Maxim MAX1771EPA, MAX1771CPA, MAX1771C-D, MAX1771MJA, MAX1771ESA Datasheet

19-0263; Rev 1; 7/95

EVALUATION KIT MANUAL

FOLLOWS DATA SHEET

12V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controller

_______________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. Order the MAX1771EVKIT-SO.

________________________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

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**

* Contact factory for dice specifications.
** Contact factory for availability and processing to MIL-STD-883B.

__________________Pin Configuration

MAX1771

__________Typical Operating Circuit

TOP VIEW

INPUT 

2V TO V

OUT

ON/OFF

OUTPUT

SHDN

REF

FB AGND GND

________________________________________________________________

CS

V+

N

EXT

MAX1771

12V

EXT

SHDN

1
2

V+

MAX1771

3

FB

4

DIP/SO

Maxim Integrated Products

Call toll free 1-800-998-8800 for free samples or literature.

8

CS

7

GND

6

AGND

5

REF

1

12V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controller

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

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

= +70°C)

A

MAX1771

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

Input Voltage Range V

Minimum Start-Up Voltage 1.8 2.0 V
Supply Current 85 110

Standby Current

Output Voltage (Note 1) V

Output Voltage Line Regulation
(Note 2)

Output Voltage Load Regulation
(Note 2)

Maximum Switch On-Time tON(max) 12 16 20
Minimum Switch Off-Time t

Efficiency 92

Reference Voltage V

REF Load Regulation 0µA ≤ I

FB Trip Point Voltage V

= 0mA, TA= T

LOAD

PARAMETER

to T

MIN

, unless otherwise noted. Typical values are at TA= +25°C.)

MAX

SYMBOL CONDITIONS MIN TYP MAX UNITS

MAX1771 (internal feedback resistors) 2.0 12.5
MAX1771C/E (external resistors) 3.0 16.5
MAX1771MJA (external resistors) 3.1 16.5

V+ = 16.5V, SHDN = 0V (normal operation)
V+ = 10.0V, SHDN ≥ 1.6V (shutdown)
V+ = 16.5V, SHDN ≥ 1.6V (shutdown)

V+ = 2.0V to 12.0V, over full load range,
Circuit of Figure 2a

V+ = 5V to 7V, V
I

= 700mA, Circuit of Figure 2a

LOAD

V+ = 6V, V
500mA, Circuit of Figure 2a

(min) 1.8 2.3 2.8

OFF

V+ = 5V, V
Circuit of Figure 2a

I

0µA

REF

FB

REF =

REF

3V ≤ V+ ≤ 16.5V 40 100
MAX1771C 1.4700 1.5 1.5300
MAX1771E 1.4625 1.5 1.5375
MAX1771M 1.4550 1.5 1.5450

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

25
4

11.52 12.0 12.48

= 12V

= 12V, I

OUT

= 12V, I

OUT

≤ 100µA

OUT

= 0mA to

LOAD

= 500mA,

LOAD

MAX1771C 1.4700 1.5 1.5300
MAX1771E 1.4625 1.5 1.5375
MAX1771M 1.4550 1.5 1.5450
MAX1771C/E 410
MAX1771M 415

5 mV/V

20 mV/A

µA
µA

µs
µs

%

V

mV

µV/VREF Line Regulation

V

_______________________________________________________________________________________

2

12V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controller

ELECTRICAL CHARACTERISTICS (continued)

(V+ = 5V, I

FB Input Current I

SHDN Input High Voltage V
SHDN Input Low Voltage V
SHDN Input Current ±1

Current-Limit Trip Level V

CS Input Current 0.01 ±1
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.

= 0mA, TA= T

LOAD

PARAMETERS

to T

MIN

, unless otherwise noted. Typical values are at TA= +25°C.)

