LTC1701/LTC1701B
1MHz Step-Down
DC/DC Converters in SOT-23
FEATURES
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DESCRIPTIO
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Tiny 5-Lead SOT-23 Package
Uses Tiny Capacitors and Inductor
High Frequency Operation: 1MHz
High Output Current: 500mA
Low R
DS(ON)
Internal Switch: 0.28Ω
High Efficiency: Up to 94%
Current Mode Operation for Excellent Line
and Load Transient Response
Short-Circuit Protected
Low Quiescent Current: 135µA (LTC1701)
Low Dropout Operation: 100% Duty Cycle
Ultralow Shutdown Current: I
Q
< 1µA
Peak Inductor Current Independent of Inductor Value
Output Voltages from 5V Down to 1.25V
The LTC
®
1701/LTC1701B are the industry’s first SOT-23
step-down, current mode, DC/DC converters. Intended for
low to medium power applications, they operate from 2.5V
to 5.5V input voltage range and switch 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-LOOP
TM
compensation
allows the transient response to be optimized over a wide
range of loads and output capacitors.
The LTC1701 incorporates automatic power saving Burst
Mode
TM
operation to reduce gate charge losses when the
load current drops below the level required for continuous
operation. The LTC1701B operates continuously to very
low load currents to provide low ripple at the expense of
light load efficiency. With no load, the LTC1701 draws only
135µA. In shutdown, both devices draw less than 1µA,
making them ideal for current sensitive applications.
Their small size and switching frequency enables a
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.
APPLICATIO S
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PDAs/Palmtop PCs
Digital Cameras
Cellular Phones
Portable Media Players
PC Cards
Handheld Equipment
TYPICAL APPLICATION
V
IN
2.5V TO
5.5V
R4
1M
L1
4.7µH
V
IN
SW
D1
LTC1701
R2
121k
V
OUT
(2.5V/
500mA)
100
95
90
85
EFFICIENCY (%)
LTC1701
V
IN
= 3.3V
V
OUT
= 2.5V
+
C1
10µF
+
I
TH
/RUN
R3
GND
5.1k
C3
330pF
V
FB
R1
121k
C2
47µF
80
75
70
65
60
LTC1701B
C1: TAIYO YUDEN JMK316BJ106ML
C2: SANYO POSCAP 6TPA47M
D1: MBRM120L
L1: SUMIDA CD43-4R7
1701 F01a
55
50
1
10
100
LOAD CURRENT (mA)
1000
1701 F01b
Figure 1. 2.5V/500mA Step-Down Regulator
U
Efficiency Curve
U
U
1
LTC1701/LTC1701B
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SW 1
GND 2
V
FB
3
4 I
TH
/RUN
5 V
IN
(Voltages Referred to GND Pin)
V
IN
Voltage (Pin 5).......................................– 0.3V to 6V
I
TH
/RUN Voltage (Pin 4) ..............................– 0.3V to 3V
V
FB
Voltage (Pin 3) ......................................– 0.3V to 3V
V
IN
– 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
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC1701ES5
LTC1701BES5
S5 PART
MARKING
LTKG
LTUD
S5 PACKAGE
5-LEAD PLASTIC SOT-23
T
JMAX
= 125°C,
θ
JA
= 250°C/W
SEE THE APPLICATION
INFORMATION SECTION
Consult factory for parts specified with wider operating temperature ranges.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 3.3V, R
ITH/RUN
= 1Meg (from V
IN
to I
TH
/RUN) unless otherwise
specified. (Note 2)
SYMBOL
V
IN
I
FB
V
FB
∆V
LINE REG
∆V
LOAD REG
PARAMETER
Operating Voltage Range
Feedback Pin Input Current
Feedback Voltage
Reference Voltage Line Regulation
Output Voltage Load Regulation
Input DC Supply Current (Note 4)
Active Mode
Sleep Mode
Shutdown
V
ITH/RUN
I
ITH/RUN
I
SW(PEAK)
R
DS(ON)
Run Threshold High
Run Threshold Low
Run Pullup Current
Peak Switch Current Threshold
Switch ON Resistance
(Note 3)
(Note 3)
V
IN
= 2.5V to 5V (Note 3)
Measured in Servo Loop, V
ITH
= 1.5V, (Note 3)
Measured in Servo Loop, V
ITH
= 1.9V, (Note 3)
V
FB
= 0V
V
FB
= 1.4V (LTC1701 only)
V
ITH/RUN
= 0V
I
TH/RUN
Ramping Down
I
TH/RUN
Ramping Up
V
ITH/RUN
= 1V
V
FB
= 0V
V
IN
= 5V, V
FB
= 0V
V
IN
= 3.3V, V
FB
= 0V
V
IN
= 2.5V, V
FB
= 0V
V
IN
= 5V, V
ITH/RUN
= 0V, V
FB
= 0V
400
0.3
50
0.9
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
2.5
1.22
TYP
MAX
5.5
±0.1
UNITS
V
µA
V
%/V
%
%
µA
µA
µA
V
V
µA
A
Ω
Ω
Ω
1.25
0.04
0.01
– 0.80
185
135
0.25
1.4
0.6
100
1.1
0.28
0.30
0.35
0.01
500
1.28
0.1
0.70
–1.50
300
200
1
1.6
300
I
SW(LKG)
t
OFF
Switch Leakage Current
Switch Off-Time
1
600
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC1701E/LTC1701BE 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.
