LT3433EFE [Linear]
High Voltage Step-Up/Step-Down DC/DC Converter; 高电压升压/降压型DC / DC转换器
| 型号： | LT3433EFE |
| 厂家： | 
Linear |
| 描述： | High Voltage Step-Up/Step-Down DC/DC Converter |
| 文件： | 总12页 (文件大小：184K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Final Electrical Specifications
LT3433
High Voltage
Step-Up/Step-Down
DC/DC Converter
September 2003
U
FEATURES
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.
■
Automatic Step-Up and Step-Down Conversion
■
Uses a Single Inductor
■
Wide 4V to 60V Input Voltage Range
■
VOUT from 3.3V to 20V
■
Dual Internal 500mA Switches
■
100µA No-Load Quiescent Current
■
Low Current Shutdown
■
±1% Output Voltage Accuracy
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.
■
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
■
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APPLICATIO S
Current limit foldback and frequency foldback help pre-
vent inductor current runaway during start-up. Program-
mablesoft-starthelpspreventoutputovershootatstart-up.
■
12V Automotive Systems
■
Wall Adapter Powered Systems
■
Battery Power Voltage Buffering
The LT3433 is available in a 16-lead thermally enhanced
TSSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
U
TYPICAL APPLICATIO
4V to 60V to 5V DC/DC Converter
with Burst Mode Operation
V
OUT
5V
L1
100µH
4V ≤ V ≤ 8.5V: 125mA
8.5V ≤ V ≤ 60V: 350mA
IN
B160A
Maximum Output
Current vs VIN
CoEv DU1352-101M
IN
Efficiency
500
400
300
200
90
80
70
60
50
40
30
20
B120A
1N4148
47µF
V
= 5V
OUT
V
SW_L
BST
BUCK
0.1µF
V
V
IN
= 13.8V
IN
SW_H PWRGND
LT3433
4V TO 60V
V
V
OUT
IN
+
1N4148
0.1µF
2.2µF
BURST_EN
V
V
IN
= 4V
BIAS
1nF
100pF
BRIDGED
V
V
SHDN
SS
C
68k
100
0
FB
100k
0.5%
305k
0.5%
SGND
0.01µF
0
20
30
(V)
40
50
60
10
0.1
1
10
100
1000
V
IN
OUTPUT CURRENT (mA)
3433 TA01
3433 TA01c
3433 TA01b
3433ia
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 notinfringe onexisting patent rights.
1
LT3433
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ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
TOP VIEW
Input Supply (VIN) .................................... –0.3V to 60V
Boosted Supply (VBST) .............. –0.3V to VSW_H + 30V
(VBST(MAX) = 80V)
ORDER PART
SGND
1
2
3
4
5
6
7
8
16
SGND
NUMBER
V
15 SW_L
BST
LT3433EFE
LT3433IFE
SW_H
14
13
12
11
10
9
PWRGND
Internal Supply (VBIAS) ............................. –0.3V to 30V
SW_H Switch Voltage.................................. –2V to 60V
SW_L Switch Voltage ............................... –0.3V to 30V
Feedback Voltage (VFB)............................... –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
V
IN
V
V
OUT
17
BURST_EN
BIAS
V
C
SHDN
SS
V
FB
FE PART MARKING
SGND
SGND
3433EFE
3433IFE
FE PACKAGE