MAX

SYMBOL CONDITIONS

MAX1771C ±20

FB

MAX1771M ±60
V+ = 2.0V to 16.5V 1.6 V

IH

V+ = 2.0V to 16.5V 0.4 V

IL

V+ = 16.5V, SHDN = 0V or V+

V+ = 5V to 16V

CS

MAX1771C/E
MAX1771M

MIN TYP MAX

85 100 115
75 100 125

UNITS

nAMAX1771E ±40

µA

mV

µA

__________________________________________Typical Operating Characteristics

(TA = +25°C, unless otherwise noted.)

EFFICIENCY vs. LOAD CURRENT

100

95
90

85
80

75

EFFICIENCY (%)

70
65

60

(BOOTSTRAPED MODE)

VIN = 10V

VIN = 5V

1



10

LOAD CURRENT (mA)

100



VIN = 3V

VIN = 8V

V

= 12V

OUT

CIRCUIT OF
FIGURE 2a

1000

10,000

MAX1771–01

EFFICIENCY vs. LOAD CURRENT

(NON-BOOTSTRAPED MODE)

100

95

VIN =10V

90

85
80

VIN = 5V

75

EFFICIENCY (%)

70
65

60

1



10



100

LOAD CURRENT (mA)

VIN = 8V

V

= 12V

OUT

CIRCUIT OF
FIGURE 2b

1000

10,000

700

600

MAX1771–02

500

400

300

LOAD CURRENT (mA)

200

100

0

LOAD CURRENT vs.

MINIMUM START-UP INPUT VOLTAGE

= 12V, CIRCUIT OF FIGURE 2a

V

OUT

EXTERNAL FET THRESHOLD
LIMITS FULL-LOAD START-UP
BELOW 3.5V

2.00

2.25 2.50 2.75 3.00 3.25 3.50

MINIMUM START-UP INPUT VOLTAGE (V)

MAX1771

MAX1771-TOC3

_______________________________________________________________________________________

3

12V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controller

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

SUPPLY CURRENT vs. TEMPERATURE

4

V

= 12V, VIN = 5V

OUT

CIRCUIT OF FIGURE 2a
BOOTSTRAPPED MODE

MAX1771

3

2

SUPPLY CURRENT (mA)

250

200

150

100

50

REFERENCE OUTPUT RESISTANCE (Ω)

SCHOTTKY DIODE

1

LEAKAGE EXCLUDED

0

-50 -25 0

-75

0

-60 -20 60 140

TEMPERATURE (°C)

REFERENCE OUTPUT RESISTANCE vs.

TEMPERATURE

-40 0 8040 120
TEMPERATURE (°C)



ENTIRE
CIRCUIT

25 50 75 100 125

10µA

50µA

20 100

100µA

0.8

MAX1771-04

0.6

0.4

SUPPLY CURRENT (mA)

0.2

0

1.506

1.504

MAX1771-07

1.502

1.500

1.498

REFERENCE (V)

1.496

1.494

1.492

SUPPLY CURRENT vs. SUPPLY VOLTAGE



V

= 12V

OUT

BOOTSTRAPPED
CIRCUIT OF
FIGURE 2a

NON-BOOTSTRAPPED



CIRCUIT OF FIGURE 2b

12

2

68

4

SUPPLY VOLTAGE (V)

REFERENCE vs. TEMPERATURE

-60 -20 60 140

20 100-40 0 8040 120

TEMPERATURE (°C)

10



EXT RISE/FALL TIME vs. SUPPLY VOLTAGE

250

MAX1771-05

200

150

100

EXT RISE/FALL TIME (ns)

50

MAX1771-08

0

2

MAXIMUM SWITCH ON-TIME vs.

16.5



16.0



ON(MAX) (µs)

t



15.5

-60

68

4

SUPPLY VOLTAGE (V)

TEMPERATURE

-30 0 30 60
TEMPERATURE (°C)

C

EXT

C

EXT

C

EXT

C

EXT



= 2200pF



= 1000pF



= 446pF



= 100pF



90

10

120 150

MAX1771-06

12

MAX1771-09

SHUTDOWN CURRENT vs. TEMPERATURE

4.0

3.5

3.0

2.5

2.0

1.5

1.0

SHUTDOWN CURRENT (µA)

0.5
0

-60 -20 60 140

_______________________________________________________________________________________

4

V+ = 8V

V+ = 4V

TEMPERATURE (°C)

V+ = 15V

20 100-40 0 8040 120

MAX1771-10

MINIMUM SWITCH OFF-TIME vs.

