Linear Technology LTC1772 Datasheet

FEATURES

Final Electrical Specifications

■

High Efficiency: Up to 94%

■

High Output Currents Easily Achieved

■

Wide VIN Range: 2.5V to 9.8V

■

Constant Frequency 550kHz Operation

■

Burst Mode

■

Low Dropout: 100% Duty Cycle

■

0.8V Reference Allows Low Output Voltages

■

Current Mode Operation for Excellent Line and Load

TM

Operation at Light Load

Transient Response

■

Low Quiescent Current: 270µA

■

Shutdown Mode Draws Only 8µA Supply Current

■

±2.5% Reference Accuracy

■

Tiny 6-Lead SOT-23 Package

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APPLICATIONS

■

One or Two Lithium-Ion-Powered Applications

■

Cellular Telephones

■

Wireless Modems

■

Portable Computers

■

Distributed 3.3V, 2.5V or 1.8V Power Systems

■

Scanners

LTC1772

Constant Frequency

Current Mode Step-Down

DC/DC Controller in SOT-23

September 1999

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DESCRIPTION

The LTC®1772 is a constant frequency current mode step­down DC/DC controller providing excellent AC and DC load
and line regulation. The device incorporates an accurate
undervoltage lockout feature that shuts down the LTC1772
when the input voltage falls below 2.0V.

The LTC1772 boasts a ±2.5% output voltage accuracy and
consumes only 270µA of quiescent current. For applica-
tions where efficiency is a prime consideration, the LTC1772
is configured for Burst Mode operation, which enhances
efficiency at low output current.

To further maximize the life of a battery source, the
external P-channel MOSFET is turned on continuously in
dropout (100% duty cycle). In shutdown, the device draws
a mere 8µA. High constant operating frequency of 550kHz
allows the use of a small external inductor.

The LTC1772 is available in a small footprint 6-lead
SOT-23.

, LTC and LT are registered trademarks of Linear Technology Corporation.

Burst Mode is a trademark of Linear Technology Corporation.

TYPICAL APPLICATION

1

ITH/RUN

10k

220pF

C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT
C2: SANYO POSCAP 6TPA47M
D1: IR10BQ015

LTC1772

2

GND

3

V

FB

Figure 1. High Efficiency Step-Down Converter

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V

IN

169k

78.7k

2.5V
TO 9.8V

V

OUT

2.5V
2A

1772 F01a

R1

0.03Ω

6

PGATE

5

V

IN

4

–

SENSE

L1: MURATA LQN6C-4R7
M1: Si3443DV
R1: DALE 0.25W

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

M1

L1

4.7µH

D1

C1
10µF
16V

+

C2
47µF
6V

100

90

80

70

EFFICIENCY (%)

60

50

40

Efficiency vs Load Current

VIN = 3.3V

1 100 1000 10000

VIN = 4.2V

VIN = 9.8V

10

LOAD CURRENT (mA)

VIN = 8.4V

VIN = 6V

V

OUT

R

SENSE

= 2.5V

= 0.03Ω

1772 F01b

1

LTC1772

ITH/RUN 1

GND 2

V

FB

3

6 PGATE
5 V

IN

4 SENSE

–

TOP VIEW

S6 PACKAGE

6-LEAD PLASTIC SOT-23

WW

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ABSOLUTE MAXIMUM RATINGS

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PACKAGE/ORDER INFORMATION

(Note 1)

Input Supply Voltage (VIN).........................– 0.3V to 10V

SENSE–, PGATE Voltages.............–0.3V to (VIN + 0.3V)

VFB, ITH/RUN Voltages ..............................–0.3V to 2.4V

PGATE Peak Output Current (<10µs) ....................... 1A

ORDER PART

NUMBER

LTC1772CS6

Storage Ambient Temperature Range ... – 65°C to 150°C

Operating Temperature Range (Note 2) ....... 0°C to 70°C

S6 PART MARKING

Junction Temperature (Note 3)............................. 150°C

Lead Temperature (Soldering, 10 sec).................. 300°C

Consult factory for Industrial and Military grade parts.