16-LEAD PLASTIC TSSOP
TJMAX = 125°C, θJA = 40°C/W, θJC = 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 ● denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
VIN = 13.8V, VFB = 1.25V, VOUT = 5V, VBURST_EN = 0V, VBST – VIN = 5V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
60
UNITS
V
V
Operating Voltage Range
Undervoltage Lockout
Undervoltage Lockout Hysteresis
Operating Voltage Range
Operating Voltage Range
●
●
4
V
V
IN
Enable Threshold
3.4
3.95
IN(UVLO)
160
mV
V
V
V
●
3.3
3.3
20
OUT
BST
V
V
< V
– V
+ 20V
●
●
75
20
V
V
BST
BST
SW_H
SW_H
I
Normal Operation
Burst Mode Operation
Shutdown
(Notes 2, 3)
●
●
●
580
100
10
940
190
25
µA
µA
µA
VIN
V
V
< 0.6V
< 0.4V
VC
SHDN
V
Internal Supply Output Voltage
Operating Voltage Range
●
●
●
2.6
2.9
20
V
V
BIAS
I
Normal Operation
Burst Mode Operation
Shutdown
660
0.1
0.1
4.5
990
µA
µA
µA
VBIAS
V
V
< 0.6V
VC
< 0.4V
SHDN
Short-Circuit Current Limit
mA
R
R
Boost Supply Switch On-Resistance
Output Supply Switch On-Resistance
Shutdown Pin Thresholds
I
I
= 500mA
= 500mA
●
●
0.8
0.6
1.2
1
Ω
Ω
SWH(ON)
SWL(ON)
SHDN
SW
SW
V
Disable
Enable
●
●
0.4
V
V
1
I
I
I
/I
Boost Supply Switch Drive Current
Output Supply Switch Drive Current
Switch Current Limit
High Side Switch On, I = 500mA
●
●
●
30
30
50
50
0.9
mA/A
mA/A
A
VBST SW
SW
/I
Low Side Switch On, I = 500mA
SW
VOUT SW
0.5
3
0.7
LIM
Foldback Current Limit
V
= 0V
0.35
5
A
FB
I
Soft-Start Output Current
●
●
9
µA
SS
V
Feedback Reference Voltage
1.224
1.215
1.231
1.238
1.245
V
V
FB
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LT3433
ELECTRICAL CHARACTERISTICS
The ● denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
VIN = 13.8V, VFB = 1.25V, VOUT = 5V, VBURST_EN = 0V, VBST – VIN = 5V, unless otherwise noted.
SYMBOL
∆V
PARAMETER
CONDITIONS
5.5V ≤ V ≤ 60V
MIN
TYP
0.002
35
MAX
0.01
100
UNITS
%/V
nA
Feedback Reference Line Regulation
●
●
●
FB
IN
I
V
Pin Input Bias Current
FB
FB
g
Error Amplifier Transconductance
Error Amplifier Voltage Gain
200
270
66
330
umhos
dB
m
A
V
I
f
/V
Control Voltage to Switch Transconductance
Operating Frequency
0.55
200
A/V
SW VC
O
V
V
> 1V
= 0V
185
170
215
230
kHz
kHz
FB
FB
●
Foldback Frequency
50
kHz
ns
t
t
Minimum Switch On Time
Minimum Switch Off Time
R = 35Ω (Note 4)
●
●
250
500
450
800
ON(MIN)
OFF(MIN)
L
R = 35Ω (Note 4)
ns
L
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 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 3: “Normal Operation” supply current specification does not include
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.
I
currents. Powering the V
pin externally reduces I supply
BIAS
BIAS CC
current.
Note 4: Minimum times are tested using the high side switch with a 35Ω
load to ground.
