Linear Technology LTC1701 Datasheet

Final Electrical Specifications

FEATURES

■

Tiny 5-Lead SOT-23 Package

■

Uses Tiny Capacitors and Inductor

■

High Frequency Operation: 1MHz

■

High Output Current: 500mA

■

Low R

■

High Efficiency: Up to 94%

■

Current Mode Operation for Excellent Line

Internal Switch: 0.28Ω

DS(ON)

and Load Transient Response

■

Short-Circuit Protected

■

Low Quiescent Current: 135µA

■

Low Dropout Operation: 100% Duty Cycle

■

Ultralow Shutdown Current: IQ < 1µA

■

Peak Inductor Current Independent of Inductor Value

■

Output Voltages from 5V Down to 1.25V

U

APPLICATIO S

LTC1701

1MHz Step-Down

DC/DC Converter in SOT-23

December 1999

U

DESCRIPTIO

The LTC®1701 is the industry’s first 5-lead SOT-23 step
down, current mode, DC/DC converter. Intended for small
to medium power applications, it operates from 2.5V to

5.5V input voltage range and switches at 1MHz, allowing
the use of tiny, low cost capacitors and inductors 2mm or
less in height. The output voltage is adjustable from 1.25V
to 5V. A built-in 0.28Ω switch allows up to 0.5A of output
current at high efficiency. OPTI-LOOPTM compensation
allows the transient response to be optimized over a wide
range of loads and output capacitors.

The LTC1701 incorporates automatic power saving Burst
ModeTM operation to reduce gate charge losses when the
load current drops below the level required for continuous
operation. With no load, the converter draws only 135µA.
In shutdown, it draws less than 1µA, making it ideal for
current sensitive applications.

■

PDAs/Palmtop PCs

■

Digital Cameras

■

Cellular Phones

■

Portable Media Players

■

PC Cards

■

Handheld Equipment

TYPICAL APPLICATIO

V

IN

2.5V TO

5.5V

+

C1

10µF

C1: TAIYO YUDEN JMK316BJ106ML
C2: SANYO POSCAP 6TPA47M
D1: MBRM120L
L1: SUMIDA CD43-4R7

Figure 1. Step-Down 2.5V/500mA Regulator

R4
1M

R3

5.1k
C3
330pF

V

IN

LTC1701

ITH/RUN

GND

SW

V

FB

U

4.7µH

D1

In dropout, the internal P-channel MOSFET switch is
turned on continuously, thereby maximizing battery life.
Its small size and switching frequency enables the com­plete DC/DC converter function to consume less than 0.3
square inches of PC board area.

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

Burst Mode and OPTI-LOOP are trademarks of Linear Technology Corporation.

L1

R2
121k

R1
121k

V

OUT

(2.5V/
500mA)

+

C2
47µF

1701 F01

100

95

90

85

EFFICIENCY (%)

80

75

70

1 10 100 1000

Efficiency Curve

VIN = 3.3V

LOAD CURRENT (mA)

1701 F01a

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.

1

LTC1701

WW

W

ABSOLUTE AXI U RATI GS

U

UUW

PACKAGE/ORDER I FOR A TIO

(Note 1)

(Voltages Referred to GND Pin)

VIN Voltage (Pin 5).......................................–0.3V to 6V

ITH/RUN Voltage (Pin 4) ..............................–0.3V to 3V

VFB Voltage (Pin 3) ......................................–0.3V to 3V

Peak Switch Current (Pin 1) ................................... 1.3A

VIN – SW (Max Switch Voltage)................8.5V to –0.3V

Operating Temperature Range (Note 2).. –40°C to 85°C

Junction Temperature (Note 5).............................125°C

TOP VIEW

SW 1

GND 2

3

V

FB

S5 PACKAGE

5-LEAD PLASTIC SOT-23

T

= 125°C, θJA = 110°C/W

JMAX

5 V

IN

4 ITH/RUN

ORDER PART

NUMBER

LTC1701ES5

S5 PART

MARKING

LTKG

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

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

Consult factory for Industrial and Military grade parts.

