LTC1622
Low Input Voltage
Current Mode Step-Down
DC/DC Controller
FEATURES
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DESCRIPTIO
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High Efficiency
Constant Frequency 550kHz Operation
V
IN
Range: 2V to 10V
Multiampere Output Currents
OPTI-LOOP
TM
Compensation Minimizes C
OUT
Selectable, Burst Mode Operation
Low Dropout Operation: 100% Duty Cycle
Synchronizable up to 750kHz
Current Mode Operation for Excellent Line and Load
Transient Response
Low Quiescent Current: 350µA
Shutdown Mode Draws Only 15µA Supply Current
±1.9%
Reference Accuracy
Available in 8-Lead MSOP
APPLICATIO S
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1- or 2-Cell Li-Ion Powered Applications
Cellular Telephones
Wireless Modems
Portable Computers
Distributed 3.3V, 2.5V or 1.8V Power Systems
Scanners
Battery-Powered Equipment
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are a trademarks of Linear Technology Corporation.
The LTC
®
1622 is a constant frequency current mode step-
down DC/DC controller providing excellent AC and DC load
and line regulation. The device incorporates an accurate
undervoltage feature that shuts the LTC1622 down when
the input voltage falls below 2V.
The LTC1622 boasts a
±1.9%
output voltage accuracy and
consumes only 350µA of quiescent current. For applica-
tions where efficiency is a prime consideration and the
load current varies from light to heavy, the LTC1622 can
be configured for Burst Mode
TM
operation. Burst Mode
operation enhances low current efficiency and extends
battery run time. Burst Mode operation is inhibited during
synchronization or when the SYNC/MODE pin is pulled low
to reduce noise and possible RF interference.
High constant operating frequency of 550kHz allows the
use of a small inductor. The device can also be synchro-
nized up to 750kHz for special applications. The high
frequency operation and the available 8-lead MSOP pack-
age create a high performance solution in an extremely
small amount of PCB area.
To further maximize the life of the battery source, the
P-channel MOSFET is turned on continuously in dropout
(100% duty cycle). In shutdown, the device draws a mere
15µA.
TYPICAL APPLICATIO
8
2
R1
10k
C3
220pF
4
470pF
V
IN
I
TH
SENSE
–
1
V
IN
2.5V TO 8.5V
R2
0.03Ω
Si3443DV L1
4.7µH
D1
IR10BQ015
R3
159k
R4
75k
L1: MURATA LQN6C-4R7
R2: DALE WSL-1206 0-03Ω
C1
10µF
10V
Efficiency vs Load Current with
Burst Mode Operation Enabled
100
V
IN
= 4.2V
90
EFFICIENCY (%)
V
IN
= 3.3V
7
PDRV
LTC1622
5
SYNC/MODE
6
80
V
IN
= 6V
70
60
50
V
OUT
= 2.5V
R
SENSE
= 0.03Ω
1
100
1000
10
LOAD CURRENT (mA)
5000
1622 F01b
V
OUT
2.5V
1.5A
RUN/SS
GND
V
FB
3
+
C2
47µF
6V
C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT
C2: SANYO POSCAP 6TPA47M
D1: INTERNATIONAL RECTIFIER IR10BQ015
1622 F01a
40
Figure 1. High Efficiency Step-Down Converter
U
V
IN
= 8.4V
U
U
1
LTC1622
ABSOLUTE
MAXIMUM
RATINGS
Input Supply Voltage (V
IN
).........................– 0.3V to 10V
RUN/SS Voltage .......................................– 0.3V to 2.4V
SYNC/MODE Voltage ................................. – 0.3V to V
IN
SENSE
–
Voltage .......................................... 2.4V to V
IN
PDRV Peak Output Current (< 10µs) ......................... 1A
Storage Ambient Temperature Range ... – 65°C to 150°C
PACKAGE/ORDER INFORMATION
ORDER PART
NUMBER
TOP VIEW
SENSE
–
1
I
TH
2
V
FB
3
RUN/SS 4
8
7
6
5
V
IN
PDRV
GND
SYNC/MODE
TOP VIEW
SENSE
–
1
I
TH
2
V
FB
3
8
7
6
5
V
IN
PDRV
GND
SYNC/MODE
LTC1622CMS8
MS8 PART MARKING
LTDB
MS8 PACKAGE
8-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 250°C/ W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL
