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1
LTC1707
High Efficiency Monolithic
Synchronous Step-Down
Switching Regulator
December 1999
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.
Figure 1b. Efficiency vs Output Load Current
22
F
16V
47pF
V
IN
*
3V TO
8.5V
1707 F01a
6
2
7
1
5
8
3
SW
V
REF
V
FB
V
IN
RUN
SYNC/MODE
I
TH
LTC1707
+
15
H
100
F
6.3V
80.6k
*V
OUT
FOLLOWS V
IN
FOR
3V < VIN < 3.3V
249k
V
OUT
3.3V
+
GND
4
OUTPUT CURRENT (mA)
1
70
EFFICIENCY (%)
90
95
100
10
100
1000
1707 F01b
85
80
75
V
OUT
= 3.3V
V
IN
= 6V
V
IN
= 3.6V
V
IN
= 8.4V
Figure 1a. High Efficiency Low Dropout Step-Down Converter
The LTC
1707 is a high efficiency monolithic current
mode synchronous buck regulator using a fixed frequency
architecture. The operating supply range is from 8.5V
down to 2.85V, making it suitable for both single and dual
lithium-ion battery-powered applications. Burst Mode op-
eration provides high efficiency at low load currents.
100% duty cycle provides low dropout operation, extend-
ing operating time in battery-powered systems.
The switching frequency is internally set at 350kHz,
allowing the use of small surface mount inductors. For
noise sensitive applications it can be externally synchro-
nized up to 550kHz. Burst Mode operation is inhibited
during synchronization or when the SYNC/MODE pin is
pulled low preventing low frequency ripple from interfer-
ing with audio circuitry. Soft-start is provided by an
external capacitor.
The internal synchronous MOSFET switch increases effi-
ciency and eliminates the need for an external Schottky
diode, saving components and board space. Low output
voltages down to 0.8V are easily achieved due to the 0.8V
internal reference. The LTC1707 comes in an 8-lead SO
package.
s
600mA Output Current (V
IN
4V)
s
High Efficiency: Up to 96%
s
Constant Frequency: 350kHz Synchronizable
to 550kHz
s
2.85V to 8.5V V
IN
Range
s
0.8V Feedback Reference Allows Low Voltage
Outputs: 0.8V
V
OUT
V
IN
s
No Schottky Diode Required
s
1.19V
1% Reference Output Pin
s
Selectable Burst Mode
TM
Operation/Pulse
Skipping Mode
s
Low Dropout Operation: 100% Duty Cycle
s
Precision 2.7V Undervoltage Lockout
s
Current Mode Control for Excellent Line and
Load Transient Response
s
Low Quiescent Current: 200
A
s
Shutdown Mode Draws Only 15
A Supply Current
s
Available in 8-Lead SO Package
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
s
Cellular Telephones
s
Portable Instruments
s
Wireless Modems
s
RF Communications
s
Distributed Power Systems
s
Single and Dual Cell Lithium
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
2
LTC1707
ORDER PART
NUMBER
S8 PART MARKING
Consult factory for Military grade parts.
1707
1707I
LTC1707CS8
LTC1707IS8
T
JMAX
= 125
C,
JA
= 110
C/ W
TOP VIEW
V
REF
SYNC/MODE
V
IN
SW
I
TH
RUN/SS
V
FB
GND
S8 PACKAGE
8-LEAD PLASTIC SO
1
2
3
4
8
7
6
5
(Note 1)
Input Supply Voltage ................................ 0.3V to 10V
I
TH
Voltage ................................................. 0.3V to 5V
RUN/SS, V
FB
Voltages ............................... 0.3V to V
IN
SYNC/MODE Voltage ................................. 0.3V to V
IN
P-Channel Switch Source Current (DC) .............. 800mA
N-Channel Switch Sink Current (DC) .................. 800mA
Peak SW Sink and Source Current ......................... 1.5A
Operating Ambient Temperature Range
Commercial ............................................ 0
C to 70
C
Industrial ........................................... 40
