Final Electrical Specifications
LT3433
High Voltage
Step-Up/Step-Down
DC/DC Converter
September 2003
FEATURES
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DESCRIPTION
The LT
®
3433 is a 200kHz fixed-frequency current mode
switching regulator that provides both step-up and step-
down regulation using a single inductor. The IC operates
over a 4V to 60V input voltage range making it suitable for
use in various wide input voltage range applications such
as automotive electronics that must withstand both load
dump and cold crank conditions.
Internal control circuitry monitors system conditions and
converts from single switch buck operation to dual switch
bridged operation when required, seamlessly changing
between step-down and step-up voltage conversion.
Optional Burst Mode
®
operation reduces no-load quies-
cent current to 100µA and maintains high efficiencies with
light loads.
Current limit foldback and frequency foldback help pre-
vent inductor current runaway during start-up. Program-
mable soft-start helps prevent output overshoot at start-up.
The LT3433 is available in a 16-lead thermally enhanced
TSSOP package.
Automatic Step-Up and Step-Down Conversion
Uses a Single Inductor
Wide 4V to 60V Input Voltage Range
V
OUT
from 3.3V to 20V
Dual Internal 500mA Switches
100µA No-Load Quiescent Current
Low Current Shutdown
±1%
Output Voltage Accuracy
200kHz Operating Frequency
Boosted Supply Pin to Saturate High Side Switch
Frequency Foldback Protection
Current Limit Foldback Protection
Current Limit Unaffected by Duty Cycle
16-lead Thermally Enhanced TSSOP Package
APPLICATIO S
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s
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12V Automotive Systems
Wall Adapter Powered Systems
Battery Power Voltage Buffering
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
L1
100µH
CoEv DU1352-101M
4V to 60V to 5V DC/DC Converter
with Burst Mode Operation
B160A
V
OUT
5V
4V
≤
V
IN
≤
8.5V: 125mA
8.5V
≤
V
IN
≤
60V: 350mA
Maximum Output
Current vs V
IN
500
V
OUT
= 5V
MAXIMUM OUTPUT CURRENT (mA)
BUCK
400
1N4148
V
BST
V
IN
4V TO 60V
0.1µF
SW_H
V
IN
1nF100pF
68k
V
FB
100k
0.5%
305k
0.5%
SGND
0.01µF
SS
BURST_EN
V
C
PWRGND
V
OUT
LT3433
SW_L
B120A
47µF
+
2.2µF
300
EFFICIENCY (%)
1N4148
V
BIAS
0.1µF
SHDN
200
BRIDGED
100
0
0
10
20
3433 TA01
30
V
IN
(V)
40
50
60
3433 TA01c
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
U
U
Efficiency
90
80
V
IN
= 13.8V
70
60
50
40
30
20
0.1
V
IN
= 4V
10
100
1
OUTPUT CURRENT (mA)
1000
3433 TA01b
3433ia
1
LT3433
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
SGND
V
BST
SW_H
V
IN
BURST_EN
V
C
V
FB
SGND
1
2
3
4
5
6
7
8
17
16 SGND
15 SW_L
14 PWRGND
13 V
OUT
12 V
BIAS
11 SHDN
10 SS
9
SGND
Input Supply (V
IN
) .................................... –0.3V to 60V
Boosted Supply (V
BST
) .............. –0.3V to V
SW_H
+ 30V
(V
BST(MAX)
= 80V)
Internal Supply (V
BIAS
) ............................. – 0.3V to 30V
SW_H Switch Voltage .................................. – 2V to 60V
SW_L Switch Voltage ............................... – 0.3V to 30V
Feedback Voltage (V
FB
) ............................... – 0.3V to 5V
Operating Junction Temperature Range (Note 5)
LT3433E (Note 6) ............................ – 40°C to 125°C
LT3433I ........................................... – 40°C to 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
NUMBER
LT3433EFE
LT3433IFE
FE PART MARKING
3433EFE
3433IFE
FE PACKAGE
16-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 40°C/W,
θ
JC
= 10°C/W
EXPOSED PAD (PIN 17) MUST BE SOLDERED TO SGND
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications that apply over the full operating temperature range, otherwise specifications are at T
A
= 25°C.
V
IN
= 13.8V, V
FB
= 1.25V, V
OUT
= 5V, V
BURST_EN
= 0V, V
BST
– V
IN
= 5V, unless otherwise noted.
