Linear Technology LTC1771 Datasheet

LTC1771

Final Electrical Specifications

Low Quiescent Current

High Efficiency Step-Down

DC/DC Controller

FEATURES

■

Very Low Standby Current: 10µA

■

Available in Space-Saving 8-Lead MSOP Package

■

High Output Currents

■

Wide VIN Range: 2.8V to 20V Operation

■

V

Range: 1.23V to 18V

OUT

■

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 ModeTM Operation

■

Short-Circuit Protected

■

Optional Programmable Soft-Start

■

Micropower Shutdown: IQ = 2µA

U

APPLICATIO S

■

Cellular Telephones and Wireless Modems

■

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

■

Portable Instruments

■

Battery-Powered Equipment

■

Battery Chargers

■

Scanners

U

February 2000

DESCRIPTIO

The LTC®1771 is a high efficiency current mode step­down DC/DC controller that draws as little as 10µA 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 2µA. 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 appli­cations where maximizing battery life is important.

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

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

Burst Mode is a trademark of Linear Technology Corporation.

TYPICAL APPLICATIO

V

IN

C

SS

0.01µF

R

C

10k

C
22OpF

RUN/SS
I

TH

V

FB

C

R1
1M
1%

Figure 1. High Efficiency Step-Down Converter

LTC1771

GND

U

V

IN

4.5V TO 18V

100

90

80

70

EFFICIENCY (%)

60

50

40

L1

15µH

10µF
25V
CER

V

OUT

3.3V

C

OUT

150µF

6.3V

2A

1771 F01

+

R

SENSE

0.05Ω

SENSE

PGATE

MODE

M1

Si6447DQ

V

IN

R2

1.64M
1%

5pF

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.

UPS5817

LTC1771 Efficiency

VIN = 5V

VIN = 10V

VIN = 15V

V

= 3.3V

OUT

= 0.05Ω

R

SENSE

0.1 1 100 1000 10000

10

LOAD CURRENT (mA)

1771 F01b

1

LTC1771

RUN/SS

I

TH

V

FB

GND

1
2
3
4

8
7
6
5

MODE
SENSE
V

IN

PGATE

TOP VIEW

MS8 PACKAGE

8-LEAD PLASTIC MSOP

TOP VIEW

MODE
SENSE
V

IN

PGATE

RUN/SS

I

TH

V

FB

GND

S8 PACKAGE

8-LEAD PLASTIC SO

1

2

3

4

8

7

6

5

WWWU

ABSOLUTE AXI U RATI GS

(Note 1)

Input Supply Voltage (VIN)........................ –0.3V to 20V

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

RUN/SS Voltage ........................... – 0.3V to (VIN + 0.3V)

MODE Voltage .......................................... –0.3V to 20V

ITH, VFB Voltage .......................................... –0.3V to 5V

SENSE Voltage (VIN > 12V)...(VIN – 12V) to (VIN + 0.3V)

SENSE Voltage (VIN ≤ 12V) .......... –0.3V to (VIN + 0.3V)

UU

W

PACKAGE/ORDER I FOR ATIO

ORDER PART

NUMBER

LTC1771EMS8

MS8 PART MARKING

T

= 125°C, θJA = 150°C/W

JMAX

LTKD

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

Operating Temperature Range (Note 3)

LTC1771E......................................... –40°C to 85°C

LTC1771I ......................................... – 40°C to 85°C

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

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

ORDER PART

NUMBER

LTC1771ES8
LTC1771IS8

S8 PART MARKING

T

= 125°C, θJA = 110°C/W

JMAX

1771
1771I

Consult factory for Military grade parts.

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VIN = 10V, V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

FB

I

FB

I

SUPPLY

∆V

LINEREG

∆V

LOADREG

I

Q

∆V

SENSE(MAX)

∆V

SENSE(MIN)

∆V

SENSE(SLEEP)

t

OFF

V

MODE

2

= open unless otherwise specified.