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TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Output Current
vs VIN
VBIAS Output Voltage
vs Temperature
VIN Supply Current
vs VIN Supply Voltage
620
590
560
530
500
2.8
2.6
2.4
2.2
500
400
300
200
V
= 5V
OUT
T
A
= 25°C
BUCK
BRIDGED
100
0
SEE TYPICAL APPLICATION
ON THE FIRST PAGE OF
THIS DATA SHEET
–50
0
50
100 125
0
15
30
(V)
45
60
0
20
30
(V)
40
50
60
10
TEMPERATURE (°C)
V
V
IN
IN
3433 G01
3433 G02
3433 G11
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LT3433
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TYPICAL PERFOR A CE CHARACTERISTICS
Error Amp Reference
vs Temperature
Soft-Start Current vs Temperature
Switch Current Limit vs VFB
7.0
6.5
6.0
5.5
5.0
4.5
4.0
1.232
1.231
1.230
1.229
1.228
700
600
500
400
300
T
= 25°C
A
–50
0
50
100 125
0.2
0.6
(V)
0.8
–50
0
50
100 125
0
0.4
V
1.0
TEMPERATURE (°C)
TEMPERATURE (°C)
FB
3433 G03
3433 G04
3433 G05
Oscillator Frequency
vs Temperature
Switch Current Limit
vs Temperature
Oscillator Frequency vs VFB
210
205
200
195
190
750
725
700
675
650
200
150
100
50
T
= 25°C
A
–50
0
50
100 125
0
–50
0
50
100 125
TEMPERATURE (°C)
0
0.2
0.4
V
0.6
(V)
0.8
1.0
TEMPERATURE (°C)
3433 G06
FB
3433 G08
3433 G07
Maximum Boost Supply Switch
Drive Current vs Boost Supply
Voltage
Maximum Output Supply Switch
Drive Current vs Output Supply
Voltage
70
65
60
55
50
45
70
65
60
55
50
45
8
8
4
5
6
7
9
10 11 12
4
5
6
7
9
10 11 12
V
– V
(V)
V
(V)
OUT
BST
SW_H
3433 G09
3433 G10
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LT3433
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PI FU CTIO S
SGND (Pins 1, 8, 9, 16): Low Noise Ground Reference.
VFB (Pin7):ErrorAmplifierInvertingInput.Thenoninvert-
ing input of the error amplifier is connected to an internal
1.231V reference. The VFB pin is connected to a resistor
divider from the converter output. Values for the resistor
connected from VOUT to VFB (RFB1) and the resistor con-
nectedfromVFB toground(RFB2)canbecalculatedtopro-
gram converter output voltage (VOUT) via the following
relation:
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 VBST pin
to the SW_H pin. A 1µF capacitor is generally adequate for
most applications.
Thechargeonthebootstrapcapacitorisrefreshedthrough
a diode, typically connected from the converter output
(VOUT), during the switch-off period. Minimum off-time
operationassuresthattheboostcapacitorisrefreshedeach
switch cycle. The LT3433 supports operational VBST sup-
ply voltages up to 75V (absolute maximum) as referenced
to ground.
VOUT = 1.231 • (RFB1 + RFB2)/RFB2
The VFB 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:
SW_H (Pin 3): Boosted Switch Output. This is the current
returnfortheboostedswitchandcorrespondstotheemitter
of the switch transistor. The boosted switch shorts the
SW_H pin to the VIN supply when enabled. The drive cir-
cuitry for this switch is boosted above the VIN supply
through the VBST pin, allowing saturation of the switch for
maximum efficiency. The “ON” resistance of the boosted
switch is 0.8Ω.
∆VOUT(BIAS) = 35nA • RFB1
The voltage on VFB also controls the LT3433 oscillator
frequencythrougha“frequency-foldback”function.When
theVFB pinvoltageisbelow0.8V,theoscillatorrunsslower
than the 200kHz typical operating frequency. The oscilla-
torfrequencyslowswithreducedvoltageonthepin, down
to 50kHz when VFB = 0V.
The VFB pin voltage also controls switch current limit
througha“current-limitfoldback”function.AtVFB=0V,the
maximum switch current is reduced to half of the normal
value. The current limit value increases linearly until VFB
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.
VIN (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 VBIAS pin is not driven
externally. This supply will be subject to high switching
transientcurrentssothispinrequiresahighqualitybypass
capacitor that meets whatever application-specific input
ripple current requirements exist. See Applications Infor-
mation.
SS(Pin10):SoftStart. Connectacapacitor(CSS)fromthis
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.
BURST_EN (Pin 5): Burst Mode Enable Pin. Shorting this
pin to SGND enables Burst Mode operation. If Burst Mode
operationisnotdesired,connectingthispinto VBIASorVOUT
will disable the burst function.