I
VFB
V
FB
V
OVL
∆V
OSENSE
V
LOADREG
I
S
PARAMETER
Feedback Current
Regulated Feedback Voltage
Output Overvoltage Lockout
Reference Voltage Line Regulation
Output Voltage Load Regulation
Input DC Supply Current
Burst Mode Inhibited
Sleep Mode
Shutdown
Shutdown
RUN/SS Threshold
Soft-Start Current Source
Oscillator Frequency
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 4.2V
CONDITIONS
(Note 3) V
FB
= 0.8V
(Note 3) Commercial Grade
(Note 3) Industrial Grade
Referenced to Nominal V
OUT
V
IN
= 4.2V to 8.5V (Note 3)
Measured in Servo Loop; V
ITH
= 0.2V to 0.625V
Measured in Servo Loop; V
ITH
= 0.9V to 0.625V
(Note 4)
V
IN
= 2.3V
V
ITH
= 0V, V
SYNC/MODE
= 2.4V
V
RUN/SS
= 0V
V
RUN/SS
= 0V, V
IN
= V
UVLO
– 0.1V
Commercial Grade
Industrial Grade
V
RUN/SS
= 0V
V
FB
= 0.8V
V
FB
= 0V
V
SYNC/MODE
Ramping Down
V
IN
Ramping Down
V
IN
Ramping Up
q
q
q
q
q
V
RUN/SS
I
RUN/SS
f
OSC
V
SYNC/MODE
SYNC/MODE Threshold
V
UVLO
Undervoltage Lockout
2
U
U
W
W W
U
W
(Note 1)
Operating Temperature Range
Commercial ............................................ 0°C to 70°C
Industrial ........................................... – 45°C to 85°C
Junction Temperature (Note 2) ............................. 125°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC1622CS8
LTC1622IS8
S8 PART MARKING
1622
1622I
RUN/SS 4
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 150°C/ W
MIN
0.785
0.780
4
TYP
10
0.8
0.8
16
0.04
0.3
– 0.3
450
350
15
4
MAX
70
0.815
0.820
25
0.08
0.5
– 0.5
UNITS
nA
V
V
%
%/V
%
%
µA
µA
µA
µA
V
V
µA
kHz
kHz
V
V
V
400
30
10
0.9
1.0
5
625
140
1.5
2.3
2.36
0.4
0.3
1
475
75
1.55
0.7
0.7
2.5
550
110
1
1.92
1.97
LTC1622
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 4.2V
SYMBOL
PDRV t
r
PDRV t
f
∆V
SENSE(MAX)
PARAMETER
Gate Drive Rise Time
Gate Drive Fall Time
Maximum Current Sense Voltage
CONDITIONS
C
LOAD
= 3000pF
C
LOAD
= 3000pF
q
ELECTRICAL CHARACTERISTICS
MIN
TYP
80
100
MAX
140
140
140
UNITS
ns
ns
mV
80
110
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
LTC1622CS8; T
J
= T
A
+ (P
D
• 150°C/W),
LTC1622CMS8; T
J
= T
A
+ (P
D
• 250°C/W)
Note 3:
The LTC1622 is tested in a feedback loop that servos V
FB
to the
feedback point for the error amplifier (V
ITH
= 0.8V).
Note 4:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Current
vs Supply Voltage
45
40
35
30
25
20
15
10
5
0
2
3
4
6
7
5
8
SUPPLY VOLTAGE (V)
9
10
SOFT-START CURRENT (µA)
SHUTDOWN CURRENT (µA)
TRIP VOLTAGE (mV)
Normalized Oscillator Frequency
vs Temperature
10.0
V
IN
= 4.2V
0.810
0.805
7.5
5.0
2.5
0
–2.5
–5.0
–7.5
–10.0
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1622 G04
UNDERVOLTAGE LOCKOUT VOLTAGE (V)
NORMALIZED FREQUENCY (%)
REFERENCE VOLTAGE (V)
U W
1622 G01
RUN/SS Current vs Supply Voltage
3.50
110
100
Maximum Current Sense Voltage
vs Duty Cycle
V
IN
= 4.2V
3.00
90
UNSYNC
80
70
60
50
40
2.50
2.00
1.50
1.00
2
3
4
5
6
7
8
9
10
SUPPLY VOLTAGE (V)
1622 G02
30
20
30
40
50 60 70 80
DUTY CYCLE (%)
90
100
1622 G03
Reference Voltage
vs Temperature
V
IN
= 4.2V
2.10
2.05
2.00
1.95
1.90
1.85
1.80
Undervoltage Lockout Voltage
vs Temperature
0.800
0.795
0.790
0.785
0.780
0.775
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1622 G05
1.75
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1622 G06
3
LTC1622
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current for
Figure 1 with Burst Mode
Operation Defeated
100
V
IN
= 3.3V
90
EFFICIENCY (%)
V
IN
= 4.2V
100mV/DIV
100mV/DIV
I
LOAD
= 50mA TO 1.2A
V
IN
= 4.2V
1622 G08
80
70
60
50
40
1
10
100
LOAD CURRENT (mA)
1000
1622 G07
V
IN
= 6V
V
IN
= 8.4V
V
OUT
= 2.5V
R
SENSE
= 0.03Ω
PIN FUNCTIONS
SENSE
–
(Pin 1):
The Negative Input to the Current Com-
parator.