SYMBOL
V
IN
V
IN(UVLO)
V
OUT
V
BST
I
VIN
PARAMETER
Operating Voltage Range
Undervoltage Lockout
Undervoltage Lockout Hysteresis
Operating Voltage Range
Operating Voltage Range
Normal Operation
Burst Mode Operation
Shutdown
Internal Supply Output Voltage
Operating Voltage Range
I
VBIAS
Normal Operation
Burst Mode Operation
Shutdown
Short-Circuit Current Limit
Boost Supply Switch On-Resistance
Output Supply Switch On-Resistance
Shutdown Pin Thresholds
Boost Supply Switch Drive Current
Output Supply Switch Drive Current
Switch Current Limit
Foldback Current Limit
I
SS
V
FB
Soft-Start Output Current
Feedback Reference Voltage
q
q
CONDITIONS
q
MIN
4
q
TYP
3.4
160
MAX
60
3.95
20
75
20
UNITS
V
V
mV
V
V
V
µA
µA
µA
V
V
µA
µA
µA
mA
Ω
Ω
V
V
mA/A
mA/A
A
A
µA
V
V
3433ia
Enable Threshold
3.3
3.3
580
100
10
2.6
660
0.1
0.1
4.5
0.8
0.6
0.4
V
BST
< V
SW_H
+ 20V
V
BST
– V
SW_H
(Notes 2, 3)
V
VC
< 0.6V
V
SHDN
< 0.4V
q
q
q
q
q
q
q
q
940
190
25
2.9
20
990
V
BIAS
V
VC
< 0.6V
V
SHDN
< 0.4V
I
SW
= 500mA
I
SW
= 500mA
Disable
Enable
High Side Switch On, I
SW
= 500mA
Low Side Switch On, I
SW
= 500mA
V
FB
= 0V
q
q
q
q
q
q
q
q
R
SWH(ON)
R
SWL(ON)
V
SHDN
I
VBST
/I
SW
I
VOUT
/I
SW
I
LIM
1.2
1
1
30
30
0.5
3
1.224
1.215
0.7
0.35
5
1.231
50
50
0.9
9
1.238
1.245
2
U
W
U
U
W W
W
LT3433
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications that apply over the full operating temperature range, otherwise specifications are at T
A
= 25°C.
V
IN
= 13.8V, V
FB
= 1.25V, V
OUT
= 5V, V
BURST_EN
= 0V, V
BST
– V
IN
= 5V, unless otherwise noted.
SYMBOL
∆V
FB
I
FB
g
m
A
V
I
SW
/V
VC
f
O
PARAMETER
Feedback Reference Line Regulation
V
FB
Pin Input Bias Current
Error Amplifier Transconductance
Error Amplifier Voltage Gain
Control Voltage to Switch Transconductance
Operating Frequency
Foldback Frequency
t
ON(MIN)
t
OFF(MIN)
Minimum Switch On Time
Minimum Switch Off Time
V
FB
> 1V
q
CONDITIONS
5.5V
≤
V
IN
≤
60V
q
q
q
MIN
TYP
0.002
35
MAX
0.01
100
330
UNITS
%/V
nA
umhos
dB
A/V
200
270
66
0.55
185
170
200
50
215
230
450
800
kHz
kHz
kHz
ns
ns
V
FB
= 0V
R
L
= 35Ω (Note 4)
R
L
= 35Ω (Note 4)
q
q
250
500
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
Supply current specification does not include switch drive
currents. Actual supply currents will be higher.
Note 3:
“Normal Operation” supply current specification does not include
I
BIAS
currents. Powering the V
BIAS
pin externally reduces I
CC
supply
current.
Note 4:
Minimum times are tested using the high side switch with a 35Ω
load to ground.
Note 5:
This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 6:
The LT3433E 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. The
LT3433I is guaranteed over the full –40°C to 125°C operating junction
temperature range.
TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Output Current
vs V
IN
500
V
OUT
= 5V
MAXIMUM OUTPUT CURRENT (mA)
BUCK
2.8
V
BIAS
OUTPUT VOLTAGE (V)
400
I
VIN
(µA)
300
200
BRIDGED
100
SEE TYPICAL APPLICATION
ON THE FIRST PAGE OF
THIS DATA SHEET
0
10
20
30
V
IN
(V)
40
50
60
3433 G11
0
U W
V
BIAS
Output Voltage
vs Temperature
620
V
IN
Supply Current
vs V
IN
Supply Voltage
T
A
= 25°C
590
2.6
560
2.4
530
2.2
–50
500
0
50
TEMPERATURE (°C)
100
125
0
15
30
V
IN
(V)
45
60
3433 G02
3433 G01
3433ia
3
LT3433
TYPICAL PERFOR A CE CHARACTERISTICS
Soft-Start Current vs Temperature
7.0
6.5
6.0
1.232
SWITCH CURRENT LIMIT (mA)
ERROR AMP REFERENCE (V)
I
SS
(µA)
5.5
5.0
4.5
4.0
–50
0
50
TEMPERATURE (°C)
Oscillator Frequency
vs Temperature
210
OSCILLATOR FREQUENCY (kHz)
205
OSCILLATOR FREQUENCY (kHz)
200
CURRENT LIMIT (mA)
195
190
–50
0
50
TEMPERATURE (°C)
Maximum Boost Supply Switch
Drive Current vs Boost Supply
Voltage
70
70
65
I
VOUT
/I
SW
(mA/A)
I
BST
/I
SW
(mA/A)
60
55
50
45
4
5
6
9
10
V
BST
– V
SW_H
(V)
7
8
11
12
4
U W
100
3433 G03
Error Amp Reference
vs Temperature
700
Switch Current Limit vs V
FB
T
A
= 25°C
1.231
600
1.230
500
1.229
400
125
1.228
–50
300
0
50
TEMPERATURE (°C)
100
125
0
0.2
0.6
0.4
V
FB
(V)
0.8
1.0
3433 G05
3433 G04
Oscillator Frequency vs V
FB
200
T
A
= 25°C
150
725
750
Switch Current Limit
vs Temperature
100
700
50
675
100
125
0
0
0.2
3433 G06
0.6
0.4
V
FB
(V)
0.8
1.0
3433 G07
650
–50
0
50
TEMPERATURE (°C)
100
125
3433 G08
Maximum Output Supply Switch
Drive Current vs Output Supply
Voltage
65
60
55
50
45
4
5
6
7
8
V
OUT
(V)
3433 G09
3433 G10
9
10
11
12
3433ia
LT3433
PI FU CTIO S
SGND (Pins 1, 8, 9, 16):
Low Noise Ground Reference.