ELECTRICAL CHARACTERISTICS

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

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

IN

I

FB

V

FB

∆V

LINE REG

∆V

LOAD REG

V

ITH/RUN

I

ITH/RUN

I

SW(PEAK)

R

DS(ON)

I

SW(LKG)

t

OFF

Operating Voltage Range 2.5 5.5 V
Feedback Pin Input Current (Note 3) ±0.1 µA
Feedback Voltage (Note 3) ● 1.22 1.25 1.28 V
Reference Voltage Line Regulation VIN = 2.5V to 5V (Note 3) 0.04 0.1 %/V
Output Voltage Load Regulation Measured in Servo Loop, V

Input DC Supply Current (Note 4)

Active Mode V
Sleep Mode V
Shutdown V

Run Threshold High I
Run Threshold Low I

Run Pullup Current V
Peak Switch Current Threshold VFB = 0V 0.9 1.1 A
Switch ON Resistance VIN = 5V, VFB = 0V 0.28 Ω

Switch Leakage Current VIN = 5V, V
Switch Off-Time 400 500 600 ns

The ● denotes the specifications which apply over the full operating

= 3.3V, R

IN

Measured in Servo Loop, V

= 0V 185 300 µA

FB

= 1.4V 135 200 µA

FB

= 0V 0.25 1 µA

ITH/RUN

Ramping Down 1.4 1.6 V

TH/RUN

Ramping Up 0.3 0.6 V

TH/RUN

= 1V 50 100 300 µA

ITH/RUN

= 3.3V, VFB = 0V 0.30 Ω

V

IN

V

= 2.5V, VFB = 0V 0.35 Ω

IN

= 0V, VFB = 0V 0.01 1 µA

ITH/RUN

= 1Meg (from VIN to ITH/RUN) unless otherwise

ITH/RUN

= 1.5V, (Note 3) 0.01 0.70 %

ITH

= 1.9V, (Note 3) –0.80 –1.50 %

ITH

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

Note 2: The LTC1701E 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.

2

Note 3: The LTC1701 is tested in a feedback loop which servos VFB to the
midpoint for the error amplifier (V

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

Note 5: T
according to the following formula:

is calculated from the ambient TA and power dissipation P

J

LTC1701ES5: T

= TA + (PD•110°C/W)

J

= 1.7V unless otherwise specified).

ITH

D

UUU

PI FU CTIO S

LTC1701

SW (Pin 1): The Switch Node Connection to the Inductor.
This pin swings from VIN to a Schottky diode (external)
voltage drop below ground. The cathode of the Schottky
diode must be closely connected to this pin.

GND (Pin 2): Ground Pin. Connect to the (–) terminal of
C

, the Schottky diode and (–) terminal of CIN.

OUT

VFB (Pin 3): Receives the feedback voltage from the
external resistive divider across the output. Nominal volt-

ITH/RUN (Pin 4): Combination of Error Amplifier Compen­sation Point and Run Control Input. The current compara­tor threshold increases with this control voltage. Nominal
voltage range for this pin is 1.25V to 2.25V. Forcing this
pin below 0.8V causes the device to be shut down. In
shutdown all functions are disabled.

VIN (Pin 5): Main Supply Pin and the (+) Input to the
Current Comparator. Must be closely decoupled to ground.

age for this pin is 1.25V.

