LTC1627
Monolithic Synchronous
Step-Down Switching Regulator
FEATURES
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DESCRIPTION
The LTC
®
1627 is a monolithic current mode synchronous
buck regulator using a fixed frequency architecture. The
operating supply range is from 8.5V down to 2.65V, making
it suitable for one or two lithium-ion battery-powered appli-
cations. Burst Mode operation provides high efficiency at
low load currents. 100% duty cycle provides low dropout
operation, which extends operating time in battery-operated
systems.
The operating frequency is internally set at 350kHz, allowing
the use of small surface mount inductors. For switching noise
sensitive applications it can be externally synchronized up to
525kHz. The SYNC/FCB control pin guarantees regulation of
secondary windings regardless of load on the main output by
forcing continuous operation. Burst Mode operation is inhib-
ited during synchronization or when the SYNC/FCB pin is
pulled low to reduce noise and RF interference. Soft-start is
provided by an external capacitor.
Optional bootstrapping enhances the internal switch drive for
single lithium-ion cell applications. The internal synchronous
switch increases efficiency and eliminates the need for an
external Schottky diode, saving components and board
space. The LTC1627 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.
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High Efficiency: Up to 96%
Constant Frequency 350kHz Operation
2.65V to 8.5V V
IN
Range
V
OUT
from 0.8V to V
IN
, I
OUT
to 500mA
No Schottky Diode Required
Synchronizable Up to 525kHz
Selectable Burst Mode
TM
Operation
Low Dropout Operation: 100% Duty Cycle
Precision 2.5V Undervoltage Lockout
Secondary Winding Regulation
Current Mode Operation for Excellent Line and
Load Transient Response
Low Quiescent Current: 200µA
Shutdown Mode Draws Only 15µA Supply Current
±1.5%
Reference Accuracy
Available in 8-Lead SO Package
APPLICATIONS
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Cellular Telephones
Portable Instruments
Wireless Modems
RF Communications
Distributed Power Systems
Scanners
Single and Dual Cell Lithium
TYPICAL APPLICATION
1
47pF
C
SS
0.1µF
2
3
4
I
TH
SYNC/FCB
8
7
6
5
L1 15µH
C
OUT
+
100µF
6.3V
V
OUT
3.3V
249k
V
IN
2.8V*
TO 8.5V
C
IN
22µF
16V
EFFICIENCY (%)
100
V
OUT
= 3.3V
95
V
IN
= 6V
90
85
80
75
70
V
IN
= 8.4V
V
IN
= 3.6V
RUN/SS
V
DR
LTC1627
V
FB
GND
V
IN
SW
+
*V
OUT
CONNECTED TO V
IN
FOR 2.8V < V
IN
< 3.3V
80.6k
1627 F01a
1
Figure 1a. High Efficiency Step-Down Converter
Figure 1b. Efficiency vs Output Load Current
U
U
U
10
100
OUTPUT CURRENT (mA)
1000
1627 F01b
1
LTC1627
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
I
TH
1
RUN/SS 2
V
FB
3
GND 4
8
7
6
5
SYNC/FCB
V
DR
V
IN
SW
Input Supply Voltage ................................ – 0.3V to 10V
Driver Supply Voltage (V
IN
– V
DR
) ........... – 0.3V to 10V
I
TH
Voltage .................................................. – 0.3V to 5V
Run/SS, V
FB
Voltages ................................ – 0.3V to V
IN
Sync/FCB Voltage ...................................... – 0.3V to V
IN
V
DR
Voltage (V
IN
≤
5V) ............................... – 5V to 0.3V
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
LTC1627CS8
LTC1627IS8
S8 PART MARKING
1627
1627I
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 110°C/ W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
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
Synchronized
Burst Mode Operation
Shutdown
Shutdown
Run/SS Threshold
Soft-Start Current Source
Auxiliary Feedback Threshold
Auxiliary Feedback Current
Oscillator Frequency
Undervoltage Lockout
R
DS(ON)
of P-Channel FET
R
DS(ON)
of N-Channel FET
SW Leakage
V
RUN/SS
= 0V
CONDITIONS
(Note 3)
(Note 3)
T
A
= 25°C, V
IN
= 5V unless otherwise specified.