V
BST
(Pin 2):
Boosted Switch Supply. This “boosted” sup-
ply rail is referenced to the SW_H pin. Supply voltage is
maintained by a bootstrap capacitor tied from the V
BST
pin
to the SW_H pin. A 1µF capacitor is generally adequate for
most applications.
The charge on the bootstrap capacitor is refreshed through
a diode, typically connected from the converter output
(V
OUT
), during the switch-off period. Minimum off-time
operation assures that the boost capacitor is refreshed each
switch cycle. The LT3433 supports operational V
BST
sup-
ply voltages up to 75V (absolute maximum) as referenced
to ground.
SW_H (Pin 3):
Boosted Switch Output. This is the current
return for the boosted switch and corresponds to the emitter
of the switch transistor. The boosted switch shorts the
SW_H pin to the V
IN
supply when enabled. The drive cir-
cuitry for this switch is boosted above the V
IN
supply
through the V
BST
pin, allowing saturation of the switch for
maximum efficiency. The “ON” resistance of the boosted
switch is 0.8Ω.
V
IN
(Pin 4):
Input Power Supply. This pin supplies power
to the boosted switch and corresponds to the collector of
the switch transistor.This pin also supplies power to most
of the IC’s internal circuitry if the V
BIAS
pin is not driven
externally. This supply will be subject to high switching
transient currents so this pin requires a high quality bypass
capacitor that meets whatever application-specific input
ripple current requirements exist. See Applications Infor-
mation.
BURST_EN (Pin 5):
Burst Mode Enable Pin. Shorting this
pin to SGND enables Burst Mode operation. If Burst Mode
operation is not desired, connecting this pin to V
BIAS
or V
OUT
will disable the burst function.
V
C
(Pin 6):
Error Amplifier Output. The voltage on the V
C
pin corresponds to the maximum switch current per oscil-
lator cycle. The error amplifier is typically configured as an
integrator circuit by connecting an RC network from this
pin to ground. This circuit typically creates the dominant
pole for the converter regulation feedback loop. Specific in-
tegrator characteristics can be configured to optimize tran-
sient response. See Applications Information.
V
FB
(Pin 7):
Error Amplifier Inverting Input. The noninvert-
ing input of the error amplifier is connected to an internal
1.231V reference. The V
FB
pin is connected to a resistor
divider from the converter output. Values for the resistor
connected from V
OUT
to V
FB
(R
FB1
) and the resistor con-
nected from V
FB
to ground (R
FB2
) can be calculated to pro-
gram converter output voltage (V
OUT
) via the following
relation:
V
OUT
= 1.231 • (R
FB1
+ R
FB2
)/R
FB2
The V
FB
pin input bias current is 35nA, so use of extremely
high value feedback resistors could cause a converter
output that is slightly higher than expected. Bias current
error at the output can be estimated as:
∆V
OUT(BIAS)
= 35nA • R
FB1
The voltage on V
FB
also controls the LT3433 oscillator
frequency through a “frequency-foldback” function. When
the V
FB
pin voltage is below 0.8V, the oscillator runs slower
than the 200kHz typical operating frequency. The oscilla-
tor frequency slows with reduced voltage on the pin, down
to 50kHz when V
FB
= 0V.
The V
FB
pin voltage also controls switch current limit
through a “current-limit foldback” function. At V
FB
= 0V, the
maximum switch current is reduced to half of the normal
value. The current limit value increases linearly until V
FB
reaches 0.6V when the normal maximum switch current
level is restored. The frequency and current-limit foldback
functions add robustness to short-circuit protection and
help prevent inductor current runaway during start-up.
SS (Pin 10):
Soft Start. Connect a capacitor (C
SS
) from this
pin to ground. The output voltage of the LT3433 error
amplifier corresponds to the peak current sense amplifier
output detected before resetting the switch output(s). The
soft-start circuit forces the error amplifier output to a zero
peak current for start-up. A 5µA current is forced from the
SS pin onto an external capacitor. As the SS pin voltage
ramps up, so does the LT3433 internally sensed peak cur-
rent limit. This forces the converter output current to ramp
from zero until normal output regulation is achieved. This
function reduces output overshoot on converter start-up.
U
U
U
3433ia
5
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