VC (Pin 6): Error Amplifier Output. The voltage on the VC
pincorrespondstothemaximumswitchcurrentperoscil-
latorcycle. Theerroramplifieristypicallyconfiguredasan
integrator circuit by connecting an RC network from this
pin to ground. This circuit typically creates the dominant
polefortheconverterregulationfeedbackloop.Specificin-
tegratorcharacteristicscanbeconfiguredtooptimizetran-
sient response. See Applications Information.
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LT3433
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PI FU CTIO S
The time from VSS = 0V to maximum available current can
this pin should be driven above 3V to assure the internal
supply is completely disabled. This pin is typically diode-
connectedtotheconverteroutputtomaximizeconversion
efficiency. This pin must be bypassed with at least a 0.1µF
ceramic capacitor to SGND.
be calculated given a capacitor CSS as:
tSS = (2.7 • 105)CSS or 0.27s/µF
SHDN (Pin 11): Shutdown. If the SHDN pin is externally
pulledbelow0.5V,lowcurrentshutdownmodeisinitiated.
Duringshutdownmode,allinternalfunctionsaredisabled,
and ICC is reduced to 10µA. This pin is intended to receive
a digital input, however, there is a small amount of input
hysteresisbuiltintotheSHDNcircuittohelpassureglitch-
free mode switching. If shutdown is not desired, connect
the SHDN pin to VIN.
V
OUT (Pin 13): Converter Output Pin. This pin voltage is
compared with the voltage on VIN internally to control
operation in single or 2-switch mode. When the ratios of
thetwovoltagesaresuchthata>75%dutycycleisrequired
forregulation,thelowsideswitchisenabled.Drivebiasfor
the low side switch is also derived directly from this pin.
PWRGND (Pin 14): High Current Ground Reference. This
isthecurrentreturnforthelowsideswitchandcorresponds
to the emitter of the low side switch transistor.
VBIAS (Pin 12): Internal Local Supply. Much of the LT3433
circuitry is powered from this supply, which is internally
regulated to 2.5V through an on-board linear regulator.
CurrentdriveforthisregulatorissourcedfromtheVIN pin.
The VBIAS supply is short-circuit protected to 5mA.
SW_L(Pin15):GroundReferencedSwitchOutput.Thispin
is the collector of the low side switch transistor. The low
sideswitchshortstheSW_LpintoPWRGNDwhenenabled.
The series impedance of the ground-referenced switch is
0.6Ω.
The VBIAS supply only sources current, so forcing this pin
abovetheregulatedvoltageallowstheuseofexternalpower
formuchoftheLT3433circuitry.Whenusingexternaldrive,
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LT3433
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BLOCK DIAGRA
V
BIAS
12
BURST
CONTROL
CIRCUITS
BIAS
BURST_EN
5
4
1.25V
V
IN
SENSE
AMPLIFIER
V
BST
2
3
COMPARATOR
BOOSTED
DRIVER
SW_H
SWITCH
CONTROL
LOGIC
SLOPE
COMP
OSCILLATOR 200kHz
SW_L
GND
15
14
7
FREQUENCY
CONTROL
DRIVER
MODE
CONTROL
V
FB
ERROR
AMPLIFIER
30%
LOAD
1.231V
V
C
6
SHDN
+
–
11
Burst Mode
CONTROL
SHUTDOWN
15%
LOAD
0.7V
SS
10
13
5µA
V
OUT
+
SGND
1, 8, 9,16
3433 BD
V
OUT
3433ia
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LT3433
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APPLICATIO S I FOR ATIO
Overview
condition as requiring a duty cycle greater than 75%. If
suchaconditionexists, asecondswitchisenabledduring
theswitchontime,whichactstopulltheoutputsideofthe
inductor to ground. This “bridged” operation allows volt-
age conversion to continue when VOUT approaches or
exceeds VIN.