Pin Limit Table

NOMINAL (V) ABSOLUTE MAX (V)

PIN NAME DESCRIPTION MIN TYP MAX MIN MAX

1 SW Switch Node –0.3 V
2 GND Ground Pin 0
3VFBOutput Feedback Pin 0 1.25 1.35 –0.3 3
4I

/RUN Error Amplifier Compensation and RUN Pin 0 2.25 –0.3 3

TH

5VINMain Power Supply 2.5 5.5 –0.3 6

–0.3 VIN + 0.3

IN

BLOCK DIAGRA

REF

REF

+

I

TH

CLAMP

–

+

ERROR

AMP

–

1.4V

/REF

+

OVER

VOLTAGE

COMP

–

ITH/RUN

V

V

V

FB

W

V

IN

50µA

SHDN

(1.25V TO 2.25V)

PULSE

STRETCHER

<0.6V

V

FB

1.5V

V

IN

1.25V

BANDGAP

REFERENCE

+

–

I

TH

COMP

V

REF

(1.25V)

CURRENT

COMP

+

–

CONTROL LOGIC

V

REF

OFF-TIMER
AND GATE

CURRENT

SENSE

AMP

GATE

DRIVER

V

IN

+

–

SW

GND

1701 BD

3

LTC1701

fO=

−
+





















VV

VV T

IN OUT

IN D OFF

1

OPERATIO

U

The LTC1701 uses a contant off-time, current mode
architecture. The operating frequency is then determined
by the off-time and the difference between VIN and V
To optimize efficiency, the LTC1701 automatically switches
between continuous and Burst Mode operation.

The output voltage is set by an external divider returned to
the VFB pin. An error amplfier compares the divided output
voltage with a reference voltage of 1.25V and adjusts the
peak inductor current accordingly.

Main Control Loop

During normal operation, the internal PMOS switch is
turned on when the VFB voltage is below the reference
voltage. The current into the inductor and the load in­creases until the current limit is reached. The switch turns
off and energy stored in the inductor flows through the
external Schottky diode into the load. After the constant
off-time interval, the switch turns on and the cycle repeats.

The peak inductor current is controlled by the voltage on
the ITH/RUN pin, which is the output of the error
amplifier.This amplifier compares the VFB pin to the 1.25V
reference. When the load current increases, the FB voltage
decreases slightly below the reference. This decrease
causes the error amplifier to increase the ITH/RUN voltage
until the average inductor current matches the new load
current.

OUT

.

The main control loop is shut down by pulling the ITH/RUN
pin to ground. When the pin is released an external resistor
is used to charge the compensation capacitor. When the
voltage at the ITH/RUN pin reaches 0.8V, the main control
loop is enabled and the error amplifier drives the ITH/RUN
pin. Soft-start can be implemented by ramping the voltage
on the ITH/RUN pin (see Applications Information sec­tion).

Low Current Operation

When the load is relatively light, the LTC1701 automati­cally switches to Burst Mode operation in which the
internal PMOS switch operates intermittently based on
load demand. The main control loop is interrupted when
the output voltage reaches the desired regulated value.
The hysteretic voltage comparator trips when ITH/RUN is
below 1.5V, shutting off the switch and reducing the
power consumed. The output capacitor and the inductor
supply the power to the load until the output voltage drops
slightly and the ITH/RUN pin exceeds 1.5V, turning on the
switch and the main control loop which starts another
cycle.

Dropout Operation

In dropout, the internal PMOS switch is turned on continu­ously (100% duty cycle) providing low dropout operation
with V
an under voltage lockout, care should be taken to shut
down the LTC1701 for VIN < 2.5V.

at VIN. Since the LTC1701 does not incorporate

OUT

U

WUU

APPLICATIO S I FOR ATIO

The basic LTC1701 application circuit is shown in
Figure 1. External component selection is driven by the
load requirement and begins with the selection of L1. Once
L1 is chosen, the Schottky diode D1 can be selected
followed by CIN and C

L Selection and Operating Frequency

The operating frequency is fixed by VIN, V
constant off-time of about 500ns. The complete expres­sion for operating frequency is given by:

4

OUT

.

OUT

and the

Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current ∆IL decreases with
higher inductance and increases with higher VIN or V



∆=

I

L





−

VVfLVV

IN OUT OUT D









VV





IN D



+



+



OUT

: