LT3436
3A, 800kHz Step-Up
Switching Regulator
FEATURES
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DESCRIPTIO
Constant 800kHz Switching Frequency
Wide Operating Voltage Range: 3V to 25V
High Efficiency 0.1Ω/3A Switch
1.2V Feedback Reference Voltage
±2%
Overall Output Voltage Tolerance
Uses Low Profile Surface Mount External
Components
Low Shutdown Current: 11µA
Synchronizable from 1MHz to 1.4MHz
Current-Mode Control
Constant Maximum Switch Current Rating
at All Duty Cycles*
Available in a Small Thermally Enhanced
TSSOP-16 Package
The LT
®
3436 is an 800kHz monolithic boost switching
regulator. A high efficiency 3A, 0.1Ω 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 range. A low
dropout internal regulator maintains consistent perfor-
mance over a wide range of inputs from 24V systems to Li-
Ion batteries. An operating supply current of 1mA main-
tains high efficiency, especially at lower output currents.
Shutdown reduces quiescent current to 11µA. Maximum
switch current remains constant at all duty cycles. Syn-
chronization capability allows an external logic level signal
to increase the internal oscillator from 1MHz to 1.4MHz.
Full cycle-by-cycle switch current limit protection and ther-
mal shutdown are provided. High frequency operation al-
lows the reduction of input and output filtering components
and permits the use of tiny chip inductors. The LT3436 is
available in an exposed pad, 16-pin TSSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
*Protectd by U.S. Patents including 6535042, 6611131, 6498466
APPLICATIO S
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DSL Modems
Portable Computers
Battery-Powered Systems
Distributed Power
TYPICAL APPLICATIO
3.9µH
5V to 12V Boost Converter
B220A
INPUT
5V
4.7µF
CERAMIC
OPEN
OR
HIGH
= ON
V
IN
LT3436
SHDN
SYNC
FB
V
SW
90.9k
V
C
EFFICIENCY (%)
OUTPUT
12V
0.9A
†
GND
10nF
470pF
4.7k
†
MAXIMUM OUTPUT CURRENT IS SUBJECT TO THERMAL DERATING.
10k
1%
22µF
CERAMIC
3436 TA01
U
Efficiency vs Load Current
90
85
80
75
70
65
60
V
IN
= 5V
V
OUT
= 12V
0
0.1
0.2
0.3 0.4 0.5 0.6
LOAD CURRENT (A)
0.7
0.8
3436 TA01b
U
U
3436fa
1
LT3436
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
GND
V
IN
SW
SW
GND
GND
NC
GND
1
2
3
4
5
6
7
8
17
16 GND
15 NC
14 SYNC
13 V
C
12 FB
11 SHDN
10 NC
9
GND
Input Voltage .......................................................... 25V
Switch Voltage ......................................................... 35V
SHDN Pin ............................................................... 25V
FB Pin Current ....................................................... 1mA
SYNC Pin Current .................................................. 1mA
Operating Junction Temperature Range (Note 2)
LT3436E .......................................... – 40°C to 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART NUMBER
LT3436EFE
FE PART MARKING
3436EFE
FE PACKAGE
16-LEAD PLASTIC TSSOP
EXPOSED PAD IS GND (PIN 17),
MUST BE SOLDERED TO PCB
T
JMAX
= 125°C,
θ
JA
= 45°C/W,
θ
JC(PAD)
= 10°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
Recommended Operating Voltage
Maximum Switch Current Limit
Oscillator Frequency
Switch On Voltage Drop
V
IN
Undervoltage Lockout
V
IN
Supply Current
V
IN
Supply Current/I
SW
Shutdown Supply Current
Feedback Voltage
FB Input Current
FB to V
C
Voltage Gain
FB to V
C
Transconductance
V
C
Pin Source Current
V
C
Pin Sink Current
V
C
Pin to Switch Current Transconductance
V
C
Pin Minimum Switching Threshold
V
C
Pin 3A I
SW
Threshold
Maximum Switch Duty Cycle
SHDN Threshold Voltage
SHDN Input Current (Shutting Down)
SHDN Threshold Current Hysteresis
SYNC Threshold Voltage
SYNC Input Frequency
SYNC Pin Resistance
I
SYNC
= 1mA
Duty Cycle = 0%
0.4V < V
C
< 0.9V
∆I
VC
=
±10µA
V
FB
= 1V
V
FB
= 1.4V
3.3V < V
IN
< 25V
I
SW
= 3A
(Note 3)
I
SW
= 0A
I
SW
= 3A
CONDITION
The
●
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.