ELECTRICAL CHARACTERISTICS

range, otherwise specifications are at TA = 25°C. VIN = 4.2V unless otherwise specified. (Note 2)

PARAMETER CONDITIONS MIN TYP MAX UNITS

Input DC Supply Current Typicals at VIN = 4.2V (Note 4)
Normal Operation 2.4V ≤ V
Sleep Mode 2.4V ≤ V
Shutdown 2.4V ≤ V
UVLO V

Undervoltage Lockout Threshold VIN Falling ● 1.6 2.0 2.3 V

V

IN

Shutdown Threshold (at ITH/RUN) ● 0.2 0.35 0.5 V
Start-Up Current Source V
Regulated Feedback Voltage (Note 5) ● 0.780 0.800 0.820 V
Output Voltage Line Regulation 2.4V ≤ VIN ≤ 9.8V (Note 5) 0.05 mV/V
Output Voltage Load Regulation ITH/RUN Sinking 5µA (Note 5) 2.5 mV/µA

VFB Input Current (Note 5) 10 50 nA
Overvoltage Protect Threshold Measured at V
Overvoltage Protect Hysteresis 20 mV
Oscillator Frequency VFB = 0.8V 500 550 650 kHz

Gate Drive Rise Time C
Gate Drive Fall Time C
Maximum Current Sense Voltage 120 mV

I

TH

V

ITH

FB

LOAD

LOAD

The ● denotes specifications that apply over the full operating temperature

≤ 9.8V 270 420 µA

IN

≤ 9.8V 230 370 µA

IN

≤ 9.8V, V

IN

< UVLO Threshold 6 10 µA

IN

Rising 1.85 2.3 2.5 V

/RUN = 0V 0.25 0.5 0.85 µA

/RUN Sourcing 5µA (Note 5) 2.5 mV/µA

= 0V 120 kHz

= 3000pF 40 ns
= 3000pF 40 ns

ITH

FB

T

= 150°C, θJA = 230°C/W

JMAX

/RUN = 0V 8 22 µA

0.820 0.860 0.895 V

LTIL

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Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Note 2: The LTC1772 is guaranteed to meet specified performance over
the 0°C to 70°C operating temperature range.

Note 3: T
dissipation P

is calculated from the ambient temperature TA and power

J

according to the following formula:

D

TJ = TA + (PD • θJ°C/W)

2

Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.

Note 5: The LTC1772 is tested in a feedback loop that servos V
output of the error amplifier.

to the

FB

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TYPICAL PERFORMANCE CHARACTERISTICS

LTC1772

Reference Voltage
vs Temperature

825

VIN = 4.2V

820
815
810
805
800
795

VOLTAGE (mV)

FB

V

790
785
780
775

–35 5

–15

–55

85

45 125

25

TEMPERATURE (°C)

105

65

1772 G01

Maximum (VIN – SENSE–) Voltage
vs Duty Cycle

130

120

110

100

90

80

TRIP VOLTAGE (mV)

70

60

50

20 30

40 50

60 70

DUTY CYCLE (%)

Normalized Oscillator Frequency
vs Temperature

10

VIN = 4.2V

8
6
4
2

0
–2
–4
–6

NORMALIZED FREQUENCY (%)

–8

–10

–35 5

–55

VIN = 4.2V
T

= 25°C

A

80 90

–15

TEMPERATURE (°C)

100

1772 G04

45 125

65

25

85

105

1772 G02

Shutdown Threshold
vs Temperature

600

VIN = 4.2V

560
520
480
440
400
360

/RUN VOLTAGE (mV)

320

TH

I

280
240
200

–35 5

–15

–55

TEMPERATURE (

Undervoltage Lockout Trip
Voltage vs Temperature

2.24
VIN = 4.2V

2.20

FALLING

V

IN

2.16

2.12

2.08

2.04

2.00

TRIP VOLTAGE (V)

1.96

1.92

1.88

1.84

–35 5

–15

–55

45 125

65

25

°C)

25

TEMPERATURE (°C)

85

105

1772 G05

85

45 125

105

65

1772 G03

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PIN FUNCTIONS

ITH/RUN (Pin 1): This pin performs two functions. It
serves as the error amplifier compensation point as well as
the run control input. The current comparator threshold
increases with this control voltage. Nominal voltage range
for this pin is 0.7V to 1.9V. Forcing this pin below 0.35V
causes the device to be shut down. In shutdown all
functions are disabled and the PGATE pin is held high.

GND (Pin 2): Ground Pin.

VFB (Pin 3): Receives the feedback voltage from an exter-

nal resistive divider across the output.
SENSE– (Pin 4): The Negative Input to the Current Com-

parator.
VIN (Pin 5): Supply Pin. Must be closely decoupled to GND

Pin 2.
PGATE (Pin 6): Gate Drive for the External P-Channel

MOSFET. This pin swings from 0V to VIN.

