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
LTC1773
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
I
TH
RUN/SS
SYNC/FCB
V
FB
GND
1
2
3
4
5
10
9
8
7
6
SW
SENSE
–
V
IN
TG
BG
Input Supply Voltage .............................. –0.3V to 10.0V
I
TH
Voltage ................................................ –0.3V to 2.5V
RUN/SS, V
FB,
SENSE
–
Voltages .................. –0.3V to V
IN
SYNC/FCB Voltage ...................................... –0.3V to V
IN
BG, TG Voltages...........................................–0.3V to V
IN
SW Voltage ...................................................– 5V to 11V
Operating Ambient Temperature Range
(Note 2) ...............................................–40°C to 85°C
Junction Temperature (Note 3) ............................. 125°C
Storage Temperature Range ..................–65°C to 150°C
Lead Temperature (Soldering, 10 sec.)................. 300°C
ORDER PART
NUMBER
LTC1773EMS
MS PART MARKING
LTMV
MS PACKAGE
10-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 120°C/W
Order Options
Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking:
http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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
Normal Mode
Burst Mode Operation
Shutdown
Shutdown
V
RUN/SS
I
RUN/SS
V
SYNC/FCB
I
SYNC/FCB
f
OSC
V
UVLO
∆
VSENSE(MAX
)
TG t
r
TG t
f
BG t
r
BG t
f
Top Gate Drive Rise Time
Top Gate Drive Fall Time
Bottom Gate Drive Rise Time
Bottom Gate Drive Fall Time
RUN/SS Threshold
Soft-Start Current Source
Auxiliary Feedback Threshold
SYNC/FCB Pull-Up Current
Oscillator Frequency
Undervoltage Lockout
The
●
denotes specifications which apply over the full operating
temperature range, T
A
= 25
°
C. V
IN
= 5V unless otherwise specified.
CONDITIONS
(Note 4)
(Note 4)
∆V
OVL
= V
OVL
– V
FB
V
IN
= 2.7V to 8.5V (Note 4)
I
TH
at 1.0V (Note 4)
I
TH
at 0.6V (Note 4)
(Note 5)
V
IN
= 5V, V
ITH
= OPEN, V
SYNC/MODE
= OPEN
V
ITH
= 0V, V
IN
= 5V, V
SYNC/MODE
= OPEN
V
RUN/SS
= 0V, 2.7V < V
IN
< 8.5V
V
RUN/SS
= 0V, V
IN
< 2.4V
0.4
V
RUN/SS
= 0V
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
Maximum Current Sense Voltage
C
LOAD
= 3000pF (Note 6)
C
LOAD
= 3000pF (Note 6)
C
LOAD
= 3000pF (Note 6)
C
LOAD
= 3000pF (Note 6)
●
●
●
●
MIN
0.788
40
TYP
20
0.80
60
0.002
0.2
–0.2
400
80
10
2
0.7
1.5
0.8
0.4
550
55
2.5
2.65
100
45
48
80
45
MAX
60
0.812
80
0.02
0.8
–0.8
600
30
5
1.0
2.5
0.84
1.0
600
2.65
2.8
115
160
150
180
150
UNITS
nA
V
mV
%/V
%
%
µA
µA
µA
µA
V
µA
V
µA
kHz
kHz
V
V
mV
ns
ns
ns
ns
0.75
0.76
0.1
500
2.35
85
1773fb
2
U
W
U
U
W W
W
LTC1773
ELECTRICAL CHARACTERISTICS
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
The LTC1773 is guaranteed to meet performance specifications from
0°C to 70°C. Specifications over the –40°C to 85°C operating temperature
range are assured by design, characterization and correlation with statistical
process controls.
Note 3:
T
J
is calculated from the ambient temperature T
A
and power dissipation
P
D
according to the following formula:
LTC1773: T
J
= T
A
+ (P
D
• 120°C/W)
Note 4:
The LTC1773 is tested in a feedback loop which servos V
FB
to the
balance point for the error amplifier (V
ITH
= 0.8V)
Note 5:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 6:
Rise and fall times are measured using 10% and 90% levels.