MIN
●
●
●
●
●
●
TYP
4
800
330
2.6
1
15
11
MAX
25
6
960
550
2.73
1.3
25
45
1.218
1.224
– 0.4
1300
– 165
165
UNITS
V
A
kHz
mV
V
mA
mA/A
µA
µA
V
V
µA
µMho
µA
µA
A/V
V
V
%
%
3
3
640
2.47
V
SHDN
= 0V, V
IN
= 25V, V
SW
= 25V
●
3V < V
IN
< 25V, 0.4V < V
C
< 0.9V
●
●
●
●
●
1.182
1.176
0
150
500
– 85
70
1.2
– 0.2
350
850
– 120
110
4.8
0.3
0.9
V
C
= 1.2V, I
SW
= 350mA
V
C
= 1.2V, I
SW
= 1A
SHDN = 60mV Above Threshold
SHDN = 100mV Below Threshold
●
●
●
85
80
1.28
–7
4
1
90
1.35
–10
7
1.5
20
1.42
–13
10
2.2
1.4
2
U
W
U
U
W W
W
V
µA
µA
V
MHz
kΩ
3436fa
LT3436
ELECTRICAL CHARACTERISTICS
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT3436E is guaranteed to meet performance specifications
from 0°C to 125°C. 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 PERFORMANCE CHARACTERISTICS
FB Voltage
1.220
1.215
SWITCH VOLTAGE (mV)
400
1.210
FB VOLTAGE (V)
350
300
250
200
150
100
50
0
T
A
= 25°C
OSCILLATOR FREQUENCY (kHz)
1.205
1.200
1.195
1.190
1.185
1.180
–50 –25
0
25
50
75
100
125
TEMPERATURE (°C)
3436 G01
SHDN Threshold
1.40
14
12
1.38
SHDN THRESHOLD (V)
V
IN
CURRENT (µA)
SHDN INPUT CURRENT (µA)
1.36
1.34
1.32
2
1.30
–50 –25
0
0
25
50
75
100
125
0
5
TEMPERATURE (°C)
3436 G04
U W
Switch On Voltage Drop
500
450
T
A
= 125°C
920
890
860
830
800
770
740
710
0
0.5
1.5
2.0
1.0
SWITCH CURRENT (A)
2.5
3.0
3436 G02
Oscillator Frequency
T
A
= –40°C
680
–50 –25
0
25
50
75
100
125
TEMPERATURE (°C)
3436 G03
SHDN Supply Current
–12
T
A
= 25°C
SHDN = 0V
–10
SHDN Input Current
10
8
6
4
SHUTTING DOWN
–8
–6
–4
–2
0
–50 –25
STARTING UP
10
15
20
INPUT VOLTAGE (V)
25
30
3436 G05
50
25
75
0
TEMPERATURE (°C)
100
125
3436 G06
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LT3436
TYPICAL PERFOR A CE CHARACTERISTICS
SHDN Supply Current
300
250
V
IN
CURRENT (µA)
T
A
= 25°C
V
IN
= 15V
SWITCH PEAK CURRENT (A)
V
IN
CURRENT (µA)
200
150
100
50
0
0
0.2
0.4 0.6 0.8 1.0
SHDN VOLTAGE (V)
PIN FUNCTIONS
GND (Pins 1, 5, 6, 8, 9, 16, 17):
Short GND pins 1, 5, 6,8,
9, 16 and the exposed pad (pin 17) 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
performance.
V
IN
(Pin 2):
This pin powers the internal circuitry and
internal regulator. Keep the external bypass capacitor
close to this pin.
SW (Pins 3, 4):
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.
SHDN (Pin 11):
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 undervoltage lockout (UVLO), preventing the
regulator from operating until the input voltage has reached
a predetermined level. Float or pull high to put the regula-
tor in the operating mode.
FB (Pin 12):
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 imped-
ance of less than 5kΩ at the FB pin is needed for this
feature to operate.
V
C
(Pin 13):
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.
SYNC (Pin 14):
The sync pin is used to synchronize the
internal oscillator to an external signal. It is directly logic
compatible and can be driven with any signal between
20% and 80% duty cycle. The synchronizing range is
equal to
initial
operating frequency, up to 1.4MHz. See
Synchronization section in Applications Information for
details. When not in use, this pin should be grounded.
3436fa
4
U W
1.2
1.4
3436 G07
Input Supply Current
1200
1000
800
600
400
200
0
MINIMUM
INPUT
VOLTAGE
T
A
= 25°C
4.0
3.5
Current Limit Foldback
T
A
= 25°C
SWITCH CURRENT
3.0
2.5
2.0
1.5
1.0
0.5
10
20
30
40
FB INPUT CURRENT (µA)
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
3436 G08
0
0
0.2
1.0
0.4
0.6
0.8
FEEDBACK VOLTAGE (V)
0
1.2
3436 G09
U
U
U
LT3436
BLOCK DIAGRAM
The LT3436 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
INPUT
2.5V BIAS
REGULATOR
SYNC
SHUTDOWN
COMPARATOR
1.35V
3µA
ERROR
AMPLIFIER
g
m
= 850µMho
V
C
Figure 1. Block Diagram
–
SHDN
+
W
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.
INTERNAL
V
CC
SLOPE COMP
Σ
0.3V
800kHz
OSCILLATOR
S
CURRENT
COMPARATOR
R
S
FLIP-FLOP
R
CURRENT SENSE
AMPLIFIER VOLTAGE
GAIN = 40
DRIVER
CIRCUITRY
SW
Q1
POWER
SWITCH
+
–
7µA
+
0.005Ω
–
+
–
FB
1.2V
GND
3436 F01
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