LTC3801/LTC3801B
Micropower
Constant Frequency Step-Down
DC/DC Controllers in ThinSOT
FEATURES
s
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DESCRIPTIO
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High Efficiency: Up to 94%
Very Low No-Load Quiescent Current:
Only 16µA (LTC3801)
High Output Currents Easily Achieved
Internal Soft-Start
Wide V
IN
Range: 2.4V to 9.8V
Low Dropout: 100% Duty Cycle
Constant Frequency 550kHz Operation
Burst Mode
®
Operation for High Efficiency
at Light Loads (LTC3801)
Burst Mode Operation Disabled for Lower Output
Ripple at Light Loads (LTC3801B)
Output Voltage as Low as 0.8V
±1.5%
Voltage Reference Accuracy
Current Mode Operation for Excellent Line and Load
Transient Response
Only 6µA Supply Current in Shutdown (LTC3801)
Low Profile (1mm) SOT-23 Package
The LTC
®
3801/LTC3801B are constant frequency cur-
rent mode step-down DC/DC controllers in a low profile
(1mm max) 6-lead SOT-23 (ThinSOT
TM
) package. The
parts provide excellent AC and DC load and line regula-
tion with
±1.5%
output voltage accuracy. The LTC3801
consumes only 195µA of quiescent current in normal
operation, dropping to 16µA under no-load conditions.
The LTC3801/LTC3801B incorporate an undervoltage lock-
out feature that shuts down the device when the input
voltage falls below 2.2V. The LTC3801 automatically
switches into Burst Mode operation at light loads which
enhances efficiency at low output current. In the LTC3801B,
Burst Mode operation is disabled for lower output ripple at
light loads.
To further maximize the life of a battery source, the
external P-channel MOSFET is turned on continuously in
dropout (100% duty cycle). High switching frequency of
550kHz allows the use of a small inductor.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
APPLICATIO S
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1- or 2-Cell Li-Ion Battery-Powered Applications
Wireless Devices
Portable Computers
Distributed Power Systems
TYPICAL APPLICATIO
LTC3801 Efficiency vs Load Current*
100
95
V
OUT
= 2.5V
V
IN
= 4.2V
V
IN
= 3.3V
550kHz Micropower Step-Down DC/DC Controller
10k
V
IN
I
TH
/RUN
LTC3801/
LTC3801B
GND
SENSE
–
V
FB
402k
866k
4.7µH
V
OUT
2.5V
2A
PGATE
0.025Ω
10µF
V
IN
2.7V TO 9.8V
EFFICIENCY (%)
90
85
80
75
70
65
60
55
50
0.1
1
10
100
1000
LOAD CURRENT (mA)
10000
V
IN
= 9.8V
V
IN
= 8.4V
V
IN
= 6.6V
220pF
+
47µF
3801 TA01
*SEE NO-LOAD I
Q
vs INPUT VOLTAGE ON THE LAST PAGE OF THIS DATA SHEET
sn3801 3801fs
U
3801 TA02
U
U
1
LTC3801/LTC3801B
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
I
TH
/RUN 1
GND 2
V
FB
3
6 PGATE
5 V
IN
4 SENSE
–
Input Supply Voltage (V
IN
)........................ – 0.3V to 10V
SENSE
–
, PGATE Voltages ............ – 0.3V to (V
IN
+ 0.3V)
V
FB
, I
TH
/RUN Voltages ............................. – 0.3V to 2.4V
PGATE Peak Output Current (<10µs) ........................ 1A
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Junction Temperature (Note 3) ............................ 150°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC3801ES6
LTC3801BES6
S6 PART MARKING
LTACR
LTAHN
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
T
JMAX
= 150°C,
θ
JA
= 230°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
Input Voltage Range
Input DC Supply Current
Normal Operation
SLEEP Mode
Shutdown
UVLO
Undervoltage Lockout Threshold
Start-Up Current Source
Shutdown Threshold (at I
TH
/RUN)
Regulated Feedback Voltage
Feedback Voltage Line Regulation
Feedback Voltage Load Regulation
V
FB
Input Current
Overvoltage Protect Threshold
Overvoltage Protect Hysteresis
Oscillator Frequency
Normal Operation
Output Short Circuit
Gate Drive Rise Time
Gate Drive Fall Time