2.30



2.25

OFF(MIN) (µs)

t



2.20

-60

TEMPERATURE

-30 0 30 60
TEMPERATURE (°C)

MAX1771-11



t



t

120 150

90

MAXIMUM SWITCH ON-TIME/

MINIMUM SWITCH OFF-TIME RATIO

8.0





7.5

7.0

OFF(MIN) RATIO



6.5

ON(MAX)/

6.0

-30 0 30 60

-60

vs. TEMPERATURE



TEMPERATURE (°C)

90

MAX1771-12

120 150

12V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controller

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 2a, TA = +25°C, unless otherwise noted.)

HEAVY-LOAD SWITCHING WAVEFORMS

A



B

C

= 5V, I

= 900mA, V

V

IN

OUT

A: EXT VOLTAGE, 10V/div
B: INDUCTOR CURRENT, 1A/div

RIPPLE, 50mV/div, AC-COUPLED

C: V

OUT

OUT

2µs/div

= 12V

LINE-TRANSIENT RESPONSE

A



V

OUT

0V
I

LIM

0A

7V



5V

MEDIUM-LOAD SWITCHING WAVEFORMS

A



B

C

= 5V, I

= 500mA, V

V

IN

OUT

A: EXT VOLTAGE, 10V/div
B: INDUCTOR CURRENT, 1A/div

RIPPLE, 50mV/div, AC-COUPLED

C: V

OUT

OUT

10µs/div

= 12V

LOAD-TRANSIENT RESPONSE



A

V

OUT

0V
I

LIM

0A


500mA


0A

MAX1771

B

= 700mA, V

I

OUT

, 5V to 7V, 2V/div

A: V

IN

RIPPLE, 100mV/div, AC-COUPLED

B: V

OUT

_______________________________________________________________________________________

0V

B

5ms/div

= 12V

OUT

= 6V, V

V

IN

OUT

A: LOAD CURRENT, 0mA to 500mA, 500mA/div

RIPPLE, 100mV/div, AC-COUPLED

B: V

OUT

5ms/div

= 12V

5

12V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controller

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 2a, TA = +25°C, unless otherwise noted.)

ENTERING/EXITING SHUTDOWN



MAX1771



A


0V

B

= 500mA, VIN = 5V

I

OUT

A: SHDN, 5V/div
B: V

OUT

2ms/div

, 5V/div

5V
0V

______________________________________________________________Pin Description

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
Active-High TTL/CMOS Logic-Level Shutdown Input. In shutdown mode, V

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.

Setting the Output Voltage

is a diode drop

OUT

section.

_______________________________________________________________________________________

6

12V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controller

_______________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 pulse­width 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

REF

1.5V

REFERENCE

MIN OFF-TIME

ONE-SHOT
Q TRIG

2.3µs

ERROR

COMPARATOR

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

, which is connected to V+) and the input voltage

OUT

range is 2V to V

. The voltage applied to the gate of

OUT

the external power transistor is switched from V
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

FB

DUAL-MODE
COMPARATOR

SHDN

V+

50mV

MAX1771

N

BIAS

CIRCUITRY

OUT

MAX1771

to

MAX ON-TIME

ONE-SHOT

QTRIG

16µs

Figure 1. Functional Diagram

_______________________________________________________________________________________

F/F

QS

R

LOW-VOLTAGE

OSCILLATOR

CURRENT-SENSE

AMPLIFIER

0.1V

2.5V

EXT

CS

7

12V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controller

VIN = 5V

C2

0.1µF

5

REF

C3

0.1µF
4

MAX1771

SHDN

3

FB

6

AGND

2

V+

MAX1771

GND

7

EXT





L1

22µH





1

8

CS

C1

68µF

D1

1N5817-22

N
Si9410DY/

MTD20N03HDL



R

SENSE

40mΩ

V

OUT

C4
300µF

Figure 2a. 12V Preset Output, Bootstrapped

VIN = 4V

C2

0.1µF

5

REF

C3

0.1µF
4

SHDN

6

AGND

V

OUT

R2 = (R1) ( -1)

