LTC1707
High Efficiency Monolithic
Synchronous Step-Down
Switching Regulator
FEATURES
s
s
s
DESCRIPTIO
s
s
s
s
s
s
s
s
s
s
s
600mA Output Current (V
IN
≥
4V)
High Efficiency: Up to 96%
Constant Frequency: 350kHz Synchronizable
to 550kHz
2.85V to 8.5V V
IN
Range
0.8V Feedback Reference Allows Low Voltage
Outputs: 0.8V
≤
V
OUT
≤
V
IN
No Schottky Diode Required
1.19V
±1%
Reference Output Pin
Selectable Burst Mode
TM
Operation/Pulse
Skipping Mode
Low Dropout Operation: 100% Duty Cycle
Precision 2.7V Undervoltage Lockout
Current Mode Control for Excellent Line and
Load Transient Response
Low Quiescent Current: 200µA
Shutdown Mode Draws Only 11µA Supply Current
Available in 8-Lead SO Package
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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
APPLICATIO S
s
s
s
s
s
s
Cellular Telephones
Portable Instruments
Wireless Modems
RF Communications
Distributed Power Systems
Single and Dual Cell Lithium
TYPICAL APPLICATIO
V
IN
*
3V TO
8.5V
100
V
OUT
= 3.3V
95
V
IN
= 6V
V
IN
= 3.6V
6
+
22µF
16V
2
V
IN
RUN
SW
V
REF
V
FB
5
8
3
15µH
+
249k
100µF
6.3V
V
OUT
3.3V
EFFICIENCY (%)
90
85
80
75
V
IN
= 8.4V
LTC1707
7
SYNC/MODE
1
47pF
I
TH
80.6k
GND
4
*V
OUT
FOLLOWS V
IN
FOR
3V < VIN < 3.3V
1707 F01a
70
1
10
100
OUTPUT CURRENT (mA)
1000
1707 F01b
Figure 1a. High Efficiency Low Dropout Step-Down Converter
Figure 1b. Efficiency vs Output Load Current
U
U
U
1
LTC1707
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
I
TH
1
RUN/SS 2
V
FB
3
GND 4
8
7
6
5
V
REF
SYNC/MODE
V
IN
SW
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
ORDER PART
NUMBER
LTC1707CS8
LTC1707IS8
S8 PART MARKING
1707
1707I
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 120°C/ W
Consult factory for Military grade parts.
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.
SYMBOL
I
VFB
V
FB
∆V
OVL
∆V
FB
V
LOADREG
I
S
PARAMETER
Feedback Current
Regulated Feedback Voltage
Output Overvoltage Lockout
Reference Voltage Line Regulation
Output Voltage Load Regulation
Input DC Bias Current
Pulse Skipping Mode
Burst Mode Operation
Shutdown
Shutdown
Run/SS Threshold
Soft-Start Current Source
SYNC/MODE Pull-Up Current
Oscillator Frequency
Undervoltage Lockout
CONDITIONS
(Note 3)
(Note 3)
∆V
OVL
= V
OVL
– V
FB
V
IN
= 3V to 8.5V (Note 3)
I
TH
Sinking 2µA (Note 3)
I
TH
Sourcing 2µA (Note 3)
(Note 4)
V
IN
= 8.5V, V
OUT
= 3.3V, V
SYNC/MODE
= 0V
V
ITH
= 0V, V
IN