The LT3433 is a high input voltage range, step-up/step-
down DC/DC converter IC using a 200kHz constant fre-
quency, currentmodearchitecture. Dualinternalswitches
allow the full input voltage to be imposed across the
switched inductor, such that both step-up and step-down
modes of operation can be realized using the same single
inductor topology.
Shutdown
The LT3433 incorporates a low current shutdown mode
where all IC functions are disabled and the VIN current is
reduced to 10µA. Pulling the SHDN pin down to 0.4V or
less activates shutdown mode.
The LT3433 has provisions for high efficiency, low load
operation for battery-powered applications. Burst Mode
operation reduces average quiescent current to 100µA in
noloadconditions.Alowcurrentshutdownmodecanalso
be activated, reducing total quiescent current to 10µA.
Burst Mode Operation
Much of the LT3433’s internal circuitry is biased from an
internal low voltage linear regulator. The output of this
regulatorisbroughtouttotheVBIAS pin, allowingbypass-
ing of the internal regulator. The associated internal
circuitry can be powered directly from the output of the
converter, increasing overall converter efficiency. Using
externally derived power also eliminates the IC’s power
dissipation associated with the internal VIN to VBIAS
regulator.
TheLT3433employslowcurrentBurstModefunctionality
to maximize efficiency during no load and low load condi-
tions. Burst Mode function is disabled by shorting the
BURST_EN pin to either VBIAS or VOUT. Burst Mode
function is enabled by shorting BURST_EN to SGND.
When the required switch current, sensed via the VC pin
voltage, is below 30% of maximum, the Burst Mode
functionisemployed.WhenthevoltageonVC dropsbelow
the 30% load level, that level of sense current is latched
intotheIC. Iftheoutputloadrequireslessthanthislatched
currentlevel,theconverterwilloverdrivetheoutputslightly
during each switch cycle. This overdrive condition forces
the voltage on the VC pin to continue to drop. When the
voltage on VC drops below the 15% load level, switching
isdisabled,andtheLT3433shutsdownmostofitsinternal
circuitry, reducing quiescent current to 100µA. When the
voltage on the VC pin climbs back to 20% load level, the IC
returns to normal operation and switching resumes.
Theory of Operation (See Block Diagram)
The LT3433 senses converter output voltage via the VFB
pin. The difference between the voltage on this pin and an
internal 1.231V reference is amplified to generate an error
voltage on the VC pin which is, in turn, used as a threshold
for the current sense comparator.
During normal operation, the LT3433 internal oscillator
runs at 200kHz. At the beginning of each oscillator cycle,
the switch drive is enabled. The switch drive stays enabled
until the sensed switch current exceeds the VC-derived
threshold for the current sense comparator and, in turn,
disables the switch driver. If the current comparator
threshold is not obtained for the entire oscillator cycle, the
switch driver is disabled at the end of the cycle for 250ns.
This minimum off-time mode of operation assures regen-
eration of the VBST bootstrapped supply.
Antislope Compensation
Most current mode switching controllers use slope com-
pensationtopreventcurrentmodeinstability. TheLT3433
is no exception. A slope compensation circuit imposes an
artificial ramp on the sensed current to increase the rising
slope as duty cycle increases. Unfortunately, this addi-
tional ramp corrupts the sensed current value, reducing
the achievable current limit value by the same amount as
the added ramp represents. As such, current limit is
typically reduced as duty cycles increase.
If the converter input and output voltages are close
together, proper operation in normal buck configuration
would require high duty cycles. The LT3433 senses this
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LT3433
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APPLICATIO S I FOR ATIO
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TheLT3433containscircuitrytoeliminatethecurrentlimit
reduction associated with slope-compensation, or anti-
slope compensation. As the slope compensation ramp is
addedtothesensedcurrent, asimilarrampisaddedtothe
current limit threshold reference. The end result is that
current limit is not compromised so the LT3433 can
provide full power regardless of required duty cycle.
switch current will be reduced by this required drive
current.
