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LTC1771

Very Low Standby Current: 10

Available in Space-Saving 8-Lead MSOP Package

High Output Currents

Wide V

Range: 2.8V to 20V Operation

OUT

Range: 1.23V to 18V

High Efficiency: Over 93% Possible

2% Output Accuracy

Very Low Dropout Operation: 100% Duty Cycle

Current Mode Operation for Excellent Line and
Load Transient Response

Defeatable Burst Mode

Operation

Short-Circuit Protected

Optional Programmable Soft-Start

Micropower Shutdown: I

= 2

The LTC

1771 is a high efficiency current mode step-

down DC/DC controller that draws as little as 10

A DC

supply current to regulate the output at no load while
maintaining high efficiency for loads up to several amps.

The LTC1771 drives an external P-channel power MOSFET
using a current mode, constant off-time architecture. An
external sense resistor is used to program the operating
current level. Current mode control provides short-circuit
protection, excellent transient response and controlled
start-up behavior. Burst Mode operation enables the
LTC1771 to maintain high efficiency down to extremely
low currents. Shutdown mode further reduces the supply
current to a mere 2

A. For low noise applications, Burst

Mode operation can be easily disabled with the MODE pin.

Wide input supply range of 2.8V to 18V (20V maximum)
and 100% duty cycle operation for low dropout make the
LTC1771 ideal for a wide variety of battery-powered appli-
cations where maximizing battery life is important.

The LTC1771's availability in both 8-lead MSOP and SO
packages provides for a minimum area solution.

Cellular Telephones and Wireless Modems

1- to 4-Cell Lithium-Ion-Powered Applications

Portable Instruments

Battery-Powered Equipment

Battery Chargers

Scanners

Figure 1. High Efficiency Step-Down Converter

Low Quiescent Current

High Efficiency Step-Down

DC/DC Controller

February 2000

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

Burst Mode is a trademark of Linear Technology Corporation.

RUN/SS

4.5V TO 18V

10k

SENSE

0.05

1.64M

R1
1M
1%

Si6447DQ

UPS5817

22OpF

5pF

25V
CER

OUT

150

6.3V

OUT

3.3V
2A

1771 F01

0.01

PGATE

MODE

LTC1771

GND

SENSE

LTC1771 Efficiency

LOAD CURRENT (mA)

EFFICIENCY (%)

100

0.1

100

1000

10000

1771 F01b

= 5V

= 10V

= 15V

OUT

= 3.3V

SENSE

= 0.05

Final Electrical Specifications

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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

LTC1771

The

denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25

= 10V, V

RUN

= open unless otherwise specified.

LTC1771EMS8

JMAX

= 125

= 150

C/ W

ORDER PART

NUMBER

Input Supply Voltage (V

)........................ 0.3V to 20V

Peak Driver Output Current < 10

s (PGATE) ............. 1A

RUN/SS Voltage ........................... 0.3V to (V

+ 0.3V)

MODE Voltage .......................................... 0.3V to 20V
I

, V

Voltage .......................................... 0.3V to 5V

SENSE Voltage (V

> 12V)... (V

12V) to (V

+ 0.3V)

SENSE Voltage (V

12V) .......... 0.3V to (V

+ 0.3V)

(Note 1)

Consult factory for Military grade parts.

Junction Temperature (Note 2) ............................ 125

Operating Temperature Range (Note 3)

LTC1771E ......................................... 40

C to 85

LTC1771I ......................................... 40

C to 85

Storage Temperature Range ................. 65

C to 150

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

LTC1771ES8
LTC1771IS8

ORDER PART

NUMBER

RUN/SS

GND

1
2
3
4

8
7
6
5

MODE
SENSE
V

PGATE

TOP VIEW

MS8 PACKAGE

8-LEAD PLASTIC MSOP

MS8 PART MARKING

TOP VIEW

MODE

SENSE

PGATE

RUN/SS

GND

S8 PACKAGE

8-LEAD PLASTIC SO

JMAX

= 125

= 110

C/ W

S8 PART MARKING

1771
1771I

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Feedback Voltage

(Note 5)

1.205

1.230

1.255

Feedback Current

(Note 5)

SUPPLY

No-Load Supply Current

= 10V, I

LOAD

= 0 (Note 6)

LINEREG

Reference Voltage Line Regulation

= 5V to 15V (Note 5)

0.003

0.03

%/V

LOADREG

Output Voltage Load Regulation

= 0.5V to 2V, Burst Disabled (Note 5)

0.25

Input DC Supply Current

(Note 4)

Active Mode (PGATE = 0V)

= 2.8V to 18V

150

235

Sleep Mode (Note 6)

= 2.8V to 18V, V

= 1.5V

Shutdown

= 2.8V to 18V, V

RUN

= 0V

Short Circuit

= 2.8V to 18V, V

= 0V

175

275

SENSE(MAX)

Maximum Current Sense Threshold

= V

REF

20mV

110

140

180

SENSE(MIN)

Minimum Current Sense Threshold

= V

REF

+ 10mV, Burst Disabled

SENSE(SLEEP)

Sleep Current Sense Threshold

= 1V

OFF

Switch Off Time

at Regulated Value

3.5

= 0V

MODE

Mode Pin Threshold

MODE

Rising

0.5

1.3

LTKD

ABSOLUTE AXI U RATI GS

PACKAGE/ORDER I FOR ATIO

ELECTRICAL CHARACTERISTICS

LTC1771

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

Note 2: T

is calculated from the ambient temperature T

and power

dissipation P

according to the following formulas:

LTC1771S8: T

= T

+ (P

)(110

C/W)

LTC1771MS8: T

= T

+ (P

)(150

C/W)

Note 3: The LTC1771E is guaranteed to meet performance specifications
from 0

C to 70

C. Specifications over the 40

C to 85

C operating

temperature range are assured by design, characterization and correlation
with statistical process controls. The LTC1771I is guaranteed and tested
over the 40

C to 85

C operating temperature range.