= 8.5V, V
SYNC/MODE
= Open
V
RUN/SS
= 0V, 3V < V
IN
< 8.5V
V
RUN/SS
= 0V, V
IN
< 3V
V
RUN/SS
Ramping Positive
V
RUN/SS
= 0V
V
SYNC/MODE
= 0V
V
FB
= 0.7V
V
FB
= 0V
V
IN
Ramping Down from 3V (0°C to 70°C)
V
IN
Ramping Up from 0V (0°C to 70°C)
V
IN
Ramping Down from 3V (–40°C to 85°C)
V
IN
Ramping Up from 0V (–40°C to 85°C)
I
SW
= –100mA
I
SW
= – 100mA
V
IN
= 4V, I
TH
= 1.4V, Duty Cycle < 40%
V
RUN/SS
= 0V
I
REF
= 0µA
0V
≤
I
REF
≤
100µA
MIN
q
ELECTRICAL CHARACTERISTICS
0.78
20
TYP
6
0.80
60
0.002
0.5
– 0.5
300
200
11
6
0.7
2.25
1.5
350
35
2.70
2.80
2.70
2.80
0.5
0.6
0.915
±10
1.19
2.3
MAX
60
0.82
110
0.01
0.8
– 0.8
UNITS
nA
V
mV
%/V
%
%
µA
µA
µA
µA
V
µA
µA
kHz
kHz
V
V
V
V
Ω
Ω
A
nA
mV
mV
320
35
1.0
3.3
2.5
385
2.85
3.00
2.85
3.00
0.7
0.8
1.10
±1000
1.202
15
V
RUN/SS
I
RUN/SS
I
SYNC/MODE
f
OSC
V
UVLO
0.4
1.2
0.5
315
2.55
2.60
2.45
2.50
R
PFET
R
NFET
I
PK
I
LSW
V
REF
∆V
REF
R
DS(ON)
of P-Channel FET
R
DS(ON)
of N-Channel FET
Peak Inductor Current
SW Leakage
Reference Output Voltage
Reference Output Load Regulation
0.70
q
q
1.178
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.
2
U
W
U
U
W W
W
LTC1707
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Input Voltage
100
95
I
LOAD
= 100mA
I
LOAD
= 300mA
I
LOAD
= 10mA
EFFICIENCY (%)
EFFICIENCY (%)
90
80
75
70
65
PULSE SKIPPING
MODE
EFFICIENCY (%)
85
80
V
OUT
= 2.5V
L = 15µH
Burst Mode OPERATION
0
2
6
4
INPUT VOLTAGE (V)
8
10
1707 G01
75
Undervoltage Lockout Threshold
vs Temperature
3.00
350
UNDERVOLTAGE LOCKOUT THRESHOLD (V)
2.95
SUPPLY CURRENT IN SHUTDOWN (µA)
DC SUPPLY CURRENT (µA)
2.90
2.85
2.80
2.75
2.70
2.65
2.60
2.55
2.50
– 50
– 25
V
IN
RAMPING UP
V
IN
RAMPING DOWN
0
25
50
75
TEMPERATURE (°C)
Reference Voltage
vs Temperature
1.200
V
IN
= 5V
OSCILLATOR FREQUENCY (kHz)
REFERENCE VOLTAGE (V)
1.195
370
360
350
340
330
320
310
OSCILLATOR FREQUENCY (kHz)
1.190
1.185
1.180
– 50
– 25
0
25
50
75
TEMPERATURE (°C)
U W
100
1707 G04
Efficiency vs Load Current
100
95
Burst Mode
90 OPERATION
85
100
Efficiency vs Load Current
V
IN
= 2.8V
95
90
85
V
IN
= 7.2V
80
75
70
V
OUT
= 2.5V
L = 15µH
Burst Mode OPERATION
1
10
100
OUTPUT CURRENT (mA)
1000
1707 G03
V
IN
= 3.6V
60
55
50
1
V
IN
= 3.6V
V
OUT
= 2.5V
L = 15µH
10
100
OUTPUT CURRENT (mA)
1000
1707 G02
DC Supply Current
vs Input Voltage
22
20
18
16
14
12
10
8
6
4
2.5
3.5
4.5
5.5
6.5
INPUT VOLTAGE (V)
7.5
8.5
PULSE SKIPPING
MODE
300
250
200
150
100
50
0
125
T
J
= 25°C
V
OUT
= 1.8V
