LTC1773
Synchronous Step-Down
DC/DC Controller
FEATURES
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DESCRIPTIO
High Efficiency: Up to 95%
Constant Frequency 550kHz Operation
V
IN
from 2.65V to 8.5V
V
OUT
from 0.8V to V
IN
OPTI-LOOP
®
Compensation Minimizes C
OUT
Synchronizable up to 750kHz
Selectable Burst Mode Operation
Low Quiescent Current: 80µA
µ
Low Dropout Operation: 100% Duty Cycle
Secondary Winding Regulation
Soft-Start
Current Mode Operation for Excellent Line and
Load Transient Response
Low Shutdown I
Q
= 10µA
±1.5%
Reference Accuracy
Precision 2.5V Undervoltage Lockout
Available in 10-Lead MSOP
The LTC
®
1773 is a current mode synchronous buck regu-
lator controller that drives external complementary power
MOSFETs using a fixed frequency architecture. The oper-
ating supply range is from 2.65V to 8.5V, making it
suitable for 1- or 2-cell 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-oper-
ated systems.
The operating frequency is internally set at 550kHz, allow-
ing the use of small surface mount inductors. For switch-
ing-noise sensitive applications, it can be synchronized up
to 750kHz. Peak current limit is user programmable with
an external high side sense resistor. A SYNC/FCB control
pin guarantees regulation of secondary windings regard-
less of load on the main output by forcing continuous
operation. Burst Mode operation is inhibited during syn-
chronization or when the SYNC/FCB pin is pulled low to
reduce noise and RF interference. Soft-start is provided by
an external capacitor.
Synchronous rectification increases efficiency and elimi-
nates the need for a Schottky diode, saving components
and board space. The LTC1773 comes in a 10-lead MSOP
package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
OPTI-LOOP and Burst Mode are registered trademarks of Linear Technology Corporation.
Protected by U.S. Patents, including 5481178, 6580258, 6304066, 6127815.
APPLICATIO S
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Cellular Telephones
RF PA Supplies
Portable Instruments
Wireless MODEMS
Distributed Power Systems
Notebook and Palm Top Computers, PDAs
Single and Dual Cell Lithium-Ion Powered Devices
TYPICAL APPLICATIO
SYNC/FCB
V
IN
2.65V TO 8.5V
100
+
V
IN
R
SENSE
0.025Ω
L1
3µH
C
IN
68µF
EFFICIENCY (%)
95
90
85
80
75
70
65
60
55
RUN/SS SENSE
–
0.1µF
47pF
220pF
30k
V
FB
I
TH
LTC1773
SW
GND
BG
Si9801DY
80.6k
169k
1773 F01
TG
V
OUT
2.5V
+
C
OUT
180µF
1
Figure 1. Step-Down Converter
U
High Efficiency
V
IN
= 3.3V
V
IN
= 5V
V
IN
= 8V
L = SUMIDA CDRH6D28-3R0
100
1000
10
OUTPUT CURRENT (mA)
5000
1773 F1b
U
U
1773fb
1
LINER [ LINEAR TECHNOLOGY ]
