LT1578/LT1578-2.5
1.5A, 200kHz Step-Down
Switching Regulator
FEATURES
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DESCRIPTIO
1.5A Switch Current
High Efficiency—Low Loss 0.2Ω Switch
Constant 200kHz Switching Frequency
4V to 15V Input VoltageRange
Minimum Output: 1.21V
Low Supply Current: 1.9mA
Low Shutdown Current: 20µA
Easily Synchronizable Up to 400kHz
Cycle-by-Cycle Current Limit
Reduced EMI Generation
Low Thermal Resistance SO-8 Package
Uses Small Low Value Inductors
The LT
®
1578 is a 200kHz monolithic buck mode switching
regulator. A 1.5A switch is included on the die along with
all the necessary oscillator, control and logic circuitry. The
topology is current mode for fast transient response and
good loop stability. The LT1578 is a modified version of the
LT1507 that has been optimized for noise sensitive appli-
cations. It will operate over a 4V to 15V input range.
In addition, the reference voltage has been lowered to al-
low the device to produce output voltages down to 1.2V.
Quiescent current has been reduced by a factor of two.
Switch on resistance has been reduced by 30%. Switch tran-
sition times have been slowed to reduce EMI generation.
The oscillator frequency has been reduced to 200kHz to
maintain high efficiency over a wide output current range.
The pinout has been changed to improve PC layout by al-
lowing the high current, high frequency switching circuitry
to be easily isolated from low current, noise sensitive con-
trol circuitry. The new SO-8 package includes a fused
ground lead that significantly reduces the thermal resistance
of the device to extend the ambient operating temperature
range. Standard surface mount external parts can be used
including the inductor and capacitors.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
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Portable Computers
Battery-Powered Systems
Battery Chargers
Distributed Power Systems
TYPICAL APPLICATION
Efficiency vs Load Current
3.3V Buck Converter
INPUT
5V TO 15V
C3*
10µF TO
50µF
90
85
+
V
IN
BOOST
C2
0.33µF
V
SW
EFFICIENCY (%)
L1**
15µH
D2
1N914
OUTPUT**
3.3V, 1.25A
80
75
70
65
60
55
50
0
0.25
0.50 0.75 1.00
LOAD CURRENT (A)
1.25
1.50
V
OUT
= 3.3V
V
IN
= 5V
L = 25µH
LT1578
OPEN = ON
SHDN
GND
FB
V
C
C
C
100pF
D1
1N5818
R1
8.66k
R2
4.99k
* RIPPLE CURRENT RATING
≥
I
OUT
/2
** INCREASE L1 TO 30µH FOR LOAD
CURRENTS ABOVE 0.6A AND TO
60µH ABOVE 1A
SEE APPLICATIONS INFORMATION
+
C1
100µF, 10V
SOLID
TANTALUM
1578 TA01
U
1578 TA02
U
U
1
LT1578/LT1578-2.5
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
V
SW
1
V
IN
2
BOOST 3
GND 4
8 SYNC
7 SHDN
6 FB/SENSE
5 V
C
Input Voltage .......................................................... 16V
BOOST Pin Above Input Voltage ............................. 10V
SHDN Pin Voltage ..................................................... 7V
SENSE Pin Voltage .................................................... 4V
FB Pin Voltage (Adjustable Part) ............................ 3.5V
FB Pin Current (Adjustable Part) ............................ 1mA
SYNC Pin Voltage ..................................................... 7V
Operating Junction Temperature Range
LT1578C ............................................... 0°C to 125° C
LT1578I ........................................... – 40°C to 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
NUMBER
LT1578CS8
LT1578IS8
LT1578CS8-2.5
LT1578IS8-2.5
S8 PART MARKING
1578
1578I
157825
578I25
S8 PACKAGE
8-LEAD PLASTIC SO
θ
JA
= 80°C/ W WITH FUSED GROUND PIN
CONNECTED TO GROUND PLANE OR
LARGE LANDS
Consult factory for Military grade parts.
The
q
denotes specifications which apply over the full operating tempera-
ture range, otherwise specifications are at T
J
= 25°C. V
IN
= 5V, V
C
= 1.5V, Boost = V
IN
+ 5V, switch open, unless otherwise noted.
PARAMETER
Feedback Voltage
Sense Voltage (Fixed 2.5)
All Conditions
Sense Pin Resistance
Reference Voltage Line Regulation
Feedback Input Bias Current
Error Amplifier Voltage Gain (Notes 2, 10)
Error Amplifier Transconductance (Note 10)
V
C
Pin to Switch Current Transconductance
Error Amplifier Source Current
Error Amplifier Sink Current
V
C
Pin Switching Threshold
V
C
Pin High Clamp
Switch Current Limit
Slope Compensation (Note 8)
Switch On Resistance (Note 7)
Maximum Switch Duty Cycle
Minimum Switch Duty Cycle (Note 9)
Switch Frequency
Switch Frequency Line Regulation
Frequency Shifting Threshold on FB Pin
Minimum Input Voltage (Note 3)
Minimum Boost Voltage (Note 4)
4.3V
≤
V
IN
≤
15V
q
q
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
All Conditions
q
MIN
1.195
1.18
2.46
2.44
5.7
TYP
1.21
2.5
9.5
0.01
0.5
400
1050
1.5
110
130
0.8
2.1
2
0.3
0.2
94
94
8
200
0
0.74
4.0
2.3
MAX
1.225
1.24
2.54
2.56
13.7
0.03
2
1300
1700
190
200
UNITS
V
V
V
V
kΩ
%/V
µA
µMho
µMho
A/ V
µA
µA
V
V
A
A
Ω
Ω
%
%
%
kHz
kHz
%/ V
V
V
V
∆I
(V
C
) =
±10µA
q
200
800
400
40
50
V
FB
= 1.1V
V
FB
= 1.4V
Duty Cycle = 0
V
C
Open, V
FB
= 1.1V, DC
≤
50%
DC = 80%
I
SW
= 1.5A
V
FB
= 1.1V
q
q
q
1.5
3.5
0.35
0.45
q
q
90
86
180
160
0.4
V
C
Set to Give 50% Duty Cycle
q
4.3V
≤
V
IN
≤
15V
∆f
= 10kHz
I
SW
≤
1.5A
q
q
q
q
220
240
0.15
1.0
4.3
3.0
2
U
W
U
U
W W
W
LT1578/LT1578-2.5
The
q
denotes specifications which apply over the full operating tempera-
ture range, otherwise specifications are at T
J
= 25°C. V
IN
= 5V, V
C
= 1.5V, Boost = V
IN
+ 5V, switch open, unless otherwise noted.