LOAD CURRENT = 0A
Supply Current in Shutdown
vs Input Voltage
V
RUN/SS
= 0V
T
J
= 85°C
T
J
= 25°C
Burst Mode
OPERATION
T
J
= – 40°C
2.5
3.5
4.5
5.5
6.5
INPUT VOLTAGE (V)
7.5
8.5
1707 G05
1707 G06
Oscillator Frequency
vs Temperature
390
380
V
IN
= 5V
390
380
370
360
350
340
330
320
310
300
– 25
0
25
50
75
TEMPERATURE (°C)
100
125
Oscillator Frequency
vs Input Voltage
100
125
300
– 50
2.5
3.5
4.5
5.5
6.5
INPUT VOLTAGE (V)
7.5
8.5
1707 G07
1707 G08
1627 G09
3
LTC1707
TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Output Current vs
Input Voltage
1000
V
OUT
= 1.8V
800
OUTPUT CURRENT (mA)
SWITCH LEAKAGE (nA)
1200
1000
800
600
400
SYNCHRONOUS
SWITCH
SWITCH RESISTANCE (Ω)
V
OUT
= 1.5V
600
V
OUT
= 5V
400
V
OUT
= 3.3V
V
OUT
= 2.5V
V
OUT
= 2.9V
T
J
= 85°C
L = 15µH
0
2.5
3.5
4.5
5.5
6.5
INPUT VOLTAGE (V)
7.5
8.5
200
Switch Resistance
vs Input Voltage
0.9
0.8
SWITCH RESISTANCE (Ω)
0.7
SYNCHRONOUS SWITCH
0.6
0.5
0.4
0.3
0.2
0.1
0
2.5
3.5
4.5
5.5
6.5
INPUT VOLTAGE (V)
7.5
8.5
MAIN SWITCH
4
U W
1707 G10
1707 G13
Switch Leakage Current
vs Temperature
1800
1600
1400
V
IN
= 8.4V
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Switch Resistance
vs Temperature
V
IN
= 5V
SYNCHRONOUS
SWITCH
MAIN
SWITCH
200
0
– 50
– 25
0
25
50
75
TEMPERATURE (°C)
MAIN
SWITCH
100
125
0
– 50
– 25
0
25
50
75
TEMPERATURE (°C)
100
125
1707 G11
1707 G12
Load Step Transient Response
I
TH
0.5V/DIV
SW
5V/DIV
V
OUT
20mV/DIV
AC COUPLED
I
LOAD
200mA/DIV
25µs/DIV
V
IN
= 5V
V
OUT
= 3.3V
L = 15µH
C
IN
= 22µF
C
OUT
= 100µF
I
LOAD
= 0mA TO 500mA
Burst Mode OPERATION
1707 G14
Burst Mode Operation
V
OUT
50mV/DIV
AC COUPLED
I
LOAD
500mA/DIV
V
IN
= 5V
V
OUT
= 3.3V
L = 15µH
C
IN
= 22µF
C
OUT
= 100µF
I
LOAD
= 50mA
10µs/DIV
1707 G15
LTC1707
PI FU CTIO S
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/µF. 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 100µA and is stable with
up to 2000pF load capacitance.
FU CTIO AL DIAGRA
BURST
DEFEAT
X
1.5µA
SYNC/MODE
7
Y = “0” ONLY WHEN X IS A CONSTANT “1”
Y
V
IN
OSC
0.6V
V
FB
3
V
REF
8
V
IN
0.8V
+
EA
0.12V
I
TH
1
S
RUN/SOFT
START
R
Q
Q
–
1.19V
REF
2.25µA
V
IN
RUN/SS 2
UVLO
TRIP = 2.7V
+
OVDET
0.86V
SHUTDOWN
–
+
I
RCMP
5 SW
4 GND
1707 BD
+
–
+
FREQ
SHIFT
–
W
U
U
U
+
–
U
U
V
IN
V
IN
SLOPE
COMP
0.4V
6 V
IN
EN
SLEEP
–
+
6Ω
I
COMP
BURST
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTI-
SHOOT-THRU
–
5
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