1
LT1961
sn1961 1961fs
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
1.5A, 1.25MHz Step-Up
Switching Regulator
s
1.5A Switch in a Small MSOP Package
s
Constant 1.25MHz Switching Frequency
s
Wide Operating Voltage Range: 3V to 25V
s
High Efficiency 0.2
Switch
s
1.2V Feedback Reference Voltage
s
±
2% Overall Output Voltage Tolerance
s
Uses Low Profile Surface Mount External
Components
s
Low Shutdown Current: 6
µ
A
s
Synchronizable from 1.5MHz to 2MHz
s
Current-Mode Loop Control
s
Constant Maximum Switch Current Rating at All Duty
Cycles*
s
Thermally Enhanced Exposed Pad Package
The LT
®
1961 is a 1.25MHz monolithic boost switching
regulator. A high efficiency 1.5A, 0.2
switch is included
on the die together with all the control circuitry required to
complete a high frequency, current-mode switching regu-
lator. Current-mode control provides fast transient re-
sponse and excellent loop stability.
New design techniques achieve high efficiency at high
switching frequencies over a wide operating voltage range.
A low dropout internal regulator maintains consistent
performance over a wide range of inputs from 24V sys-
tems to Li-Ion batteries. An operating supply current of
1mA maintains high efficiency, especially at lower output
currents. Shutdown reduces quiescent current to 6
µ
A.
Maximum switch current remains constant at all duty
cycles. Synchronization allows an external logic level
signal to increase the internal oscillator from 1.5MHz to
2MHz.
The LT1961 is available in an exposed pad, 8-pin MSOP
package. Full cycle-by-cycle switch current limit protec-
tion and thermal shutdown are provided. High frequency
operation allows the reduction of input and output filtering
components and permits the use of chip inductors.
s
DSL Modems
s
Portable Computers
s
Battery-Powered Systems
s
Distributed Power
, LTC and LT are registered trademarks of Linear Technology Corporation.
Efficiency vs Load Current
5V to 12V Boost Converter
*Patent Pending
LT1961
V
IN
V
OUT
12V
0.5A*
V
IN
5V
1961 TA01
6800pF
100pF
6.8k
10k
1%
90.9k
UPS120
10
µ
F
CERAMIC
2.2
µ
F
CERAMIC
V
SW
FB
SHDN
OPEN
OR
HIGH
= ON
GND
V
C
SYNC
*MAXIMUM OUTPUT CURRENT IS SUBJECT TO THERMAL DERATING.
6.8
µ
H
2
6
7
3,4
8
5
1
LOAD CURRENT (mA)
0
EFFICIENCY (%)
90
85
80
75
70
65
60
100
200
300
400
1961 TA01a
500
V
IN
= 5V
V
OUT
= 12V
2
LT1961
sn1961 1961fs
PARAMETER
CONDITION
MIN
TYP
MAX
UNITS
Recommended Operating Voltage
q
3
25
V
Maximum Switch Current Limit
q
1.5
2
3
A
Oscillator Frequency
3.3V < V
IN
< 25V
q
1
1.5
MHz
Switch On Voltage Drop
I
SW
= 1.5A
q
310
500
mV
V
IN
Undervoltage Lockout
(Note 3)
q
2.47
2.6
2.73
V
V
IN
Supply Current
I
SW
= 0A
q
0.9
1.3
mA
V
IN
Supply Current/I
SW
I
SW
= 1.5A
27
mA/A
Shutdown Supply Current
V
SHDN
= 0V, V
IN
= 25V, V
SW
= 25V
6
20
µ
A
q
45
µ
A
Feedback Voltage
3V < V
IN
< 25V, 0.4V < V
C
< 0.9V
1.182
1.2
1.218
V
q
1.176
1.224
V
FB Input Current
q
0
0.2
0.4
µ
A
FB to V
C
Voltage Gain
0.4V < V
C
< 0.9V
150
350
FB to V
C
Transconductance
I
VC
=
±
10
µ
A
q
500
850
1300
µ
Mho
V
C
Pin Source Current
V
FB
= 1V
q
85
120
165
µ
A
V
C
Pin Sink Current
V
FB
= 1.4V
q
70
110
165
µ
A
V
C
Pin to Switch Current Transconductance
2.4
A/V
V
C
Pin Minimum Switching Threshold
Duty Cycle = 0%
0.3
V
V
C
Pin 1.5A I
SW
Threshold
0.9
V
Maximum Switch Duty Cycle
V
C
= 1.2V, I
SW
= 100mA
q
80
90
%
V
C
= 1.2V, I
SW
= 1A, 25
°
C
T
A
125
°
C
75
80
%
V
C
= 1.2V, I
SW
= 1A, T
A
25
°
C
70
75
%
SHDN Threshold Voltage
