LTC1778/LTC1778-1
Wide Operating Range,
No R
SENSE
TM
Step-Down Controller
FEATURES
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
DESCRIPTIO
No Sense Resistor Required
True Current Mode Control
Optimized for High Step-Down Ratios
t
ON(MIN)
≤
100ns
Extremely Fast Transient Response
Stable with Ceramic C
OUT
Dual N-Channel MOSFET Synchronous Drive
Power Good Output Voltage Monitor (LTC1778)
Adjustable On-Time (LTC1778-1)
Wide V
IN
Range: 4V to 36V
±1%
0.8V Voltage Reference
Adjustable Current Limit
Adjustable Switching Frequency
Programmable Soft-Start
Output Overvoltage Protection
Optional Short-Circuit Shutdown Timer
Micropower Shutdown: I
Q
< 30µA
Available in a 16-Pin Narrow SSOP Package
The LTC
®
1778 is a synchronous step-down switching
regulator controller optimized for CPU power. The con-
troller uses a valley current control architecture to deliver
very low duty cycles with excellent transient response
without requiring a sense resistor. Operating frequency is
selected by an external resistor and is compensated for
variations in V
IN
.
Discontinuous mode operation provides high efficiency
operation at light loads. A forced continuous control pin
reduces noise and RF interference, and can assist second-
ary winding regulation by disabling discontinuous opera-
tion when the main output is lightly loaded.
Fault protection is provided by internal foldback current
limiting, an output overvoltage comparator and optional
short-circuit shutdown timer. Soft-start capability for sup-
ply sequencing is accomplished using an external timing
capacitor. The regulator current limit level is user program-
mable. Wide supply range allows operation from 4V to 36V
at the input and from 0.8V up to (0.9)V
IN
at the output.
, LTC and LT are registered trademarks of Linear Technology Corporation.
No R
SENSE
is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 5481178, 6100678, 6580258, 5847554, 6304066
APPLICATIO S
■
■
Notebook and Palmtop Computers
Distributed Power Systems
TYPICAL APPLICATIO
R
ON
1.4MΩ
C
SS
0.1µF
RUN/SS
C
C
500pF
I
TH
R
C
20k
SGND
LTC1778
INTV
CC
BG
I
ON
V
IN
TG
SW
BOOST
M1
Si4884
C
B
0.22µF
D
B
CMDSH-3
M2
Si4874
C
VCC
4.7µF
L1
1.8µH
EFFICIENCY (%)
C
IN
10µF
50V
×3
V
IN
5V TO 28V
90
+
D1
B340A
R2
30.1k
C
OUT
180µF
4V
×2
V
OUT
2.5V
10A
+
PGOOD
PGND
V
FB
R1
14k
1778 F01a
Figure 1. High Efficiency Step-Down Converter
1778fb
U
Efficiency vs Load Current
100
V
OUT
= 2.5V
V
IN
= 5V
V
IN
= 25V
80
70
60
0.01
1
0.1
LOAD CURRENT (A)
10
1778 F01b
U
U
1
LINER [ LINEAR TECHNOLOGY ]
