Electrical Specifications Subject to Change
LTC1779
250mA Current Mode
Step-Down DC/DC Converter
in SOT-23
June 2000
FEATURES
s
s
s
s
s
s
s
s
s
DESCRIPTIO
s
s
s
High Efficiency: Up to 94%
250mA Output Current
Wide V
IN
Range: 2.5V to 9.8V
550kHz Constant Frequency Operation
Burst Mode
TM
Operation at Light Load
Low Dropout: 100% Duty Cycle
0.8V Reference Allows Low Output Voltages
±2.5%
Reference Accuracy
Current Mode Operation for Excellent Line and Load
Transient Response
Low Quiescent Current: 110µA
Shutdown Mode Draws Only 8µA Supply Current
Tiny 6-Lead SOT-23 Package
The LTC
®
1779 is a constant frequency current mode step-
down DC/DC converter providing excellent AC and DC load
and line regulation. The device incorporates an accurate
undervoltage lockout feature that shuts down the LTC1779
when the input voltage falls below 2.0V.
The LTC1779 boasts a
±2.5%
output voltage accuracy and
consumes only 110µA of quiescent current. For applica-
tions where efficiency is a prime consideration, the LTC1779
is configured for Burst Mode operation, which enhances
efficiency at low output current.
To further maximize the life of a battery source, the
internal P-channel MOSFET is turned on continuously in
dropout (100% duty cycle). In shutdown, the device draws
a mere 8µA. High constant operating frequency of 550kHz
allows the use of a small external inductor.
The LTC1779 is available in a small footprint 6-lead
SOT-23.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
APPLICATIO S
s
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One or Two Lithium-Ion-Powered Applications
Cellular Telephones
Wireless Modems
Portable Computers
Distributed 3.3V, 2.5V or 1.8V Power Systems
Scanners
TYPICAL APPLICATIO
C3
0.1µF
1
20k
100pF
3
V
FB
SENSE
–
4
I
TH
/RUN
LTC1779
GND
V
IN
SW
6
D1
R1
10Ω
+
L1
22µH
C1
10µF
16V
C2
47µF
6V
V
IN
2.5V
TO 9.8V
V
OUT
2.5V
100mA
100
90
V
IN
= 3.3V
80
EFFICIENCY (%)
+
5
169k
2
70
V
IN
= 9.8V
60
50
78.7k
1779 F01a
C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT
C2: SANYO POSCAP 6TPA47M
D1: IR10BQ015
L1: COILTRONICS UP1B220
40
30
0.1
Figure 1. LTC1779 High Efficiency 2.5V/100mA Step-Down Converter
Specifications on this data sheet are preliminary only, and subject to change
without notice. Contact the manufacturer before finalizing a design using this part.
U
Efficiency vs Load Current
V
IN
= 6V
V
OUT
= 2.5V
R
SENSE
= 10Ω
1
10
100
LOAD CURRENT (mA)
1000
1779 F01b
U
U
1
LTC1779
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
I
TH
/RUN 1
GND 2
V
FB
3
6 SW
5 V
IN
4 SENSE
–
Input Supply Voltage (V
IN
).........................– 0.3V to 10V
SENSE
–
, SW Voltages .................. – 0.3V to (V
IN
+ 0.3V)
V
FB
, I
TH
/RUN Voltages ..............................– 0.3V to 2.4V
SW Peak Output Current (< 10µs) .......................... 0.5A
Storage Ambient Temperature Range ... – 65°C to 150°C
Operating Temperature Range (Note 2) ....... 0°C to 70°C
Junction Temperature (Note 3) ............................. 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC1779CS6
S6 PART MARKING
LTLP
S6 PACKAGE
6-LEAD PLASTIC SOT-23
T
JMAX
= 150°C,
θ
JA
= 230°C/ W
Consult factory for Industrial and Military grade parts.
The
q
denotes specifications that apply over the full operating temperature
range, otherwise specifications are at T
A
= 25°C. V
IN
= 4.2V unless otherwise specified. (Note 2)
PARAMETER
Input DC Supply Current
Normal Operation
Shutdown
UVLO
Undervoltage Lockout Threshold
Shutdown Threshold (at I
TH
/RUN)
Start-Up Current Source
Regulated Feedback Voltage
Output Voltage Line Regulation
Output Voltage Load Regulation
V
FB
Input Current
Overvoltage Protect Threshold
Overvoltage Protect Hysteresis
Overtemperature Protect Threshold
Overtemperature Protect Hysteresis
Oscillator Frequency
R
DS(ON)
of Internal P-Channel FET
Maximum Current Sense Voltage
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC1779 is guaranteed to meet specified performance over
the 0°C to 70°C operating temperature range.