IDRIVE = DC • 2 • ISW(MAX) • ISWDRIVE(MAX)
Using 50mA/A for the required drive current for each
switch yields the portion of switch current used to drive
the switches is:
I
SW(DRIVE) = DC • 2 • ISW(MAX) • 0.05/(1 – DC)
Mode Switching
Removing drive currents from the available maximum
switch current yields:
The LT3433 senses operational duty cycle by directly
monitoring VIN and VOUT. Voltage drops associated with
pass and catch diodes are estimated internally such that
mode switching occurs when the duty cycle required for
continuous buck operation is greater than 75%. If such a
condition exists, a second switch is enabled during the
switch on time, changing operation to a dual switch
bridgedconfiguration.Becausethevoltageavailableacross
the switched inductor is greater in bridged mode, duty
cycle will decrease.
ISW(MAX)' = ISW(MAX) • [1 – DC • 2 • ISW(MAX)
•
0.05/(1 – DC)]
where ISW(MAX)' is maximum switch current available to
the load during bridged operation. The maximum load
current can then be calculated as:
ILOAD(MAX) = ISW(MAX)' • (1 – DC)
which reduces to:
The output current in bridged mode is not continuous, so
switch currents are considerably higher than while oper-
ating in buck mode. In order to maximize available output
power, continuous operation and low ripple currents are
recommended. Switch currents will increase by a factor of
1/(1–DC)duringbridgedmode,sothismodeofoperation
is typically the gating item for converter drive capability.
ILOAD(MAX) = [0.5A – (∆IL/2)] • (1 – 1.1 • DC)
Design Equations
V
IN
SW_H
LT3433
SW_L
IOUT(MAX) = ISW(MAX) • (1 – DC)
= [0.5A – (∆IL / 2)] • (1 – DC)
L
V
OUT
where ∆IL is the ripple current in the inductor.
It is also important to note that IOUT cannot be considered
equivalent to ILOAD during bridged operation. Most of the
converter’s switch drive power is derived from the gener-
ated output supply, so IOUT must also accommodate this
current requirement. During single-switch buck opera-
tional conditions, switch drive current is negligible in
terms of output current; however, during bridged opera-
tion, these currents can become significant. These output
derived switch drive currents will increase the current
loading on VIN by the same 1/(1 – DC) factor as the switch
currents. Asmaximumswitchcurrentisreferencedtothat
coming from the VIN supply, the available maximum
3433 AI01
Constants:
VSWH = voltage drop across boosted switch
VSWL = voltage drop across grounded switch
VF = forward drop of external Schottky diodes
f0 = operating frequency
Duty Cycle (continuous operation):
DCBUCK = (VOUT + 2VF)/(VIN – VSWH + VF)
DCBRIDGED = (VOUT + 2VF)/(VOUT + VIN + 2VF – VSWH
– VSWL
)
3433ia
9
LT3433
W U U
U
APPLICATIO S I FOR ATIO
Discontinuousoperationoccurswhentheripplecurrentin
Ripple current:
the inductor is greater than twice the load current (ILOAD
)
V
OUT
+ 2V • 1− DC
in buck mode, or greater than ILOAD/(1 – DC) during
bridged mode. Current mode instability is not a concern
duringdiscontinuousoperationsoinductorvaluessmaller
than LMIN can be used. If such a small inductor is used,
however, it must be assured that the converter never
enters continuous operation at duty cycles greater than
50% to prevent current mode instability.
(
=
F) (
)
∆IL(P−P)
L• fO
Inductor Selection
TheprimarycriterionforinductorvalueselectioninLT3433
applications is the ripple current created in that inductor.
Design considerations for ripple current are the amount of
output ripple and the ability of the internal slope compen-
sation waveform to prevent current mode instability.