V

REF

V

= 1.5V

REF



2

V+

MAX1771

GND

7

EXT

22µH





1

8

CS

3

FB

L1

1N5817-22

N

Si9410DY/
MTD20N03HDL

R

SENSE

40mΩ

R1

28k

C1

47µF

D1

C4
200µF

R2

140k

C5

100pF

Figure 2c. 9V Output, Bootstrapped

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

= 12V

@ 0.5A

V

OUT

= 9V

VIN = 5V

C1

68µF

C3

0.1µF

R2 = (R1) ( -1)

V

= 1.5V

REF

C2

0.1µF
2

V

V

REF

SHDN

AGND

OUT
REF

V+

MAX1771

GND

7



5

4

6

EXT

L1

22µH





1

8

CS

3

FB

D1

1N5817-22

N
MTD20N03HDL

R

SENSE

40mΩ

R1

18k

300µF

C4

R2

127k

C5

100pF

V

OUT

@ 0.5A

= 12V

Figure 2b. 12V Output, Non-Bootstrapped

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.

_______________________________________________________________________________________

8

12V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controller

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

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
If a decaying voltage source (such as a battery) is
used, see the additional notes in the

Operation

section.

The MAX1771’s output voltage can be adjusted from
very high voltages down to 3V, using external resistors

Setting the Output Voltage

Typical Operating Characteristics

Low Input Voltage

FB

MAX1771

GND

Figure 3. Adjustable Output Circuit

R2

R1

C5*

R1 = 10k TO 500k

V

OUT

R2 = R1 ( -1)

V

REF

= 1.5V

V

REF

* SEE TEXT FOR VALUE



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

V

OUT

REF

)

where V

equals 1.5V.

REF

R2 = (R1) (––––– -1

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

Use the theoretical output current curves shown in
Figures 4a–4d to select R

. They were derived

SENSE

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

. The voltage drop across the diode was

SENSE

assumed to be 0.5V, and the drop across the power
switch r

and coil resistance was assumed to be

DS(ON)

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

V

OUT

SENSE

MAX1771

_______________________________________________________________________________________

9

12V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controller

3.5
V

= 5V

OUT

L = 22µH

3.0

R

= 20mΩ

SENSE

2.5

R

= 25mΩ

SENSE

2.0

MAX1771

1.5

1.0

MAXIMUM OUTPUT CURRENT (A)

0.5

0

2345

R

SENSE

R

SENSE

INPUT VOLTAGE (V)

R

SENSE

= 35mΩ

= 50mΩ

= 100mΩ

Figure 4a. Maximum Output Current vs. Input Voltage

= 5V)

(V

OUT

3.5

V

= 15V

OUT

L = 22µH

3.0

R

= 20mΩ

SENSE

R

2.5

2.0

1.5

1.0

MAXIMUM OUTPUT CURRENT (A)

0.5

0

Figure 4c. Maximum Output Current vs. Input Voltage

= 15V)

(V

OUT

= 25mΩ

SENSE

R

= 35mΩ

SENSE

R

= 50mΩ

SENSE

R

= 100mΩ

SENSE

2 4 6 8 10 12 14 16

INPUT VOLTAGE (V)

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.

3.5
V

= 12V

OUT

L = 22µH

3.0

R

= 20mΩ

SENSE

R

2.5

2.0

1.5

1.0

MAXIMUM OUTPUT CURRENT (A)

0.5

0

= 25mΩ

SENSE

R

= 35mΩ

SENSE

R

= 50mΩ

SENSE

R

= 100mΩ

SENSE

2 4 6 8 10 12

INPUT VOLTAGE (V)

Figure 4b. Maximum Output Current vs. Input Voltage
(V

= 12V)

OUT

0.8
V

= 24V

OUT

L =150µH

0.6

R

= 50mΩ

SENSE

0.4

0.2

MAXIMUM OUTPUT CURRENT (A)

0

2

61014

INPUT VOLTAGE (V)

R

R

SENSE

SENSE

= 100mΩ

= 200mΩ

Figure 4d. Maximum Output Current vs. Input Voltage

= 24V)

(V

OUT

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.

