a voltage detector
(such as the MAX8211) to put the IC in shutdown mode
once the input supply voltage falls below a predetermined minimum value. Excessive loads with low input
voltages can also cause this condition.
Starting Up Under Load
The Typical Operating Characteristics show the Start-
Up Voltage vs. Load Current graph for bootstrappedmode operation. This graph depends on the type
of power switch used. The MAX1771 is not designed to
start up under full load in bootstrapped mode with low
input voltages.
Layout Considerations
Due to high current levels and fast switching waveforms, which radiate noise, proper PC board layout is
essential. Protect sensitive analog grounds by using a
star ground configuration. 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. Place input bypass capacitor C2 as close as possible to V+ and GND.
Excessive noise at the V+ input may falsely trigger the
timing circuitry, resulting in short pulses at EXT. If this
occurs it will have a negligible effect on circuit efficiency. If desired, place a 4.7µF directly across the V+ and
GND pins (in parallel with the 0.1µF C2 bypass capacitor) to reduce the noise at V+.
Other Application Circuits
4 Cells to 5V (or 3 Cells to 3.3V), 500mA
Step-Up/Down Converter
The circuit shown in Figure 5 generates 5V (or 3.3V) at
500mA with 85% efficiency, from an input voltage that
varies above and below the output. The output couples
to the switching circuitry via a capacitor. This configuration offers two advantages over flyback-transformer
and step-up linear-regulator circuits: smooth regulation
as the input passes through the output, and no output
current in shutdown.
This circuit requires two inductors, which can be wound
on one core with no regard to coupling since they do
not work as a transformer. L1 and L2 can either be
wound together (as with the Coiltronics CTX20-4) or
kept as two separate inductors; both methods provide
equal performance. Capacitors C2 and C3 should be
low-ESR types for best efficiency. A 1µF ceramic
capacitor will work at C2, but with about 3% efficiency
loss. C2’s voltage rating must be greater than the maximum input voltage. Also note that the LX switch must
withstand a voltage equal to the sum of the input and
output voltage; for example, when converting 11V to
5V, the switch must withstand 16V.
LX switch pulses are captured by Schottky diode D2 to
boost V+ to (V
OUT
+ VIN). This improves efficiency with
a low input voltage, but also limits the maximum input
supply to 11V. If the input voltage does not fall below 4V
and if a 3V logic threshold FET is used for Q1, you may
omit D2 and connect V+ directly to the input supply.
12V Output Buck/Boost
The circuit in Figure 6 generates 12V from a 4.5V to
16V input. Higher input voltages are possible if you
MAX1771
SHDN
R1
0.1
Ω
REF
AGND
R2
†
R3
†
C5
47pF
GND
4
3V = OFF
5
FB
Q1**
SEE TEXT FOR FURTHER COMPONENT INFO
**V
IN
MAY BE LOWER THAN INDICATED IF THE SUPPLY IS NOT
**REQUIRED TO START UNDER FULL LOAD
**MOTOROLA MMFT3055ELT1
†
FOR 5V: R2 = 200kΩ, R3 = 470k
Ω
3.3V: R2 = 100kΩ, R3 = 20k
Ω
3
6
EXT
CS
1
L1
20µH
1 CTX20-4
8
2
D2
1N5817
D1
1N5817
C3
220µF
10V
C1
2.2µF
C2
47µF
16V
C4
0.1µF
V+
V
IN
*
3V TO 11V
V
OUT
5V
500mA
7
L2
Figure 5. Step-Up/Down for a 5V/3.3V Output
Page 14
MAX1771
carefully observe the component voltage ratings, since
some components must withstand the sum of the input
and output voltage (27V in this case). The circuit operates as an AC-coupled boost converter, and does not
change operating modes when crossing from buck to
boost. There is no instability around a 12V input.
Efficiency ranges from 85% at medium loads to about
82% at full load. Also, when shutdown is activated
(SHDN high) the output goes to 0V and sources no current. A 1µF ceramic capacitor is used for C2. A larger
capacitor value improves efficiency by about 1% to 3%.
D2 ensures start-up for this AC-coupled configuration
by overriding the MAX1771’s Dual-Mode feature, which
allows the use of preset internal or user-set external
feedback. When operating in Dual-Mode, the IC first
tries to use internal feedback and looks to V+ for its
feedback signal. However, since V+ may be greater
than the internally set feedback (12V for the MAX1771),
the IC may think the output is sufficiently high and not
start. D2 ensures start-up by pulling FB above ground
and forcing the external feedback mode. In a normal
(not AC-coupled) boost circuit, D2 isn’t needed, since
the output and FB rise as soon as input power is
applied.
Transformerless -48V to +5V at 300mA
The circuit in Figure 7 uses a transformerless design to
supply 5V at 300mA from a -30V to -75V input supply.