MIN
q
TYP
20
0.80
60
0.002
0.5
– 0.5
450
200
15
6
MAX
60
0.812
110
0.01
0.8
– 0.8
UNITS
nA
V
mV
%/V
%
%
µA
µA
µA
µA
V
µA
V
µA
kHz
kHz
V
V
Ω
Ω
nA
0.788
20
∆V
OVL
= V
OVL
– V
FB
V
IN
= 2.8V 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, Frequency = 525kHz
V
ITH
= 0V, V
IN
= 8.5V, V
SYNC/FCB
= Open
V
RUN/SS
= 0V, 2.65V < V
IN
< 8.5V
V
RUN/SS
= 0V, V
IN
< 2.65V
320
35
1.0
3.3
0.835
2.5
385
2.65
2.80
0.7
0.8
±1000
V
RUN/SS
I
RUN/SS
V
SYNC/FCB
I
SYNC/FCB
f
OSC
V
UVLO
R
PFET
R
NFET
I
LSW
0.4
1.2
0.755
0.5
315
q
q
0.7
2.25
0.8
1.5
350
35
2.50
2.65
0.5
0.6
±10
V
SYNC/FCB
Ramping Negative
V
SYNC/FCB
= 0V
V
FB
= 0.8V
V
FB
= 0V
V
IN
Ramping Down from 3V
V
IN
Ramping Up from 0V
(V
IN
– V
DR
) = 5V, I
SW
= 100mA
I
SW
= – 100mA
V
RUN/SS
= 0V
2.4
The
q
denotes specifications which apply over the full operating
temperature range.
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 LTC1627 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
LTC1627
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Input Voltage
100
100
95
95
I
LOAD
= 100mA
I
LOAD
= 300mA
I
LOAD
= 10mA
EFFICIENCY (%)
EFFICIENCY (%)
90
V
DR
= – V
IN
85
80
75
70
V
IN
= 3.6V
V
OUT
= 2.5V
L = 15µH
Burst Mode OPERATION
1
10
100
OUTPUT CURRENT (mA)
1000
1627 G02
EFFICIENCY (%)
85
80
V
OUT
= 2.5V
L = 15µH
V
DR
= 0V
Burst Mode OPERATION
0
2
6
4
INPUT VOLTAGE (V)
8
10
1627 G01
75
Efficiency vs Load Current
100
V
IN
= 2.8V
95
EFFICIENCY (%)
90
85
V
IN
= 7.2V
80
75
70
1
10
100
OUTPUT CURRENT (mA)
1000
1627 G04
UNDERVOLTAGE LOCKOUT THRESHOLD (V)
BATTERY VOLTAGE (V)
V
IN
= 3.6V
V
OUT
= 2.5V
L = 15µH
V
DR
= 0V
Burst Mode OPERATION
Supply Current in Shutdown
vs Input Voltage
22
SUPPLY CURRENT IN SHUTDOWN (µA)
V
RUN/SS
= 0V
T
J
= 85°C
T
J
= 25°C
0.808
0.806
FORCED CONTINUOUS THRESHOLD VOLTAGE (V)
20
16
14
12
10
8
6
4
2.5
3.5
REFERENCE VOLTAGE (V)
18
T
J
= – 40°C
4.5
5.5
6.5
INPUT VOLTAGE (V)
U W
7.5
1627 G07
Efficiency vs Load Current
100
95
V
DR
= 0V
90
90
85
Efficiency vs Load Current
V
IN
= 3.6V
V
OUT
= 2.5V
L = 15µH
V
DR
= 0V
Burst Mode
OPERATION
SYNCHRONIZED
AT 525kHz
80
FORCED
CONTINUOUS
75
70
1
10
100
OUTPUT CURRENT (mA)
1000
1627 G03
Undervoltage Lockout Threshold
vs Temperature
2.75
2.70
2.65
2.60
2.55
2.50
2.45
2.40
2.35
2.30
– 50
– 25
0
25
50
75
TEMPERATURE (°C)
100
125
V
IN
RAMPING DOWN
V
IN
RAMPING UP
550
500
450
400
350
300
250
200
DC Supply Current*
vs Input Voltage
T
J
= 25°C
V
OUT
= 1.8V
SYNCHRONIZED AT 525kHz
Burst Mode OPERATION
150
100
2.5
3.5
4.5
5.5
6.5
INPUT VOLTAGE (V)
7.5
8.5
1627 G05
1627 G06
*DOES NOT INCLUDE GATE CHARGE CURRENT
Reference Voltage
vs Temperature
V
IN
= 5V
Forced Continuous Threshold
Voltage vs Temperature
0.808
0.806
0.804
0.802
0.800
0.798
0.796
0.794
0.792
0.790
– 50
– 25
0
25
50
75
TEMPERATURE (°C)
100
125
V
IN
= 5V
0.804
0.802
0.800
0.798
0.796
0.794
0.792
8.5
0.790
– 50
– 25
0
25
50
75
TEMPERATURE (°C)
100
125
1627 G08
1627 G09
3
LTC1627
TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
vs Temperature