I
TH
(Pin 2):
Error Amplifier Compensation Point. The
current comparator threshold increases with this control
voltage. Nominal voltage range for this pin is 0V to 1.2V.
V
FB
(Pin 3):
Receives the feedback voltage from an exter-
nal resistive divider across the output capacitor.
RUN/SS (Pin 4):
Combination of Soft-Start and Run
Control Inputs. A capacitor to ground at this pin sets the
ramp time to full output current. The time is approximately
0.45s/µF. Forcing this pin below 0.4V causes all circuitry
to be shut down.
SYNC/MODE (Pin 5):
This pin performs three functions.
Greater than 2V on this pin allows Burst Mode operation
at low load currents, while grounding or applying a clock
signal on this pin defeats Burst Mode operation. An
external clock between 625kHz and 750kHz applied to this
pin forces the LTC1622 to operate at the external clock
frequency.
Do not attempt to synchronize below 625kHz.
Pin 5 has an internal 1µA pull-up current source.
GND (Pin 6):
Ground Pin.
PDRV (PIN 7):
Gate Drive for the External P-Channel
MOSFET. This pin swings from 0V to V
IN
.
V
IN
(Pin 8):
Main Supply Pin. Must be closely decoupled
to ground Pin 6.
4
U W
Load Step Transient Response
Burst Enabled
Load Step Transient Response
Burst Inhibited
I
LOAD
= 50mA TO 1.2A
V
IN
= 4.2V
1622 G09
U
U
U
LTC1622
FUNCTIONAL DIAGRA
V
IN
BURST DEFEAT
1µA
X
SLOPE
COMP
OSC
0.36V
Y
SYNC/
5
MODE
0.3V
V
FB
3
–
+
–
FREQ
SHIFT
V
IN
0.8V
V
REF
2.5µA
EN
SENSE
–
1
8
V
IN
0.8V
REFERENCE
V
REF
0.8V
UVLO
TRIP = 1.97V
6
SHUTDOWN
GND
V
IN
RUN/SS 4
g
m
= 0.5m
RUN/
SOFT-START
+
V
REF
+ 0.12V
–
OV
OPERATIO
(Refer to Functional Diagram)
Main Control Loop
The LTC1622 is a constant frequency current mode switch-
ing regulator. During normal operation, the external
P-channel power MOSFET is turned on each cycle when
the oscillator sets the R
S
latch (R
S1
) and turned off when
the current comparator (I
COMP
) resets the latch. The peak
inductor current at which I
COMP
resets the R
S
latch is
controlled by the voltage on the I
TH
pin, which is the output
of the error amplifier EA. An external resistive divider
connected between V
OUT
and ground allows EA to receive
an output feedback voltage V
FB
. 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
voltage to
increase until the average inductor current matches the
new load current.
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 2.5µA
Ω
W
Y = “0” ONLY WHEN X IS A CONSTANT “1”
OTHERWISE Y = “1”
V
CC
U
U
U
+
+
EA
0.12V
–
+
BURST
2
I
TH
S
R
Q
R
S1
SLEEP
+
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
–
ICOMP
–
V
IN
PDRV
7
1622 BD
current source to charge up the soft-start capacitor C
SS
.
When C
SS
reaches 0.7V, the main control loop is enabled
with the I
TH
voltage clamped at approximately 5% of its
maximum value. As C
SS
continues to charge, I
TH
is gradu-
ally released allowing normal operation to resume.
Comparator OV guards against transient overshoots
> 16% by turning off the P-channel power MOSFET and
keeping it off until the fault is removed.
Burst Mode Operation
The LTC1622 can be enabled to go into Burst Mode
operation at low load currents simply by leaving the SYNC/
MODE pin open or connecting it to a voltage of at least 2V.
In this mode, the peak current of the inductor is set as if
V
ITH
= 0.36V (at low duty cycles) even though the voltage
at the I
TH
pin is at lower value. If the inductor’s average
current is greater than the load requirement, the voltage at
5