3

LTC1772

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FUNCTIONAL DIAGRA

–

SENSE

V

IN

5

4

4

+

ICMP

–

OSC

FREQ

FOLDBACK

SHORT-CIRCUIT

DETECT

V

IN

+

0.3V

GND

2

–

SLOPE
COMP

–

+

0.5µA

V

IN

VOLTAGE

REFERENCE

UNDERVOLTAGE

LOCKOUT

0.3V

0.15V

V

REF

0.8V

0.35V

V

RS1
R

Q

S

+

–

I

/RUN

1

TH

+

–

BURST

CMP

SHDN

CMP

SWITCHING
LOGIC AND

BLANKING

CIRCUIT

SLEEP

SHDN

UV

OVP

EAMP

IN

PGATE

6

+

V

REF

+

–

60mV

V

REF

+

0.8V

–

1.2V

V

FB

3

V

IN

1772FD

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OPERATIO

Main Control Loop

The LTC1772 is a constant frequency current mode switch­ing regulator. During normal operation, the external
P-channel power MOSFET is turned on each cycle when
the oscillator sets the RS latch (RS1) and turned off when
the current comparator (ICMP) resets the latch. The peak
inductor current at which ICMP resets the RS latch is
controlled by the voltage on the ITH/RUN pin, which is the
output of the error amplifier EAMP. An external resistive
divider connected between V
EAMP to receive an output feedback voltage VFB. When the

(Refer to Functional Diagram)

and ground allows the

OUT

4

load current increases, it causes a slight decrease in V

FB

relative to the 0.8V reference, which in turn causes the
ITH/RUN voltage to increase until the average inductor
current matches the new load current.

The main control loop is shut down by pulling the ITH/RUN
pin low. Releasing ITH/RUN allows an internal 0.5µA
current source to charge up the external compensation
network. When the ITH/RUN pin reaches 0.35V, the main
control loop is enabled with the ITH/RUN voltage then
pulled up to its zero current level of approximately 0.7V.
As the external compensation network continues to charge

OPERATIO

LTC1772

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(Refer to Functional Diagram)

up, the corre

sponding output current trip level follows,

allowing normal operation.
Comparator OVP guards against transient overshoots

>7.5% by turning off the external P-channel power
MOSFET and keeping it off until the fault is removed.

Burst Mode Operation

The LTC1772 enters Burst Mode operation at low load
currents. In this mode, the peak current of the inductor is
set as if V

/RUN = 1V (at low duty cycles) even though

ITH

the voltage at the ITH/RUN pin is at a lower value. If the
inductor’s average current is greater than the load require­ment, the voltage at the ITH/RUN pin will drop. When the
ITH/RUN voltage goes below 0.85V, the sleep signal goes
high, turning off the external MOSFET. The sleep signal
goes low when the ITH/RUN voltage goes above 0.925V
and the LTC1772 resumes normal operation. The next
oscillator cycle will turn the external MOSFET on and the
switching cycle repeats.

Dropout Operation

Short-Circuit Protection

When the output is shorted to ground, the frequency of the
oscillator will be reduced to about 120kHz. This lower
frequency allows the inductor current to safely discharge,
thereby preventing current runaway. The oscillator’s fre­quency will gradually increase to its designed rate when
the feedback voltage again approaches 0.8V.

Overvoltage Protection

As a further protection, the overvoltage comparator in the
LTC1772 will turn the external MOSFET off when the
feedback voltage has risen 7.5% above the reference
voltage of 0.8V. This comparator has a typical hysteresis
of 20mV.

Slope Compensation and Inductor’s Peak Current

The inductor’s peak current is determined by:

V

I

PK

=

ITH

R

10

SENSE

()

When the input supply voltage decreases towards the
output voltage, the rate of change of inductor current
during the ON cycle decreases. This reduction means that
the external P-channel MOSFET will remain on for more
than one oscillator cycle since the inductor current has not
ramped up to the threshold set by EAMP. Further reduc­tion in input supply voltage will eventually cause the
P-channel MOSFET to be turned on 100%, i.e., DC. The
output voltage will then be determined by the input voltage
minus the voltage drop across the MOSFET, the sense
resistor and the inductor.

Undervoltage Lockout

To prevent operation of the P-channel MOSFET below safe
input voltage levels, an undervoltage lockout is incorpo­rated into the LTC1772. When the input supply voltage
drops below approximately 2.0V, the P-channel MOSFET
and all circuitry is turned off except the undervoltage
block, which draws only several microamperes.

when the LTC1772 is operating below 40% duty cycle.
However, once the duty cycle exceeds 40%, slope com­pensation begins and effectively reduces the peak induc­tor current. The amount of reduction is given by the curves
in Figure 2.

110
100

90
80

(%)

70
60

OUT(MAX)

/I

50

OUT

SF = I

Figure 2. Maximum Output Current vs Duty Cycle

I

= 0.4I

RIPPLE

AT 5% DUTY CYCLE

40

I

= 0.2I

30
20
10

RIPPLE

AT 5% DUTY CYCLE

VIN = 4.2V

0 70 80 90 1006010 20 30 40 50

DUTY CYCLE (%)

PK

PK

1772 F02

5