Note 3:
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
•
θ
J
°C/W)
V
FB
= 0.8V
V
FB
= 0V
V
IN
= 4.2V, I
SW
= 100mA
TBD
V
ITH
/RUN = 0V
(Note 5)
2.4V
≤
V
IN
≤
9.8V (Note 5)
I
TH
/RUN Sinking 5µA (Note 5)
I
TH
/RUN Sourcing 5µA (Note 5)
(Note 5)
Measured at V
FB
TBD
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
Typicals at V
IN
= 4.2V (Note 4)
2.4V
≤
V
IN
≤
9.8V
2.4V
≤
V
IN
≤
9.8V, V
ITH
/RUN = 0V
V
IN
< UVLO Threshold
V
IN
Falling
V
IN
Rising
q
q
MIN
TYP
110
8
7
MAX
TBD
TBD
TBD
TBD
TBD
TBD
TBD
TBD
UNITS
µA
µA
µA
V
V
V
µA
V
mV/V
mV/µA
mV/µA
TBD
TBD
TBD
TBD
TBD
2.0
2.1
0.35
0.5
0.800
0.12
4.2
4.2
5
0.860
20
170
15
550
90
0.6
120
TBD
TBD
TBD
TBD
Note 4:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5:
The LTC1779 is tested in a feedback loop that servos V
FB
to the
output of the error amplifier.
2
U
nA
V
mV
°C
°C
kHz
kHz
Ω
mV
W
U
U
W W
W
LTC1779
TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage
vs Temperature
825
820
815
V
IN
= 4.2V
15
12
NORMALIZED FREQUENCY (%)
V
FB
VOLTAGE (mV)
805
800
795
790
785
780
775
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1779 G01
TRIP VOLTAGE (V)
810
Shutdown Threshold
vs Temperature
600
560
520
V
IN
= 4.2V
1.85
1.70
1.55
1.40
I
TH
/RUN VOLTAGE (mV)
480
440
400
360
320
280
240
200
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1779 G04
R
DS(ON)
(Ω)
1.25
1.10
0.95
0.80
0.65
0.50
T
A
= 125°C
R
DS(ON)
(Ω)
PI FU CTIO S
I
TH
/RUN (Pin 1):
This pin performs two functions. It
serves as the error amplifier compensation point as well as
the run control input. The current comparator threshold
increases with this control voltage. Nominal voltage range
for this pin is 0.7V to 1.9V. Forcing this pin below 0.35V
causes the device to be shut down. In shutdown all
functions are disabled and the internal P-channel MOSFET
is turned off. The SW pin will be high impedance.
GND (Pin 2):
Ground Pin.
V
FB
(Pin 3):
Receives the feedback voltage from an exter-
nal resistive divider across the output.
SENSE
–
(Pin 4):
The Negative Input to the Current Com-
parator. Can be connected to V
IN
for default minimum
peak current of 250mA. Connecting a resistor between
SENSE
–
and V
IN
specifies a lower peak current. (See
Applications Information for specifying resistor value.)
V
IN
(Pin 5):
Supply Pin. Must be closely decoupled to GND
Pin 2.
SW (Pin 6):
Switching Node and Drain of Internal
P-Channel Power MOSFET. Connects to external induc-
tor and catch diode.
U W
Normalized Oscillator Frequency
vs Temperature
V
IN
= 4.2V
2.20
2.16
2.12
2.08
2.04
2.00
1.96
1.92
1.88
1.84
5 25 45 65 85 105 125
TEMPERATURE (°C)
1779 G02
Undervoltage Lockout Trip
Voltage vs Temperature
V
IN
FALLING
9
6
3
0
–3
–6
–9
–12
–15
–55 –35 –15
1.80
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1779 G03
R
DS(ON)
of Internal P-Channel FET
vs Input Voltage
1.85
I
SW
= 100mA
SENSE
–
= V
IN
R
DS(ON)
of Internal P-Channel FET
vs Temperature
I
SW
= 100mA
1.70 SENSE
–
= V
IN
1.55
1.40
1.25
1.10
0.95
0.80
0.65
0.50
V
IN
= 9.8V
V
IN
= 8.4V
V
IN
= 6V
V
IN
= 2.4V
V
IN
= 4.2V
T
A
= 25°C
T
A
= –55°C
0.35
2
3
4
5
7
8
6
INPUT VOLTAGE (V)
9
10
0.35
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1779 G06
1779 G05
U
U
U
3
LTC1779
FU CTIO AL DIAGRA
+
ICMP
OVERTEMP
DETECT
2Ω
RS1
OSC
SLOPE
COMP
R
Q
S
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
V
IN
1×
24×
SW
6
FREQ
FOLDBACK
SHORT-CIRCUIT
DETECT
0.15V
V
IN
EAMP
0.5µA
–
+
V
IN
0.3V
–
VOLTAGE
REFERENCE
GND
2
UNDERVOLTAGE
LOCKOUT
V
REF
0.8V
OPERATIO
(Refer to Functional Diagram)
Main Control Loop
The LTC1779 is a constant frequency current mode switch-
ing regulator. During normal operation, the internal
P-channel power MOSFET is turned on each cycle when
the oscillator sets the RS latch (RS1) and turned off when
the current comparator (ICMP) resets the latch. The peak
inductor current at which ICMP resets the RS latch is
controlled by the voltage on the I
TH
/RUN pin, which is the
output of the error amplifier EAMP. An external resistive
divider connected between V
OUT
and ground allows the
EAMP to receive an output feedback voltage V
FB
. When the
4
–
+
W
SENSE
–
4
V
IN
5
0.3V
U
U
U
+
–
BURST
CMP
SLEEP
OVP
+
–
V
REF
+
60mV
+
–
V
REF
0.8V
V
FB
3
V
IN
1 I
TH
/RUN
0.35V
+
SHDN
CMP
SHDN
UV
–
1.2V
1779FD
load current increases, it causes a slight decrease in V
FB
relative to the 0.8V reference, which in turn causes the
I
TH
/RUN voltage to increase until the average inductor
current matches the new load current.