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
V
IN
= 5V
100
V
OUT
= 2.5V
SEE FIGURE 1
90
100
90
EFFICIENCY (%)
EFFICIENCY (%)
V
IN
= 5V
80
70
60
V
IN
= 8V
EFFICIENCY (%)
80
70
60
50
1
Burst Mode
OPERATION
SYNC(750kHz)
FORCED CONTINUOUS
10
100
1000
OUTPUT CURRENT (mA)
5000
Load Regulation
0
R
SENSE
= 0.025Ω
SEE FIGURE 1
– 0.05
NORMALIZED V
OUT
(%)
INPUT CURRENT (µA)
V
IN
-V
OUT
(mV)
– 0.10
– 0.15
– 0.20
– 0.25
– 0.30
0
0.5
2.5
1.0 1.5 2.0
LOAD CURRENT (A)
U W
3.0
Efficiency vs Load Current
100
V
OUT
= 2.5V
SEE FIGURE 1
V
IN
= 3.3V
Efficiency vs Input Voltage
V
OUT
= 2.5V
SEE FIGURE 1
95
I
OUT
= 1A
90
85
I
OUT
= 100mA
80
50
1
10
100
1000
OUTPUT CURRENT (mA)
10,000
1773 G02
75
2
4
6
8
INPUT VOLTAGE (V)
10
1773 G03
1773 G01
V
IN
-V
OUT
Dropout Voltage
vs Load Current
400
R
SENSE
= 0.025Ω
350 V
OUT
= 1.8V
Si9801DY
300
250
200
150
100
50
0
3.5
Input and Shutdown Currents
vs Input Voltage
500
450
400
350
300
250
200
150
100
50
0
SHUTDOWN
2
4
8
6
INPUT VOLTAGE (V)
10
1773 G06
SYNC TO 750kHz
V
OUT
= 1.8V
Si9801DY
R
SENSE
= 0.025Ω
L = CDRH6D28-3RO
Burst Mode OPERATION
0
500
1500
2000
1000
LOAD CURRENT (mA)
2500
1773 G05
1773 G16
1773fb
3
LTC1773
TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Current Sense
Threshold vs V
RUN/SS
MAXIMUM CURRENT SENSE THRESHOLD (mV)
MAXIMUM CURRENT SENSE THRESHOLD (mV)
100
80
60
40
20
0
0.5
MAXIMUM CURRENT SENSE THRESHOLD (mV)
120
1.0
1.5
2.0 2.5 3.0
V
RUN/SS
(V)
Oscillator Frequency vs
Temperature
560
2.5
RUN/SS CURRENT (µA)
550
FREQUENCY (kHz)
540
530
520
– 60 –40 –20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
1773 G10
Start-Up
V
OUT
2V/DIV
SEE FIGURE 9
V
OUT
100mV/DIV
V
RUN/SS
1V/DIV
I
L
2A/DIV
V
IN
= 5V
V
OUT
= 2.5V
40ms/DIV
4
U W
3.5
4.0
1773 G07
Maximum Current Sense
Threshold vs Temperature
105
Maximum Current Sense
Threshold vs V
ITH
120
100
80
60
Burst Mode
OPERATION
40
20
0
100
95
FORCED
CONTINUOUS
0
0.2
0.4
0.6 0.8 1.0
V
ITH
(V)
1.2
1.4
1.6
85
– 60 – 40 – 20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
1773 G08
1773 G09
RUN/SS Pin Current vs
Temperature
Burst Mode Operation
I
LOAD
= 100mA
SEE FIGURE 9
V
OUT
20mV/DIV
2.0
1.5
1.0
I
L
500mA/DIV
0.5
– 60 –40 –20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
1773 G11
V
IN
= 5V
V
OUT
= 2.5V
20µs/DIV
1773 G12
Load Step (Burst Mode
Operation)
SEE FIGURE 9
Load Step (Continuous
Mode)
SEE FIGURE 9
V
OUT
100mV/DIV
I
L
2A/DIV
I
L
2A/DIV
1773 G13
V
IN
= 5V
100µs/DIV
V
OUT
= 2.5V
100mA TO 5A LOAD STEP
1773 G14
100µs/DIV
V
IN
= 5V
V
OUT
= 2.5V
100mA TO 5A LOAD STEP
1773 G15
1773fb
LTC1773
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.
Under high duty cycle and nearing current limit, I
TH
can
swing up to 2.4V.
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
0.8s/µF. Forcing this pin below 0.4V shuts down all the
circuitry.
SYNC/FCB (Pin 3):
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 to a resistive divider from
the secondary output. To synchronize with an external
clock, apply a TTL/CMOS compatible clock with a fre-
quency between 585kHz and 750kHz. To select Burst
Mode operation, tie SYNC/FCB to V
IN
. Grounding this pin
forces continuous operation.
V
FB
(Pin 4):
Feedback Pin. Receives the feedback voltage
from an external resistive divider across the output. Do not
use more than 0.01µF of feedforward capacitance from FB
to the output.
GND (Pin 5):
Ground Pin.
BG (Pin 6):
Bottom Gate Driver of External N-Channel
Power MOSFET. This pin swings from 0V to V
IN
.
TG (Pin 7):
Top Gate Driver of External P-Channel Power
MOSFET. This pin swings from 0V to V
IN
.
V
IN
(Pin 8) :
Main Supply Pin. Must be closely decoupled
to GND (pin 5).
SENSE
–
(Pin 9):
The Negative Input to the Current Com-
parator. A sense resistor between this pin and V
IN
sets the
peak current in the top switch. Connect this pin to the
source of the external P-Channel power MOSFET.
SW (Pin 10):
Switch Node Connection to Inductor. This
pin connects to the drains of the external main and
synchronous power MOSFET switches.
FUNCTIONAL DIAGRA
0.4µA
BURST
DEFEAT
X
SYNC/FCB 3
–
SYNC
DEFEAT
V
FB
4
FREQ
SHIFT
1.5µA
0.8V REF
2
RUN/SS
+
EA
0.22V
I
TH
1
RUN/
RUN/SOFT
SOFT-START
START
S
R
–
Q
Q
UVLO
TRIP = 2.5V
–
OVDET
0.86V
SHUTDOWN
+
+
0.8V
–
FCB
+
Figure 2.
–
+
+
0.8V
–
0.6V
W
U
U
U
U
U
Y
Y = “0” ONLY WHEN X IS A CONSTANT “1”
SLOPE
COMP
OSC
0.4V
8 V
IN
9 SENSE
–
EN
–
50mV
–
SLEEP
I
COMP
+
+
BURST
COMP
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
7 TG
ANTI
SHOOT-THRU
6 BG
10 SW
I
RCMP
5 GND
1773 FD
1773fb
5
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