q
1.28
1.35
1.42
V
SHDN Input Current (Shutting Down)
SHDN = 60mV Above Threshold
q
7
10
13
µ
A
SHDN Threshold Current Hysteresis
SHDN = 100mV Below Threshold
4
7
10
µ
A
SYNC Threshold Voltage
1.5
2.2
V
SYNC Input Frequency
1.5
2
MHz
SYNC Pin Resistance
I
SYNC
= 1mA
20
k
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
ELECTRICAL CHARACTERISTICS
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
Input Voltage .......................................................... 25V
Switch Voltage ......................................................... 35V
SHDN Pin ............................................................... 25V
FB Pin Current ....................................................... 1mA
SYNC Pin Current .................................................. 1mA
Operating Junction Temperature Range (Note 2)
LT1961E .......................................... 40
°
C to 125
°
C
Storage Temperature Range ................ 65
°
C to 150
°
C
Lead Temperature (Soldering, 10 sec)................. 300
°
C
ORDER PART NUMBER
(Note 1)
T
JMAX
= 125
°
C,
JA
= 50
°
C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
LTQY
MS8E PART MARKING
LT1961EMS8E
GROUND PAD CONNECTED
TO LARGE COPPER AREA
The
q
denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at T
A
= 25
°
C. V
IN
= 15V, V
C
= 0.8V, SHDN, SYNC and switch open unless otherwise noted.
1
2
3
4
V
IN
SW
GND
GND
8
7
6
5
SYNC
V
C
FB
SHDN
TOP VIEW
MS8E PACKAGE
8-LEAD PLASTIC MSOP
3
LT1961
sn1961 1961fs
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LT1961E is guaranteed to meet performance specifications
from 0
°
C to 125
°
C junction temperature. Specifications over the 40
°
C to
125
°
C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls.
Note 3: Minimum input voltage is defined as the voltage where the internal
regulator enters lockout. Actual minimum input voltage to maintain a
regulated output will depend on output voltage and load current. See
Applications Information.
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
FB vs Temperature
Switch On Voltage Drop
Oscillator Frequency
SHDN I
P
Current vs Temperature
SHDN Threshold vs Temperature
SHDN Supply Current vs V
IN
ELECTRICAL CHARACTERISTICS
TEMPERATURE (
°
C)
50
25
0
25
50
75
100
125
FB VOLTAGE (V)
1961 G01
1.22
1.21
1.20
1.19
1.18
SWITCH CURRENT (A)
0
0.5
1
1.5
SWITCH VOLTAGE (mV)
1961 G02
400
350
300
250
200
150
100
50
0
125
°
C
25
°
C
40
°
C
TEMPERATURE (
°
C)
50
25
0
25
50
75
100
125
FREQUENCY (MHz)
1961 G03
1.5
1.4
1.3
1.2
1.1
T
A
= 25
°
C
TEMPERATURE (
°
C)
50
25
0
25
50
75
100
125
SHDN THRESHOLD (V)
1961 G04
1.40
1.38
1.36
1.34
1.32
1.30
V
IN
(V)
0
5
10
15
20
25
30
V
IN
CURRENT (
µ
A)
1961 G05
7
6
5
4
3
2
1
0
T
A
= 25
°
C
SHDN = 0V
TEMPERATURE (
°
C)
50
25
0
25
50
75
100
125
SHDN INPUT (
µ
A)
1961G06
12
10
8
6
4
2
0
STARTING UP
SHUTTING DOWN
4
LT1961
sn1961 1961fs
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
FB: The feedback pin is used to set output voltage using an
external voltage divider that generates 1.2V at the pin with
the desired output voltage. If required, the current limit
can be reduced during start up when the FB pin is below
0.5V (see the Current Limit Foldback graph in the Typical
Performance Characteristics section). An impedance of
less than 5k
at the FB pin is needed for this feature to
operate.
V
IN
: This pin powers the internal circuitry and internal
regulator. Keep the external bypass capacitor close to this
pin.