The MAX1771 is biased such that its ground connections are made to the -48V input. The IC’s supply voltage (at V+) is set to about 9.4V (with respect to -48V)
by a zener-biased emitter follower (Q2). An N-channel
FET (Q1) is driven in a boost configuration. Output regulation is achieved by a transistor (Q3), which level
shifts a feedback signal from the 5V output to the IC’s
FB input. Conversion efficiency is typically 82%.
When selecting components, be sure that D1, Q1, Q2,
Q3, and C6 are rated for the full input voltage plus a
reasonable safety margin. Also, if D1 is substituted, it
should be a fast-recovery type with a trrless than 30ns.
R7, R9, C8, and D3 are optional and may be used to
soft start the circuit to prevent excessive current surges
at power-up.
Battery-Powered LCD Bias Supply
The circuit in Figure 8 boosts two cells (2V min) to 24V
for LCD bias or other positive output applications.
Output power is boosted from the battery input, while
V+ voltage for the MAX1771 is supplied by a 5V or 3.3V
logic supply.
5V, 1A Boost Converter
The circuit in Figure 9 boosts a 2.7V to 5.5V input to a
regulated 5V, 1A output for logic, RF power, or PCMCIA
applications. Efficiency vs. load current is shown in the
adjacent graph.
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
14
______________________________________________________________________________________
MAX1771
SHDN
R1
0.1
Ω
REF
AGND
R2
200k
Ω
1%
R3
28k
Ω
1%
GND
4
ON
OFF
5
FB
NOTE: HIGHCURRENT GND
Q1**
*SEE TEXT FOR FURTHER
COMPONENT INFORMATION
**Q1 = MOTOROLA MMFT3055ELT1
L1 + L2 = ONE COILTRONICS CTX20-4
3
6
EXT
CS
1
L1
20µH
8
2
D1
1N5819
D2*
1N4148
C3
100µF
16V
C1
33µF
16V
C2*
1µF
C5
0.1µF
V+
V
IN
4.5V TO 15V
V
OUT
12V
250mA
L2*
20µH
7
C4
100µF
16V
NOTE: KEEP ALL TRACES CONNECTED
TO PIN 3 AS SHORT AS POSSIBLE
Figure 6. 12V Buck/Boost from a 4.5V to 15V Input
Page 15
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
______________________________________________________________________________________
15
MAX1771
SHDN
R1
0.15
Ω
R7
200
Ω
REF
AGND
GND
4
5
Q1
MTD6N10
6
-48V
EXT
CS
1
L1
D03340
220µH–680µH
8
3
1
2
D1
MBRS1100T3
D2
CMPZ5240/
MMBZ5240BL
D3
CMPSH-3
C8
1µF
+5V
300mA
7
C5
0.1µF
C6
10µF
100V
FB
3
V+
Q3
MMBT8599LT1
2
C7
220pF
R2
47k
Ω
1%
R3
16k
Ω
1%
C4
2.2µF
20V
R5
1k
Ω
R6
200k
Ω
R4
100k
Ω
C1
220µF
10V
C3
0.33µF
Q2
MMBT8099LT1
R9
5.1k
Ω
C2
220µF
10V
GNDREF
MAX1771
V+
EXTL
CS
4
SHDN
0.1µF
3.3V OR 5V
LOGIC
SUPPLY
BATTERY
INPUT
2V TO 12V
8
6, 7
5
R
SENSE
0.2Ω
1N5817
1
2
L1
22µH
OUTPUT
ADJ = 12V TO 24V
(AS SHOWN)
N
47µF
0.1µF
FB
3
R3
10kΩ
10kΩ
R2
150kΩ
MMFT3055ELT1
OFF
ON
Figure 7. -48V Input to 5V Output at 300mA, Without a Transformer
Figure 8. 2V Input to 24V Output LCD Bias
Page 16
MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
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
© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
___________________Chip Topography
V+
FB
0.126"
(3.200mm)
0.080"
(2.032mm)
EXT
CS
GND
AGND
SHDN REF
TRANSISTOR COUNT: 501
SUBSTRATE CONNECTED TO V+
100
50
1m 10m 100m 1
EFFICIENCY vs. LOAD CURRENT
60
LOAD CURRENT (A)
EFFICIENCY (%)
70
80
90
VIN = 3V
MAX1771
GND
76
1N5820
330µF
0.1µF
4
5
1
8
2
3
OUTPUT
5V
1A
0.1µF
232k
Ω
100k
Ω
0.04
Ω
150µF
22µH
MTD20N03HDL
INPUT
2.7V TO 5.5V
CS
EXT
V+
FB
100pF
AGND
SHDN
REF
OFF
ON
VIN = 4V
Figure 9. 5V/1A Boost Converter
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