C to 85
C
Junction Temperature (Note 2) ............................. 125
C
Storage Temperature Range ................ 65
C to 150
C
Lead Temperature (Soldering, 10 sec)................. 300
C
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
VFB
Feedback Current
(Note 3)
6
60
nA
V
FB
Regulated Feedback Voltage
(Note 3)
q
0.78
0.80
0.82
V
V
OVL
Output Overvoltage Lockout
V
OVL
= V
OVL
V
FB
20
60
110
mV
V
FB
Reference Voltage Line Regulation
V
IN
= 3V to 8.5V (Note 3)
0.002
0.01
%/V
V
LOADREG
Output Voltage Load Regulation
I
TH
Sinking 2
A (Note 3)
0.5
0.8
%
I
TH
Sourcing 2
A (Note 3)
0.5
0.8
%
I
S
Input DC Bias Current
(Note 4)
Pulse Skipping Mode
V
IN
= 8.5V, V
OUT
= 3.3V, V
SYNC/MODE
= 0V
300
A
Burst Mode Operation
V
ITH
= 0V, V
IN
= 8.5V, V
SYNC/MODE
= Open
200
320
A
Shutdown
V
RUN/SS
= 0V, 3V < V
IN
< 8.5V
11
35
A
Shutdown
V
RUN/SS
= 0V, V
IN
< 3V
6
A
V
RUN/SS
Run/SS Threshold
V
RUN/SS
Ramping Positive
0.4
0.7
1.0
V
I
RUN/SS
Soft-Start Current Source
V
RUN/SS
= 0V
1.2
2.25
3.3
A
I
SYNC/MODE
SYNC/MODE Pull-Up Current
V
SYNC/MODE
= 0V
0.5
1.5
2.5
A
f
OSC
Oscillator Frequency
V
FB
= 0.7V
315
350
385
kHz
V
FB
= 0V
35
kHz
V
UVLO
Undervoltage Lockout
V
IN
Ramping Down from 3V (0
C to 70
C)
2.55
2.70
2.85
V
V
IN
Ramping Up from 0V (0
C to 70
C)
2.60
2.80
3.00
V
V
IN
Ramping Down from 3V (40
C to 85
C)
2.45
2.70
2.85
V
V
IN
Ramping Up from 0V (40
C to 85
C)
2.50
2.80
3.00
V
R
PFET
R
DS(ON)
of P-Channel FET
I
SW
= 100mA
0.5
0.7
R
NFET
R
DS(ON)
of N-Channel FET
I
SW
= 100mA
0.6
0.8
I
PK
Peak Inductor Current
V
IN
= 4V, I
TH
= 1.4V, Duty Cycle < 40%
0.70
0.915
1.10
A
I
LSW
SW Leakage
V
RUN/SS
= 0V
10
1000
nA
V
REF
Reference Output Voltage
I
REF
= 0
A
q
1.178
1.19
1.202
mV
V
REF
Reference Output Load Regulation
0V
I
REF
100
A
q
2.3
15
mV
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
T
J
= T
A
+ (P
D
110
C/W)
Note 3: The LTC1707 is tested in a feedback loop that servos V
FB
to the
balance point for the error amplifier (V
ITH
= 0.8V).
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
IN
= 5V unless otherwise specified.
ABSOLUTE AXI U RATI GS
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PACKAGE/ORDER I FOR ATIO
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ELECTRICAL CHARACTERISTICS
3
LTC1707
Efficiency vs Input Voltage
INPUT VOLTAGE (V)
0
EFFICIENCY (%)
90
95
100
8
1707 G01
85
80
75
2
4
6
10
V
OUT
= 2.5V
L = 15
H
Burst Mode OPERATION
I
LOAD
= 100mA
I
LOAD
= 300mA
I
LOAD
= 10mA
OUTPUT CURRENT (mA)
1
70
65
60
55
50
EFFICIENCY (%)
90
95
100
10
100
1000
1707 G02
85
80
75
Burst Mode
OPERATION
PULSE SKIPPING
MODE
V
IN
= 3.6V
V
OUT
= 2.5V
L = 15
H
OUTPUT CURRENT (mA)
1
70
EFFICIENCY (%)
90
95
100
10
100
1000
1707 G03
85
80
75
V
OUT
= 2.5V
L = 15
H
Burst Mode OPERATION
V
IN
= 2.8V
V
IN
= 3.6V
V
IN
= 7.2V
Efficiency vs Load Current
Efficiency vs Load Current
Undervoltage Lockout Threshold
vs Temperature
TEMPERATURE (
C)
50
25
2.50
2.55
UNDERVOLTAGE LOCKOUT THRESHOLD (V)
2.60
2.70
2.75
2.80
75
100
3.00
1707 G04
2.65
0
25
50
125
2.85
2.90
2.95
V
IN
RAMPING UP
V
IN
RAMPING DOWN
DC Supply Current
vs Input Voltage
INPUT VOLTAGE (V)
2.5
DC SUPPLY CURRENT (
A)
50
100
150
6.5
350
1707 G05
0
4.5
3.5
7.5
5.5
8.5
200
250
300
T
J
= 25
C
V
OUT
= 1.8V
LOAD CURRENT = 0A
Burst Mode
OPERATION
PULSE SKIPPING
MODE
Supply Current in Shutdown
vs Input Voltage
INPUT VOLTAGE (V)
2.5
4
SUPPLY CURRENT IN SHUTDOWN (
A)
6
10
12
14
6.5
22
1707 G06
8
4.5
3.5
7.5
5.5
8.5
16
18
20
V
RUN/SS
= 0V
T
J
= 85
C
T
J
= 25