The

denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25

= 10V, V

RUN

= open unless otherwise specified.

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

RUN/SS

RUN/SS Pin Threshold

RUN/SS

Rising

0.5

1.0

RUN

Source Current

RUN

= 0V, V

= 2.8V to 18V

0.3

PGATE t

, t

PGATE Transition Time (Note 7)
Rise Time

LOAD

= 2000pF

Fall Time

LOAD

= 2000pF

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

Note 5: The LTC1771 is tested in a feedback loop that servos V

to the

balance point for the error amplifier (V

ITH

= 1.23V).

Note 6: No-load supply current consists of sleep mode current (9

typical) plus a small switching component necessary to overcome
Schottky diode leakage and feedback resistor current.

Note 7: t

and t

measured at 10% to 90% levels.

RUN/SS (Pin 1): The voltage level on this pin controls
shutdown/run mode (ground = shutdown, open/high =
run). Connecting an external capacitor to this pin provides
soft-start.

(Pin 2): Error Amplifier Compensation Point. The

current comparator threshold increases with this control
voltage. Nominal voltage range for this pin is 0V to 3V.

(Pin 3): Feedback of Output Voltage for Comparison

to Internal 1.23V Reference. An external resistive divider
across the output is returned to this pin.

GND (Pin 4): Ground Pin.

PGATE (Pin 5): High Current Gate Driver for External
P-Channel MOSFET Switch. Voltage swing is from ground
to V

(Pin 6): Main Input Voltage Supply Pin.

SENSE (Pin 7): Current Sense Input for Monitoring Switch
Current. Maximum switch current and Burst Mode
threshold is programmed with an external resistor be-
tween SENSE and V

MODE (Pin 8): Burst Mode Enable/Disable Pin. Connect-
ing this pin to V

(or above 2V) enables Burst Mode

operation, while connecting this pin to ground disables
Burst Mode operation. Do not leave floating.

ELECTRICAL CHARACTERISTICS

PI FU CTIO S

LTC1771

(Refer to Functional Block Diagram)

Main Control Loop

The LTC1771 uses a constant off-time, current mode
step-down architecture. During normal operation, the
P-channel MOSFET is turned on at the beginning of each
cycle and turned off when the current comparator C
triggers the 1-shot timer. The external MOSFET switch
stays off for the 3.5

s 1-shot duration and then turns back

on again to begin a new cycle. The peak inductor current
at which C triggers the 1-shot is controlled by the voltage
on Pin 3 (I

), the output of the error amplifier EA. An

external resistive divider connected between V

OUT

and

ground allows EA to receive an output feedback voltage
V

. When the load current increases, it causes a slight

decrease in V

relative to the 1.23V reference, which in

turn causes the I

voltage to increase until the average

inductor current matches the new load current.

The main control loop is shut down by pulling Pin 1
(RUN/SS) low. Releasing RUN/SS allows an internal 1

current source to charge soft-start capacitor C

. When

reaches 1V, the main control loop is enabled with the

voltage clamped at approximately 40% of its maxi-

mum value. As C

continues to charge, I

is gradually

released allowing normal operation to resume.

Burst Mode Operation

The LTC1771 provides outstanding low current efficiency
and ultralow no-load supply current by using Burst Mode
operation when the MODE pin is pulled above 2V. During
Burst Mode operation, short burst cycles of normal switch-
ing are followed by a longer idle period with the switch off
and the load current is supplied by the output capacitor.
During this idle period, only the minimum required cir-
cuitry--1.23V reference and error amp--are left on, and
the supply current is reduced to 9

A. At no load, the output

capacitor is still discharged very slowly by leakage current
in the Schottky diode and feedback resistor current result-
ing in very low frequency burst cycles that add a few more
microamps to the supply current.

Burst Mode operation is provided by clamping the mini-
mum I

voltage at 1V which represents about 25% of

maximum load current. If the load falls below this level, i.e.
the I

voltage tries to fall below 1V, the burst comparator

B switches state signaling the LTC1771 to enter sleep
mode. During this time, EA is reduced to 10% of its normal
operating current and the external compensation capaci-
tor is disconnected and clamped to 1V so that the EA can
drive its output with the lower available current. As the load
discharges the output capacitor, the internal I

voltage

increases. When it exceeds 1V the burst comparator exits
sleep mode, reconnects the external compensation com-
ponents to the error amplifier output, and returns EA to full
power along with the other necessary circuitry. This
scheme (patent pending) allows the EA to be reduced to
such a low operating current during sleep mode without
adding unacceptable delay to wake up the LTC1771 due to
the compensation capacitor on I

required for stability in

normal operation.

Burst Mode operation can be disabled by pulling the
MODE pin to ground. In this mode of operation, the burst
comparator B is disabled and the I

voltage allowed to go

all the way to 0V. The load can now be reduced to about 1%
of maximum load before the loop skips cycles to maintain
regulation. This mode provides a low noise output spec-
trum, useful for reducing both audio and RF interference,
at the expense of reduced efficiency at light loads.

Off-Time

The off-time duration is 3.5

s when the feedback voltage

is close to the reference voltage; however, as the feedback
voltage drops, the off-time lengthens and reaches a maxi-
mum value of about 70

s when V

is zero. This ensures

that the inductor current has enough time to decay when
the reverse voltage across the inductor is low such as
during short circuit, thus protecting the MOSFET and
inductor.

OPERATIO

Part Number LTC1771