Design Example
V
IN(MIN) = 4V, VOUT = 5V, L = 150µH
The LT3433 maximizes available dynamic range using a
slope compensation generator that generates a continu-
ously increasing slope as duty cycle increases. The slope
compensationwaveformiscalibratedat80%dutycycleto
compensate for ripple currents up to 12.5% of IMAX, or
~60mA.
Using VF = 0.75V yields:
DC = (VOUT + 2VF)/(VOUT + VIN + 2VF – VSWH – VSWL
= (5V + 1.5V)/(4V + 5V + 1.5V – 0.6V – 0.5V)
= 0.69
∆IL = (VOUT + 2VF) • (1 – DC) • (L • f0)–1
= (5V + 1.5V) • (1 – 0.69) • (150µH • 200kHz)–1
= 67mA
)
Ripple current can be calculated as:
V
OUT
+ 2V • 1− DC
(
=
F) (
)
∆IL(P−P)
L• fO
ILOAD(MAX) = ISW(MAX) • (1 – 1.1 • DC)
= [0.5A – (1/2 • 0.07)](1 – 1.1 • 0.69) = 0.112A
This relation can be used to determine minimum induc-
tance sizes for various values of VOUT using the DC = 80%
calibration:
LMIN = (VOUT + 1.5V) • (1 – 0.8) 60mA • 200kHz)
V
OUT
L
MIN
4V
5V
92µH
108µH
175µH
225µH
9V
12V
3433ia
10
LT3433
U
TYPICAL APPLICATIO
Burst Only Low Noise 5V Maintenance Supply
D
D
S1
B160A
S2
B160A
L1
33µH
D1
1N4148
V
SW_L
BST
C1
0.1µF
SW_H PWRGND
LT3433
V
IN
4V TO 60V
V
V
IN
OUT
D2
1N4148
+
C7
2.2µF
BURST_EN
V
BIAS
C6 100pF
C2
0.1µF
V
C
SHDN
SS
V
FB
V
OUT
R2
510k
5%
R1
5V
IN
SHDN
OUT
BYP
SGND
C4
0.01µF
C5
2.2µF
2.2M
LT1761-5
10mA
5%
GND
C3
10µF
3433 TA03
U
PACKAGE DESCRIPTIO
FE Package
16-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation BB
4.90 – 5.10*
(.193 – .201)
3.58
(.141)
3.58
(.141)
16 1514 13 12 1110
9
6.60 ±0.10
4.50 ±0.10
2.94
(.116)
SEE NOTE 4
2.94
(.116)
6.40
BSC
0.45 ±0.05
1.05 ±0.10
0.65 BSC
5
7
8
1
2
3
4
6
RECOMMENDED SOLDER PAD LAYOUT
1.10
(.0433)
MAX
4.30 – 4.50*
(.169 – .177)
0° – 8°
0.65
(.0256)
BSC
0.45 – 0.75
0.09 – 0.20
0.05 – 0.15
(.018 – .030)
(.0036 – .0079)
(.002 – .006)
0.195 – 0.30
(.0077 – .0118)
FE16 (BB) TSSOP 0203
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS 4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
MILLIMETERS
(INCHES)
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
3433ia
11
LT3433
U
TYPICAL APPLICATIO
4V-60V to 5V at 100mA DC/DC Converter Burst Disabled
D
S2
D
L1
100µH
S1
B160A
B160A
V
OUT
5V
C6
47µF
100mA
D2
1N4148
V
SW_L
BST
C1
0.1µF
SW_H PWRGND
LT3433
V
IN
V
V
IN
OUT
4V TO 60V
D1
1N4148
+
C7
2.2µF
BURST_EN
V
BIAS
C5 1nF
C4 100pF
R1 68k
C2
0.1µF
V
C
SHDN
SS
V
FB
R3
305k
0.5%
R2
C3
0.01µF
SGND
100k
0.5%
3433 TA02
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V : 1.5V to 80V, V
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I
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3433ia
LT/TP 0903 1K REV A • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
12
LINEAR TECHNOLOGY CORPORATION 2003
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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