C
T
J
= 40
C
Reference Voltage
vs Temperature
Oscillator Frequency
vs Temperature
TEMPERATURE (
C)
50
25
1.180
REFERENCE VOLTAGE (V) 1.185
1.190
75
100
1.200
1707 G07
0
25
50
125
1.195
V
IN
= 5V
Oscillator Frequency
vs Input Voltage
TEMPERATURE (
C)
50
25
300
OSCILLATOR FREQUENCY (kHz)
310
330
340
350
75
100
390
1707 G08
320
0
25
50
125
360
370
380
V
IN
= 5V
INPUT VOLTAGE (V)
2.5
300
OSCILLATOR FREQUENCY (kHz)
310
330
340
350
6.5
390
1627 G09
320
4.5
3.5
7.5
5.5
8.5
360
370
380
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
4
LTC1707
Maximum Output Current vs
Input Voltage
Switch Leakage Current
vs Temperature
Switch Resistance
vs Input Voltage
Load Step Transient Response
INPUT VOLTAGE (V)
2.5
0
OUTPUT CURRENT (mA)
200
400
600
6.5
1000
1707 G10
4.5
3.5
7.5
5.5
8.5
800
T
J
= 85
C
L = 15
H
V
OUT
= 5V
V
OUT
= 1.5V
V
OUT
= 2.5V
V
OUT
= 2.9V
V
OUT
= 3.3V
V
OUT
= 1.8V
TEMPERATURE (
C)
50
25
0
SWITCH LEAKAGE (nA)
200
600
800
1000
75
100
1800
1707 G11
400
0
25
50
125
1200
1400
1600
V
IN
= 8.4V
SYNCHRONOUS
SWITCH
MAIN
SWITCH
Switch Resistance
vs Temperature
TEMPERATURE (
C)
50
25
0
SWITCH RESISTANCE (
)
0.1
0.3
0.4
0.5
75
100
0.9
1707 G12
0.2
0
25
50
125
0.6
0.7
0.8
V
IN
= 5V
SYNCHRONOUS
SWITCH
MAIN
SWITCH
Burst Mode Operation
INPUT VOLTAGE (V)
2.5
0
SWITCH RESISTANCE (
)
0.1
0.3
0.4
0.5
6.5
0.9
1707 G13
0.2
4.5
3.5
7.5
5.5
8.5
0.6
0.7
0.8
SYNCHRONOUS SWITCH
MAIN SWITCH
I
TH
0.5V/DIV
V
OUT
50mV/DIV
AC COUPLED
I
LOAD
500mA/DIV
25
s/DIV
1707 G14
SW
5V/DIV
V
OUT
20mV/DIV
AC COUPLED
I
LOAD
200mA/DIV
10
s/DIV
1707 G15
FIGURE 1A
V
IN
= 5V
FIGURE 1A
I
LOAD
= 50mA
V
IN
= 5V
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
5
LTC1707
I
TH
(Pin 1): Error Amplifier Compensation Point. The
current comparator threshold increases with this control
voltage. Nominal voltage range for this pin is 0V to 1.2V.
RUN/SS (Pin 2): Combination of Soft-Start and Run
Control Inputs. A capacitor to ground at this pin sets the
ramp time to full current output. The time is approximately
0.5s/
F. Forcing this pin below 0.4V shuts down the
LTC1707.
V
FB
(Pin 3): Feedback Pin. Receives the feedback voltage
from an external resistive divider across the output.
GND (Pin 4): Ground Pin.
SW (Pin 5): Switch Node Connection to Inductor. This pin
connects to the drains of the internal main and synchro-
nous power MOSFET switches.
V
IN
(Pin 6):
Main Supply Pin. Must be closely decoupled
to GND, Pin 4.
SYNC/MODE (Pin 7): This pin performs two functions:
1) synchronize with an external clock and 2) select be-
tween two modes of low load current operation. To
synchronize with an external clock, apply a TTL/CMOS
compatible clock with a frequency between 385kHz and
550kHz. To select Burst Mode operation, float the pin or
tie it to V
IN
. Grounding Pin 7 forces pulse skipping mode
operation.
V
REF
(Pin 8): The Output of a 1.19V
1% Precision
Reference. May be loaded up to 100
A and is stable with
up to 2000pF load capacitance.
V
FB
V
IN
3
V
REF
2
1
6
5
4
Q
Q
R
S
0.12V
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
0.6V
I
TH
RUN/SS
0.8V
0.4V
0.86V
SYNC/MODE
Y = "0" ONLY WHEN X IS A CONSTANT "1"
SHUTDOWN
SLEEP
6
1.5
A
2.25
A
V
IN
V
IN
V
IN
V
IN
SW
GND
1707 BD
V
IN
BURST
EN
7
OSC
FREQ
SHIFT
1.19V
REF
UVLO
TRIP = 2.7V
Y
X
BURST
DEFEAT
SLOPE
COMP
+
OVDET
+
+
+
+
+
EA
I
COMP
+
I
RCMP
RUN/SOFT
START
ANTI-
SHOOT-THRU
8
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FU CTIO AL DIAGRA
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