Note 3:
The LTC1701/LTC1701B are tested in a feedback loop which
servos V
FB
to the midpoint for the error amplifier without R
ITH/RUN
= 1MHz
(V
ITH
= 1.7V unless otherwise specified).
Note 4:
Dynamic supply current is higher due to the internal gate charge
being delivered at the switching frequency.
Note 5:
T
J
is calculated from the ambient T
A
and power dissipation P
D
according to the following formula:
LTC1701ES5/LTC1701BES5: T
J
= T
A
+ (P
D
• 250°C/W)
2
U
µA
ns
W
U
U
W W
W
LTC1701/LTC1701B
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
100
V
OUT
= 2.5V
95
EFFICIENCY (%)
100
V
IN
= 3.3V
EFFICIENCY (%)
90
V
IN
= 5.0V
85
80
75
70
LTC1701
LTC1701B
1
10
100
LOAD CURRENT (mA)
1000
1701 • G01
SUPPLY CURRENT (µA)
Switch Resistance vs
Supply Voltage
370
350
ON RESISTANCE (mΩ)
V
OUT
ERROR (%)
330
310
290
270
250
V
OUT
ERROR (%)
2
3
5
4
SUPPLY VOLTAGE (V)
Dropout Characteristics
3.4
3.3
3.2
V
OUT
(V)
I
LOAD
= 100mA
3.1
I
LOAD
= 200mA
3.0
2.9
I
LOAD
= 500mA
2.8
2.7
2.6
3.0
3.2
3.4
V
IN
(V)
701 • G07
V
OUT
= 3.3V
FIGURE 1
3.6
3.8
U W
6
1701 • G04
Efficiency vs Input Voltage
V
OUT
= 2.5V
I
LOAD
=100mA
DC Supply Current
300
250
200
150
SLEEP
100
50
0
ACTIVE
95
90
85
80
75
70
65
60
2
3
4
LTC1701
LTC1701B
I
LOAD
=10mA
5
6
1701 • G02
2
3
INPUT VOLTAGE (V)
5
4
INPUT VOLTAGE (V)
6
1701 • G03
Load Regulation
0.60
0.40
0.20
0.00
–0.20
–0.40
–0.60
–0.80
–1.00
–1.20
–1.40
0
400
200
LOAD CURRENT (mA)
600
1701 • G05
Line Regulation
0.30
V
OUT
= 5.0V
0.25
I
LOAD
= 200mA
0.20
0.15
0.10
I
LOAD
= 400mA
0.05
0
V
OUT
= 3.3V
2
3
4
V
IN
(V)
5
6
1701 • G06
Start-Up
V
OUT
1V/DIV
Transient Response
V
OUT
50mV/DIV
AC COUPLED
I
TH
2V/DIV
I
L
500mA/DIV
I
L
200mA/DIV
V
IN
= 3.3V, V
OUT
= 2.5V
CIRCUIT OF FIGURE 1
R
LOAD
= 6Ω
1701 G08
V
IN
= 3.3V, V
OUT
= 2.5V
CIRCUIT OF FIGURE 1
I
LOAD
= 100mA TO 500mA STEP
1701 G09
3
LTC1701/LTC1701B
PI FU CTIO S
SW (Pin 1):
The Switch Node Connection to the Inductor.