.

______________________________________________________________________________________

10

12V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controller

Use an N-channel MOSFET power transistor with the

Power Transistor Selection

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
and reverse transfer capacitance (C

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

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
C

RSS

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:

The bypass capacitor on V+ (C2) must instantaneously
furnish the gate charge without excessive droop (e.g.,
less than 200mV):

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 VGS, and a guaranteed VTHof less than 3V. Figure
2b’s application circuit uses an MTD20N03HDL logic­level N-FET with a guaranteed threshold voltage (VTH)
of 2V.

Typical Operating Characteristics

to minimize these losses.

I

GATE

= (500kHz) (17nC) = 8.5mA.

(max)

Q

∆V+ = ——

C2

g

RSS

).

DS(ON)

DS(ON)

.

and low

),

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
down voltage exceeds V
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.

, and that its break-

SENSE

. For high-temperature

OUT

Capacitor Selection

Output Filter Capacitor

Diode Selection

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

______________________________________________________________________________________

11

12V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controller

Table 1. Component Suppliers

PRODUCTION INDUCTORS CAPACITORS TRANSISTORS

Sumida

CD54 series
CDR125 series

Surface Mount

MAX1771

Through Hole

SUPPLIER PHONE FAX

AVX USA: [phone] [phone]
Central

Semiconductor
Coilcraft USA: [phone] [phone]
Coiltronics USA: [phone] [phone]

Matsuo

Motorola USA: [phone] [phone]
Nichicon USA: [phone] [phone]
Nihon USA: [phone] [phone]

Sanyo

Siliconix USA: [phone] [phone]
Sprague USA: [phone] [phone]

Sumida

Coiltronics

CTX20 series

Coilcraft

DO3316 series
DO3340 series

Sumida

RCH855 series
RCH110 series

USA: [phone] [phone]

USA: [phone] [phone]
Japan: 81-6-337-6450 81-6-337-6456

USA: [phone] [phone]
Japan: 81-7-2070-1005 81-7-2070-1174

USA: [phone] [phone]
Japan: 81-3-3607-5111 81-3-3607-5144

Matsuo

267 series

Sprague

595D series

AVX

TPS series

Sanyo

OS-CON series

Nichicon

PL series

Siliconix

Si9410DY
Si9420DY (high voltage)

Motorola

MTP3055EL
MTD20N03HDL
MMFT3055ELT1
MTD6N1O
MMBT8099LT1
MMBT8599LT1

DIODES

Central Semiconductor

CMPSH-3
CMPZ5240

Nihon

EC11 FS1 series (high­speed silicon)

Motorola

MBRS1100T3
MMBZ5240BL

Motorola

1N5817–1N5822
MUR115 (high voltage)
MUR105 (high-speed
silicon)

______________________________________________________________________________________

12

12V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controller

C1

2.2µF

2

MAX1771

AGND

7

R2

V+

EXT

CS

FB

6

3

†

4

3V = OFF

Figure 5. Step-Up/Down for a 5V/3.3V Output

SHDN

5

C4

REF

0.1µF

GND

SEE TEXT FOR FURTHER COMPONENT INFO

VIN MAY BE LOWER THAN INDICATED IF THE SUPPLY IS NOT 

**

REQUIRED TO START UNDER FULL LOAD

**

**MOTOROLA MMFT3055ELT1

†

FOR 5V: R2 = 200kΩ, R3 = 470kΩ

3.3V: R2 = 100kΩ, R3 = 20kΩ

1


8

D2
1N5817

R3

C5

47pF

3V TO 11V

†

V

IN

Q1**

*

L1
20µH
1 CTX20-4

47µF

R1

0.1Ω

16V

V

OUT

D1

5V

1N5817

500mA

C2

L2

C3
220µF
10V

__________Applications Information

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
VGSvoltages. To avoid this condition, make sure V
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.