RUN

Feedback Voltage (Note 5) ● 1.205 1.230 1.255 V
Feedback Current (Note 5) ● 110 nA
No-Load Supply Current VIN = 10V, I
Reference Voltage Line Regulation VIN = 5V to 15V (Note 5) ● 0.003 0.03 %/V
Output Voltage Load Regulation ITH = 0.5V to 2V, Burst Disabled (Note 5) ● 0.25 1 %
Input DC Supply Current (Note 4)

Active Mode (PGATE = 0V) V
Sleep Mode (Note 6) V
Shutdown V
Short Circuit V

Maximum Current Sense Threshold VFB = V
Minimum Current Sense Threshold VFB = V
Sleep Current Sense Threshold ITH = 1V 50 mV
Switch Off Time VFB at Regulated Value 3.5 µs

Mode Pin Threshold V

= 2.8V to 18V 150 235 µA

IN

= 2.8V to 18V, VFB = 1.5V 9 15 µA

IN

= 2.8V to 18V, V

IN

= 2.8V to 18V, VFB = 0V 175 275 µA

IN

REF

REF

= 0V 70 µs

V

FB

Rising ● 0.5 1.3 2 V

MODE

= 0 (Note 6) 10 µA

LOAD

= 0V 2 6 µA

RUN

– 20mV ● 110 140 180 mV
+ 10mV, Burst Disabled –25 mV

LTC1771

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VIN = 10V, V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

RUN/SS

I

RUN

PGATE tr, t

= open unless otherwise specified.

RUN

RUN/SS Pin Threshold V
Source Current V
PGATE Transition Time (Note 7)

f

Rise Time C
Fall Time C

Rising ● 0.5 1.0 2 V

RUN/SS

= 0V, VIN = 2.8V to 18V 0.3 1 3 µA

RUN

= 2000pF 80 ns

LOAD

= 2000pF 90 ns

LOAD

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

Note 2: T
dissipation P

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.

is calculated from the ambient temperature TA and power

J

according to the following formulas:

D

LTC1771S8: TJ = TA + (PD)(110°C/W)
LTC1771MS8: T

U

= TA + (PD)(150°C/W)

J

UU

PI FU CTIO S

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.

ITH (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.

VFB (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.

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

balance point for the error amplifier (V
Note 6: No-load supply current consists of sleep mode current (9µA

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

Note 7: t

and tf measured at 10% to 90% levels.

r

= 1.23V).

ITH

to the

FB

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

VIN (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 be­tween SENSE and VIN.

MODE (Pin 8): Burst Mode Enable/Disable Pin. Connect­ing this pin to VIN (or above 2V) enables Burst Mode
operation, while connecting this pin to ground disables
Burst Mode operation. Do not leave floating.

3

LTC1771

UU

W

FUNCTIONAL BLOCK DIAGRA

V

IN

+

EA

–

1V

1V

1µA

ON

10% CURRENT

–

B

+

SOFT-START

2V

C

SS

RUN/SS

1

MODE

(BURST ENABLE)

8

1.23V

V

OUT

*

I

TH

2

R

C

C

C

GND

4

SLEEP

READY

READY

1.23V

REFERENCE

10% CURRENT

V

IN

C

ON

BLANKING

22k R

–

+

250k

V

IN

6

SENSE

7

SW

5

+

SENSE

V

IN

C

IN

*

OPTIONAL FOR FOLDBACK

CURRENT LIMITING

MODE

ON TRIGGER

1-SHOT

STRETCH

3.5µs

L

V

OUT

V

FB

3

+

C

OUT

1771 BD

4

OPERATIO

LTC1771

U

(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 3.5µs 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 (ITH), the output of the error amplifier EA. An
external resistive divider connected between V
ground allows EA to receive an output feedback voltage
VFB. When the load current increases, it causes a slight
decrease in VFB relative to the 1.23V reference, which in
turn causes the ITH 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 1µA
current source to charge soft-start capacitor CSS. When
CSS reaches 1V, the main control loop is enabled with the
ITH voltage clamped at approximately 40% of its maxi­mum value. As CSS continues to charge, ITH 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 switch­ing 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 cir­cuitry—1.23V reference and error amp—are left on, and
the supply current is reduced to 9µA. At no load, the output
capacitor is still discharged very slowly by leakage current
in the Schottky diode and feedback resistor current result­ing in very low frequency burst cycles that add a few more
microamps to the supply current.

OUT

and

Burst Mode operation is provided by clamping the mini­mum ITH voltage at 1V which represents about 25% of
maximum load current. If the load falls below this level, i.e.
the ITH 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 capaci­tor 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 ITH voltage
increases. When it exceeds 1V the burst comparator exits
sleep mode, reconnects the external compensation com­ponents 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 ITH 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 ITH 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 spec­trum, useful for reducing both audio and RF interference,
at the expense of reduced efficiency at light loads.

Off-Time

The off-time duration is 3.5µs when the feedback voltage
is close to the reference voltage; however, as the feedback
voltage drops, the off-time lengthens and reaches a maxi­mum value of about 70µs when VFB 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.

5