The main control loop is shut down by pulling the I
TH
/RUN
pin low. Releasing I
TH
/RUN allows an internal 0.5µA
current source to charge up the external compensation
network. When the I
TH
/RUN pin reaches 0.35V, the main
control loop is enabled with the I
TH
/RUN voltage then
pulled up to its zero current level of approximately 0.7V.
As the external compensation network continues to charge
LTC1779
OPERATIO
up, the corresponding output current trip level follows,
allowing normal operation.
Comparator OVP guards against transient overshoots
> 7.5% by turning off the internal P-channel power
MOSFET and keeping it off until the fault is removed.
Burst Mode Operation
The LTC1779 enters Burst Mode operation at low load
currents. In this mode, the peak current of the inductor is
set as if V
ITH
/RUN = 1V (at low duty cycles) even though
the voltage at the I
TH
/RUN pin is at a lower value. If the
inductor’s average current is greater than the load require-
ment, the voltage at the I
TH
/RUN pin will drop. When the
I
TH
/RUN voltage goes below 0.85V, the sleep signal goes
high, turning off the internal MOSFET. The sleep signal
goes low when the I
TH
/RUN voltage goes above 0.925V
and the LTC1779 resumes normal operation. The next
oscillator cycle will turn the internal MOSFET on and the
switching cycle repeats.
Dropout Operation
When the input supply voltage decreases towards the
output voltage, the rate of change of inductor current
during the ON cycle decreases. This reduction means that
the internal P-channel MOSFET will remain on for more
than one oscillator cycle since the inductor current has not
ramped up to the threshold set by EAMP. Further reduc-
tion in input supply voltage will eventually cause the
P-channel MOSFET to be turned on 100%, i.e., DC. The
output voltage will then be determined by the input voltage
minus the voltage drop across the MOSFET, the sense
resistor and the inductor.
Undervoltage Lockout
To prevent operation of the P-channel MOSFET below safe
input voltage levels, an undervoltage lockout is incorpo-
rated into the LTC1779. When the input supply voltage
drops below approximately 2.0V, the P-channel MOSFET
and all circuitry is turned off except the undervoltage
block, which draws only several microamperes.
SF = I
OUT
/I
OUT(MAX)
(%)
U
(Refer to Functional Diagram)
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator will be reduced to about 90kHz. This lower
frequency allows the inductor current to safely discharge,
thereby preventing current runaway. The oscillator’s fre-
quency will gradually increase to its designed rate when
the feedback voltage again approaches 0.8V.
Overvoltage Protection
As a further protection, the overvoltage comparator in the
LTC1779 will turn the internal MOSFET off when the
feedback voltage has risen 7.5% above the reference
voltage of 0.8V. This comparator has a typical hysteresis
of 20mV.
Slope Compensation and Inductor’s Peak Current
The inductor’s peak current is determined by:
I
PK
=
M( V
ITH/RUN
– 0.7)
10 R
SENSE
+
2
Ω
(
)
when the LTC1779 is operating below 40% duty cycle.
However, once the duty cycle exceeds 40%, slope com-
pensation begins and effectively reduces the peak induc-
tor current. The amount of reduction is given by the curves
in Figure 2.
110
100
90
80
70
60
50
40
30
20
10
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1779 F02
I
RIPPLE
= 0.4I
PK
AT 5% DUTY CYCLE
I
RIPPLE
= 0.2I
PK
AT 5% DUTY CYCLE
V
IN
= 4.2V
Figure 2. Maximum Output Current vs Duty Cycle
5