390
380
V
IN
= 5V
V
SYNC/FCB
= 0V
390
380
MAXIMUM OUTPUT LOAD CURRENT (mA)
OSCILLATOR FREQUENCY (kHz)
OSCILLATOR FREQUENCY (kHz)
370
360
350
340
330
320
310
300
– 50
– 25
0
25
50
75
TEMPERATURE (°C)
100
125
Switch Leakage Current
vs Temperature
1800
SYNCHRONOUS SWITCH LEAKAGE (nA)
1600
1400
1200
1000
800
600
400
200
V
IN
= 8.4V
V
DR
= 0V
SWITCH RESISTANCE (Ω)
SWITCH RESISTANCE (Ω)
SYNCHRONOUS
SWITCH
0
– 50
– 25
0
25
50
75
TEMPERATURE (°C)
Load Step Transient Response
I
TH
0.5V/DIV
I
TH
0.5V/DIV
V
OUT
50mV/DIV
AC COUPLED
I
LOAD
500mA/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
1627 G16
4
U W
1627 G10
Oscillator Frequency
vs Input Voltage
V
SYNC/FCB
= 0V
Maximum Output Load Current
vs Input Voltage
1100
1000
900
800
700
600
500
400
300
200
2.5
3.5
4.5
5.5
6.5
INPUT VOLTAGE (V)
V
OUT
= 2.5V
L = 15µH
7.5
8.5
V
DR
= 0V
V
DR
= –V
IN
370
360
350
340
330
320
310
300
2.5
3.5
4.5
5.5
6.5
INPUT VOLTAGE (V)
7.5
8.5
1627 G11
1627 G12
Switch Resistance
vs Temperature
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
– 50
– 25
0
25
50
75
TEMPERATURE (°C)
100
125
MAIN
SWITCH
V
IN
= 5V
V
DR
= 0V
SYNCHRONOUS
SWITCH
0.9
0.8
0.7
Switch Resistance
vs Input Voltage
V
DR
= 0V
SYNCHRONOUS SWITCH
0.6
0.5
MAIN SWITCH
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
100
125
1627 G13
1627 G14
1627 G15
Load Step Transient Response
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
FORCED CONTINUOUS MODE
1627 G17
Burst Mode Operation
V
OUT
50mV/DIV
AC COUPLED
I
LOAD
500mA/DIV
10µs/DIV
V
IN
= 5V
V
OUT
= 3.3V
L = 15µH
C
IN
= 22µF
C
OUT
= 100µF
I
LOAD
= 50mA
1627 G18
LTC1627
PIN FUNCTIONS
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 all the
circuitry.
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.
V
DR
(Pin 7):
Top Driver Return Pin. This pin can be
bootstrapped to go below ground to improve efficiency at
low V
IN
(see Applications Information).
SYNC/FCB (Pin 8):
Multifunction Pin. This pin performs
three functions: 1) secondary winding feedback input, 2)
external clock synchronization and 3) Burst Mode opera-
tion or forced continuous mode select. For secondary
winding applications connect a resistive divider from the
secondary output. To synchronize with an external clock
apply a TTL/CMOS compatible clock with a frequency
between 385kHz and 525kHz. To select Burst Mode opera-
tion, float the pin or tie it to V
IN
. Grounding Pin 8 forces
continuous operation (see Applications Information).
FUNCTIONAL DIAGRA
BURST
DEFEAT
X
1.5µA
SYNC/FCB
8
Y
Y = “0” ONLY WHEN X IS A CONSTANT “1”
V
IN
SLOPE
COMP
OSC
0.4V
0.6V
V
FB
3
V
IN
0.8V
+
EA
0.12V
I
TH
1
S
RUN/SOFT
START
R
Q
Q
–
0.8V
REF
2.25µA
V
IN
RUN/SS 2
UVLO
TRIP = 2.5V
+
OVDET
0.86V
SHUTDOWN
0.8V
–
+
I
RCMP
5 SW
4 GND
1627 BD
FCB
+
–
+
FREQ
SHIFT
–
W
+
–
U
U
U
+
–
U
U
V
IN
V
IN
6 V
IN
EN
SLEEP
–
+
6Ω
I
COMP
BURST
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTI-
SHOOT-THRU
7 V
DR
–
5
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