Peak Current Sense Voltage
The
q
indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 4.2V unless otherwise noted. (Note 2)
CONDITIONS
q
MIN
2.4
TYP
MAX
9.8
300
30
15
17
2
2.3
2.2
1.5
3.0
0.95
0.812
0.812
UNITS
V
µA
µA
µA
µA
µA
V
V
µA
µA
V
V
V
mV/V
mV/µA
mV/µA
nA
V
mV
kHz
kHz
ns
ns
mV
mV
mV
ms
Typicals at V
IN
= 4.2V (Note 4)
2.4V
≤
V
IN
≤
9.8V, V
ITH
/RUN = 1.3V
2.4V
≤
V
IN
≤
9.8V (LTC3801 Only)
2.4V
≤
V
IN
≤
9.8V, V
ITH
/RUN = 0V (LTC3801)
2.4V
≤
V
IN
≤
9.8V, V
ITH
/RUN = 0V (LTC3801B)
V
IN
< UVLO Threshold
V
IN
Rising
V
IN
Falling
V
ITH
/RUN = 0V (LTC3801)
V
ITH
/RUN = 0V (LTC3801B)
V
ITH
/RUN Rising
0°C
≤
T
A
≤
85°C (Note 5)
–40°C
≤
T
A
≤
85°C (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
q
q
q
q
0.5
1.0
0.3
0.788
0.780
0.850
195
16
6
8
1
1.8
1.7
1
2
0.6
0.800
0.800
0.05
2
2
2
0.880
40
550
210
40
40
117
104
26
0.6
10
0.910
Peak Current Sense Voltage in Burst Mode Operation
Default Soft-Start Time
V
FB
= 0.8V
V
FB
= 0V
C
LOAD
= 3000pF
C
LOAD
= 3000pF
Duty Cycle < 40% (Note 6)
LTC3801
LTC3801B
LTC3801 Only
500
650
q
q
109
95
125
113
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC3801ES6/LTC3801BES6 are guaranteed to meet specifica-
tions 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:
T
J
= T
A
+ (P
D
•
θ
JA
°C/W)
Note 4:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5:
The LTC3801/LTC3801B are tested in a feedback loop that servos
V
FB
to the output of the error amplifier while maintaining I
TH
/RUN at the
midpoint of the current limit range.
Note 6:
Peak current sense voltage is reduced dependent on duty cycle as
given in Figure 1.
sn3801 3801fs
2
U
W
U
U
W W
W
LTC3801/LTC3801B
TYPICAL PERFOR A CE CHARACTERISTICS
Input DC Supply Current (Normal)
vs Input Voltage
225
T
A
= 25°C
V
ITH
/RUN = 1.3V
20
215
205
I
IN
(µA)
I
IN
(µA)
I
IN
(µA)
195
185
175
2
3
4
5
6
V
IN
(V)
3801 G01
7
8
Undervoltage Lockout Threshold
vs Temperature
2.2
2.0
V
ITH
/RUN (mV)
V
IN
RISING
V
IN
FALLING
1.6
600
V
FB
(mV)
V
IN
(V)
1.8
1.4
1.2
–50
–30
30
–10 10
50
TEMPERATURE (°C)
Regulated Feedback Voltage
vs Input Voltage
812
808
804
800
796
792
788
2
3
4
5
T
A
= 25°C
600
590
580
570
f
OSC
(kHz)
f
OSC
(kHz)
V
FB
(mV)
7
6
V
IN
(V)
8
U W
9
70
3801 G04
Input DC Supply Current (SLEEP)
vs Input Voltage (LTC3801 Only)
T
A
= 25°C
15
Input DC Supply Current
(Shutdown) vs Input Voltage
T
A
= 25°C
V
ITH
/RUN = 0V
18
16
12
LTC3801B
9
LTC3801
14
6
12
3
10
10
2
3
4
5
6
V
IN
(V)
3801 G02
0
7
8
9
10
2
3
4
5
6
V
IN
(V)
3801 G03
7
8
9
10
Shutdown Threshold
vs Temperature
800
V
IN
= 4.2V
Regulated Feedback Voltage
vs Temperature
812
808
V
IN
= 4.2V
700
804
800
796
500
792
400
–50
90
–30
50
–10 10
30
TEMPERATURE (°C)
70
90
788
–50 –30
30
50
–10 10
TEMPERATURE (°C)
80
90
3801 G05
3801 G06
Oscillator Frequency
vs Temperature
V
IN
= 4.2V
Oscillator Frequency
vs Input Voltage
560
T
A
= 25°C
555
560
550
540
530
520
510
550
545
9
10
500
–50
540
–30
30
–10 10
50
TEMPERATURE (°C)
70
90
2
3
4
5
7
6
V
IN
(V)
8
9
10
3801 G07
3801 G08
3801 G09
sn3801 3801fs
3
LTC3801/LTC3801B
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. Nominal voltage range for this pin is
0.7V to 1.9V. Forcing this pin below 0.6V causes the
device to be shut down. In shutdown, all functions are
disabled and the PGATE pin is held high.
GND (Pin 2):
Ground Pin.
V
FB
(Pin 3):
Receives the feedback voltage from an exter-
nal resistor divider across the output.
SENSE
–
(Pin 4):
Current Sense Pin. An external sense
resistor is connected between this pin and V
IN
(Pin 5).
V
IN
(Pin 5):
Supply Pin. This pin must be closely de-
coupled to GND (Pin 2).
PGATE (Pin 6):
Gate Drive for the External P-Channel
MOSFET. This pin swings from 0V to V
IN
.