GND: Short GND pins 3 and 4 and the exposed pad on the
PCB. The GND is the reference for the regulated output, so
load regulation will suffer if the "ground" end of the load
is not at the same voltage as the GND of the IC. This
condition occurs when the load current flows through the
metal path between the GND pins and the load ground
point. Keep the ground path short between the GND pins
and the load and use a ground plane when possible. Keep
the path between the input bypass and the GND pins short.
The exposed pad should be attached to a large copper area
to improve thermal resistance.
V
SW
: The switch pin is the collector of the on-chip power
NPN switch and has large currents flowing through it.
Keep the traces to the switching components as short as
possible to minimize radiation and voltage spikes.
SYNC: The sync pin is used to synchronize the internal
oscillator to an external signal. It is directly logic compat-
ible and can be driven with any signal between 20% and
80% duty cycle. The synchronizing range is equal to
initial
operating frequency, up to 2MHz. See Synchronization
section in Applications Information for details. When not
in use, this pin should be grounded.
SHDN: The shutdown pin is used to turn off the regulator
and to reduce input drain current to a few microamperes.
The 1.35V threshold can function as an accurate under-
voltage lockout (UVLO), preventing the regulator from
operating until the input voltage has reached a predeter-
mined level. Float or pull high to put the regulator in the
operating mode.
V
C
: The V
C
pin is the output of the error amplifier and the
input of the peak switch current comparator. It is normally
used for frequency compensation, but can do double duty
as a current clamp or control loop override. This pin sits
at about 0.3V for very light loads and 0.9V at maximum
load.
SHDN Supply Current
Input Supply Current
Current Limit Foldback
PI
N
FU
N
CTIO
N
S
U
U
U
SHUTDOWN VOLTAGE (V)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
V
IN
CURRENT (
µ
A)
1961 G07
300
250
200
150
100
50
0
T
A
= 25
°
C
V
IN
= 15V
INPUT VOLTAGE (V)
0
5
10
15
20
25
30
V
IN
CURRENT (
µ
A)
1961 G08
1200
1000
800
600
400
200
0
MINIMUM
INPUT
VOLTAGE
T
A
= 25
°
C
FEEDBACK VOLTAGE (V)
0
0.2
0.4
0.6
0.8
1
1.2
SWITCH PEAK CURRENT (A)
1961 G09
2.0
1.5
1.0
0.5
0
FB INPUT CURRENT (
µ
A)
40
30
20
10
0
FB CURRENT
SWITCH CURRENT
T
A
= 25
°
C
5
LT1961
sn1961 1961fs
amplifier commands current to be delivered to the output
rather than voltage. A voltage fed system will have low
phase shift up to the resonant frequency of the inductor
and output capacitor, then an abrupt 180
°
shift will occur.
The current fed system will have 90
°
phase shift at a much
lower frequency, but will not have the additional 90
°
shift
until well beyond the LC resonant frequency. This makes
it much easier to frequency compensate the feedback loop
and also gives much quicker transient response.
A comparator connected to the shutdown pin disables the
internal regulator, reducing supply current.
The LT1961 is a constant frequency, current-mode boost
converter. This means that there is an internal clock and
two feedback loops that control the duty cycle of the power
switch. In addition to the normal error amplifier, there is a
current sense amplifier that monitors switch current on a
cycle-by-cycle basis. A switch cycle starts with an oscilla-
tor pulse which sets the R
S
flip-flop to turn the switch on.
When switch current reaches a level set by the inverting
input of the comparator, the flip-flop is reset and the
switch turns off. Output voltage control is obtained by
using the output of the error amplifier to set the switch
current trip point. This technique means that the error
Figure 1. Block Diagram
BLOCK DIAGRA
M
W
+
+
V
IN
2.5V BIAS
REGULATOR
1.25MHz
OSCILLATOR
SW
FB
V
C
GND
GND
1767 F01
SLOPE COMP
0.01
INTERNAL
V
CC
CURRENT SENSE
AMPLIFIER VOLTAGE
GAIN = 40
SYNC
SHDN
SHUTDOWN
COMPARATOR
CURRENT
COMPARATOR
ERROR
AMPLIFIER
g
m
= 850
µ
Mho
R
S
FLIP-FLOP
DRIVER
CIRCUITRY
S
R
0.3V
Q1
POWER
SWITCH
1.2V
+
+
1.35V
3µ
A
7
µ
A
1
8
5
7
6
3
4
2
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