LTC1773 Datasheet, PDF (1/20 Pages) Linear Technology – Synchronous Step-Down DC/DC Controller

FEATURES
■ _{High Efficiency: Up to 95%}
■ _{Constant Frequency 550kHz Operation}
■ _{VIN from 2.65V to 8.5V}
■ _{VOUT from 0.8V to VIN}
■ _OPTI-LOOP^® _{Compensation Minimizes COUT}
■ _{Synchronizable up to 750kHz}
■ _{Selectable Burst Mode Operation}
■ _{Low Quiescent Current: 80µA}
■ _{Low Dropout Operation: 100% Duty Cycle}
■ _{Secondary Winding Regulation}
■ _Soft-Start
■ _{Current Mode Operation for Excellent Line and}
Load Transient Response
■ _{Low Shutdown IQ = 10µA}
■ _{±1.5% Reference Accuracy}
■ _{Precision 2.5V Undervoltage Lockout}
■ _{Available in 10-Lead MSOP}
U
APPLICATIO S
■ _{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}
LTC1773
Synchronous Step-Down
DC/DC Controller
DESCRIPTIO
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.
TYPICAL APPLICATIO
V_IN
2.65V TO 8.5V
SYNC/FCB VIN
RUN/SS SENSE^–
0.1µF
I_TH
TG
47pF
30k
LTC1773
SW
220pF
V_{FB GND} BG
80.6k
R_SENSE
0.025Ω
L1
3µH
Si9801DY
169k
Figure 1. Step-Down Converter
+
C_IN
68µF
V_OUT
2.5V
+ _COUT
180µF
1773 F01
100
95
90
85
80
75
70
65
60
55
1
High Efficiency
V_IN = 3.3V
V_IN = 5V
V_IN = 8V
L = SUMIDA CDRH6D28-3R0
10
100
1000 5000
OUTPUT CURRENT (mA)
1773 F1b
1773fb
1