This pin swings from V
IN
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
OUT
, the Schottky diode and (–) terminal of C
IN
.
V
FB
(Pin 3):
Receives the feedback voltage from the
external resistive divider across the output. Nominal volt-
age for this pin is 1.25V.
I
TH
/RUN (Pin 4):
Combination of Error Amplifier Compen-
sation Point and Run Control Input. The current compara-
tor 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.
V
IN
(Pin 5):
Main Supply Pin and the (+) Input to the
Current Comparator. Must be closely decoupled to ground.
Pin Limit Table
PIN
1
2
3
4
5
NAME
SW
GND
V
FB
I
TH
/RUN
V
IN
DESCRIPTION
Switch Node
Ground Pin
Output Feedback Pin
Error Amplifier Compensation and RUN Pin
Main Power Supply
0
0
2.5
MIN
– 0.3
0
1.25
1.35
2.25
5.5
– 0.3
– 0.3
– 0.3
3
3
6
NOMINAL (V)
TYP
MAX
V
IN
ABSOLUTE MAX (V)
MIN
MAX
V
IN
– 8.5
V
IN
+ 0.3
BLOCK DIAGRA
V
REF
+
I
TH
/REF
CLAMP
–
I
TH
/RUN
V
REF
SHDN
+
ERROR
AMP
V
FB
–
SW
+
1.4V
–
OVER
VOLTAGE
COMP
PULSE
STRETCHER
V
FB
< 0.6V
4
W
U
U
U
V
IN
V
IN
50µA
1.25V
BANDGAP
REFERENCE
V
REF
(1.25V)
V
IN
+
+
CURRENT
COMP
CURRENT
SENSE
AMP
1.5V
+
I
TH
COMP
–
–
V
REF
–
(1.25V TO 2.25V)
(LTC1701 only)
OFF-TIMER
AND GATE
CONTROL LOGIC
GATE
DRIVER
GND
1701 BD
LTC1701/LTC1701B
OPERATIO
The LTC1701 uses a contant off-time, current mode archi-
tecture. The operating frequency is then determined by the
off-time and the difference between V
IN
and V
OUT
.
The output voltage is set by an external divider returned to
the V
FB
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 V
FB
voltage is below the reference
voltage. The current into the inductor and the load in-
creases 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 I
TH
/RUN pin, which is the output of the error
amplifier.This amplifier compares the V
FB
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 I
TH
/RUN voltage
until the average inductor current matches the new load
current.
The main control loop is shut down by pulling the I
TH
/RUN
pin to ground. When the pin is released an external resistor
is used to charge the compensation capacitor. When the
voltage at the I
TH
/RUN pin reaches 0.8V, the main control
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 C
IN
and C
OUT
.
L Selection and Operating Frequency
The operating frequency is fixed by V
IN
, V
OUT
and the
constant off-time of about 500ns. The complete expres-
sion for operating frequency is given by:
U
W
U
U
U
loop is enabled and the error amplifier drives the I
TH
/RUN
pin. Soft-start can be implemented by ramping the voltage
on the I
TH
/RUN pin (see Applications Information sec-
tion).
Low Current Operation
To optimize efficiency 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 inter-
rupted when the output voltage reaches the desired regu-
lated value. The hysteretic voltage comparator trips when
I
TH
/RUN is below 1.5V, shutting off the switch and reduc-
ing the power consumed. The output capacitor and the
inductor supply the power to the load until the output
voltage drops slightly and the I
TH
/RUN pin exceeds 1.5V,
turning on the switch and the main control loop which
starts another cycle.
For reduced output ripple, the LTC1701B doesn't use
Burst Mode operation and operates continuously down to
very low currents where the part starts skipping cycles.
Dropout Operation
In dropout, the internal PMOS switch is turned on continu-
ously (100% duty cycle) providing low dropout operation
with V
OUT
at V
IN
. Since the LTC1701 does not incorporate
an under voltage lockout, care should be taken to shut
down the LTC1701 for V
IN
< 2.5V.
V
IN
−
V
OUT
1
f
O
=
V
IN
+
V
D
T
OFF
Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current
∆I
L
decreases with
higher inductance and increases with higher V
IN
or V
OUT
:
V
−
V
V
+
V
∆
I
L
=
IN OUT
OUT D
fL
V
IN
+
V
D
where V
D
is the output Schottky diode forward drop.
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