FU CTIO AL DIAGRA
V
IN
UNDERVOLTAGE
LOCKOUT
UV
5
1µA (LTC3801)
2µA (LTC3801B)
1
0.15V
ERROR
AMPLIFIER
1.2V
4
–
+
+
BURST
DEFEAT
(LTC3801B)
+
–
I
TH
/RUN
SHUTDOWN
COMPARATOR
0.3V
+
SHDN
I
TH
BUFFER
I
LIM
–
RS R S
LATCH
Q
SLEEP
COMPARATOR
SLEEP
550kHz
OSCILLATOR
V
IN
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
0V
SOFT-START
CLAMP
OVERVOLTAGE
COMPARATOR
SHORT-CIRCUIT
DETECT
FREQUENCY
FOLDBACK
PGATE
6
–
0.88V
0.225V
0.8V
GND
2
3801 FD
–
+
W
–
U
U
U
U
U
4
SENSE
–
15mV (LTC3801B)
VOLTAGE
REFERENCE
0.8V
BURST
DEFEAT
(LTC3801B)
BURST
CLAMP
SLOPE
COMPENSATION
CURRENT
COMPARATOR
+
+
0.3V
–
V
FB
3
sn3801 3801fs
LTC3801/LTC3801B
OPERATIO
Main Control Loop (Normal Operation)
The LTC3801/LTC3801B are constant frequency current
mode step-down switching regulator controllers. During
normal operation, an external P-channel MOSFET is turned
on each cycle when the oscillator sets the RS latch and
turned off when the current comparator resets the latch.
The peak inductor current at which the current comparator
trips is controlled by the voltage on the I
TH
/RUN pin, which
is the output of the error amplifier. The negative input to
the error amplifier is the output feedback voltage V
FB
which is generated by an external resistor divider con-
nected between V
OUT
and ground. When the 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 to ground. Releasing the I
TH
/RUN pin allows an
internal 1µA current source (2µA on LTC3801B) to charge
up the external compensation network. When the I
TH
/
RUN pin voltage reaches approximately 0.6V, the main
control loop is enabled and the I
TH
/RUN voltage is pulled
up by a clamp to its zero current level of approximately
one diode voltage drop (0.7V). As the external compensa-
tion network continues to charge up, the corresponding
peak inductor current level follows, allowing normal op-
eration. The maximum peak inductor current attainable is
set by a clamp on the I
TH
/RUN pin at 1.2V above the zero
current level (approximately 1.9V).
Burst Mode Operation (LTC3801 Only)
The LTC3801 incorporates Burst Mode operation at low
load currents (<25% of I
MAX
). In this mode, an internal
clamp sets the peak current of the inductor at a level cor-
responding to an I
TH
/RUN voltage 0.3V above its zero
current level (approximately 1V), even though the actual
I
TH
/RUN voltage is lower. When the inductor’s average
current is greater than the load requirement, the voltage at
the I
TH
/RUN pin will drop. When the I
TH
/RUN voltage falls
to 0.15V above its zero current level (approximately 0.85V),
the sleep comparator will trip, turning off the external
MOSFET. In sleep, the input DC supply current to the IC is
reduced to 16µA from 195µA in normal operation. With the
switch held off, average inductor current will decay to zero
U
(Refer to the Functional Diagram)
and the load will eventually cause the error amplifier out-
put to start drifting higher. When the error amplifier output
rises to 0.225V above its zero current level (approximately
0.925V), the sleep comparator will untrip and normal op-
eration will resume. The next oscillator cycle will turn the
external MOSFET on and the switching cycle will repeat.
Low Load Current Operation (LTC3801B Only)
Under very light load current conditions, the I
TH
/RUN pin
voltage will be very close to the zero current level of 0.85V.
As the load current decreases further, an internal offset at
the current comparator input will ensure that the current
comparator remains tripped (even at zero load current)
and the regulator will start to skip cycles, as it must, in
order to maintain regulation. This behavior allows the
regulator to maintain constant frequency down to very
light loads, resulting in less low frequency noise genera-
tion over a wide load current range.
Figure 1 illustrates this result for the circuit on the front
page of this data sheet using both an LTC3801 (in Burst
Mode operation) and an LTC3801B (with Burst Mode
operation disabled). At an output current of 100mA, the
LTC3801 exhibits an output ripple of 81.6mV
P-P
, whereas
the LTC3801B has an output ripple of only 17.6mV
P-P
. At
lower output current levels, the improvement is even
greater. This comes at a tradeoff of lower efficiency for the
non Burst Mode part at light load currents (see Figure 2).
Also notice the constant frequency operation of the
LTC3801B, even at 5% of maximum output current.
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
at some input-output differential, the external P-channel
MOSFET will remain on for more than one oscillator cycle
(start dropping off-cycles) since the inductor current has
not ramped up to the threshold set by the error amplifier.
Further reduction in input supply voltage will eventually
cause the external 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
sense resistor, the MOSFET and the inductor.
sn3801 3801fs
5
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