The

Typical Operating Characteristics

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.

Low Input Voltage Operation

EXT

Starting Up Under Load

show the Start-

Due to high current levels and fast switching wave-

Layout Considerations

forms, 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

+ VIN). This improves efficiency with

OUT

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

______________________________________________________________________________________

13

12V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controller

tries to use internal feedback and looks to V+ for its

V

IN

4.5V TO 15V

C1

OFF

4

ON

MAX1771

5

C5

0.1µF

NOTE: KEEP ALL TRACES CONNECTED
TO PIN 3 AS SHORT AS POSSIBLE

SHDN

REF

V+

MAX1771

R3
28k
1%

2

33µF

16V

EXT

AGND

GND

FB

CS

3

D2*
1N4148

L1†
20µH

D1

C2*
1µF

1N5819

1

Q1**

8

6
7

L2*
20µH

R1

0.1Ω

R2

200k

1%

*SEE TEXT FOR FURTHER 
COMPONENT INFORMATION
**Q1 = MOTOROLA MMFT3055ELT1

†

L1 + L2 = ONE COILTRONICS CTX20-4

C4

C3

100µF

100µF

16V

16V

NOTE: HIGH-
CURRENT GND

Figure 6. 12V Buck/Boost from a 4.5V to 15V Input

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

V

OUT

12V

250mA

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.

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.

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.

Battery-Powered LCD Bias Supply

5V, 1A Boost Converter

______________________________________________________________________________________

14

12V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controller

L1

D03340

220µH–680µH

10µF
100V

EXT

AGND

C6

1

2


8

CS

2

V+

-48V

6

0.1µF

4

SHDN

C5

5

REF

3

MAX1771

FB

7

GND

Figure 7. -48V Input to 5V Output at 300mA, Without a Transformer

MBRS1100T3

3

Q1
MTD6N10

1

Q2

MMBT8099LT1

R1

0.15Ω

D1

5.1k

C8

R9

1µF

R6

R5

200k

1k

D3

CMPSH-3

R7

200Ω

2.2µF
20V

D2
CMPZ5240/
MMBZ5240BL

C4

C7
220pF

Q3

MMBT8599LT1

R2
47k

1%

R4
100k

R3
16k

1%

C1
220µF
10V

C2
220µF
10V

+5V

300mA

C3

0.33µF

MAX1771

BATTERY
INPUT
2V TO 12V

3.3V OR 5V
LOGIC

SUPPLY

ON

OFF



0.1µF

Figure 8. 2V Input to 24V Output LCD Bias

______________________________________________________________________________________

4

0.1µF

SHDN

MAX1771

L1



22µH

SENSE

0.2Ω

1N5817

N

MMFT3055ELT1





2

V+

1

EXTL

8

CS

R

3

FB

GNDREF

5

6, 7

Adj. = 12V TO 24V

(AS SHOWN)

R2
150k

R3

10k

10k

OUTPUT

47µF

15

12V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controller

INPUT

2.7V TO 5.5V

150µF



4

SHDN

ON

OFF

0.1µF



5

REF

MAX1771

Figure 9. 5V/1A Boost Converter

22µH

MAX1771

AGND

GND

76

EXT

CS

V+

FB









1
8

2

0.1µF

3

1N5820

MTD20N03HDL

0.04Ω

232k

100k

OUTPUT

100pF

5V
1A

330µF



EFFICIENCY vs. LOAD CURRENT

100

90

VIN = 4V

80

70

EFFICIENCY (%)

60

50

1m 10m 100m 1





VIN = 3V

LOAD CURRENT (A)

___________________Chip Topography

EXT

V+

CS

FB

SHDN REF

0.080"

(2.032mm)

TRANSISTOR COUNT: 501
SUBSTRATE CONNECTED TO V+

______________________________________________________________________________________

16

0.126"

(3.200mm)

GND
AGND