PARAMETER
Boost Current (Note 5)
V
IN
Supply Current (Note 6)
Shutdown Supply Current
Lockout Threshold
Shutdown Thresholds
Synchronization Threshold
Synchronizing Range
SYNC Pin Input Resistance
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
Gain is measured with a V
C
swing equal to 200mV above the
switching threshold level to 200mV below the upper clamp level.
Note 3:
Minimum input voltage is not measured directly, but is guaranteed
by other tests. It is defined as the voltage where internal bias lines are still
regulated so that the reference voltage and oscillator frequency remain
constant. Actual minimum input voltage to maintain a regulated output will
depend on output voltage and load current. See Applications Information.
Note 4:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
Note 5:
Boost current is the current flowing into the boost pin with the pin
held 5V above input voltage. It flows only during switch on time.
Note 6:
Input supply current is the bias current drawn by the input pin
with switching disabled.
CONDITIONS
I
SW
= 0.5A
I
SW
= 1.5A
V
SHDN
= 0V, V
IN
≤
15V, V
SW
= 0V, V
C
Open
q
ELECTRICAL CHARACTERISTICS
MIN
q
q
q
TYP
9
27
1.9
20
2.42
0.37
0.45
1.5
40
V
C
Open
V
C
Open Device Shutting Down
Device Starting Up
q
q
q
2.34
0.13
0.25
250
MAX
18
50
2.7
50
75
2.50
0.60
0.7
2.2
400
UNITS
mA
mA
mA
µA
µA
V
V
V
V
kHz
kΩ
Note 7:
Switch on resistance is calculated by dividing V
IN
to V
SW
voltage
by the forced current (1.5A). See Typical Performance Characteristics for
the graph of switch voltage at other currents.
Note 8:
Slope compensation is the current subtracted from the switch
current limit at 80% duty cycle. See Maximum Output Load Current in the
Applications Information section for further details.
Note 9:
Minimum on-time is 400ns typical. For a 200kHz operating
frequency this means the minimum duty cycle is 8%. In frequency
foldback mode, the effective duty cycle will be less than 8%.
Note 10:
Transconductance and voltage gain refer to the internal amplifier
exclusive of the voltage divider. To calculate gain and transconductance
referred to the sense pin on the fixed voltage parts, divide values shown by
the ratio 2.5/1.21.
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Voltage Drop
0.5
125°C
SWITCH PEAK CURRENT (A)
25°C
0.3
–20°C
0.2
0.4
SWITCH VOLTAGE (V)
1.5
MINIMUM
FEEDBACK VOLTAGE (V)
0.1
0
0
0.25
0.50 0.75 1.00
SWITCH CURRENT (A)
1.25
1.50
U W
1576 G01
Switch Peak Current Limit
2.5
TYPICAL
2.0
1.22
1.23
Feedback Pin Voltage
1.21
1.0
0.5
1.20
0
0
20
60
40
DUTY CYCLE (%)
80
100
1576 G02
1.19
0
25
50
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
125
1576 G03
3
LT1578/LT1578-2.5
TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Pin Bias Current
(V
SHDN
= Lockout Threshold)
4
AT 2.44V STANDBY THRESHOLD
(CURRENT FLOWS OUT OF PIN)
SHDN PIN CURRENT (µA)
SHDN PIN CURRENT (µA)
3
140
120
100
80
60
40
20
CURRENT REQUIRED TO FORCE
SHUTDOWN (FLOWS OUT OF PIN).
AFTER SHUTDOWN, CURRENT
DROPS TO A FEW
µA
125
SHUTDOWN PIN VOLTAGE (V)
2
1
0
0
25
50
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
Standby Thresholds
2.46
INPUT SUPPLY CURRENT (µA)
SHUTDOWN PIN VOLTAGE (V)
20
V
SHDN
= 0V
15
INPUT SUPPLY CURRENT (µA)
2.45
2.44
ON
2.43
STANDBY
2.42
2.41
2.40
50
100
–50 –25
25
75
0
JUNCTION TEMPERATURE (°C)
Error Amplifier Transconductance
2000
PHASE
1500
GAIN (µMho)
TRANSCONDUCTANCE (µMho)
150
GAIN
100
V
C
SWITCHING FREQUENCY (kHz)
OR CURRENT (µA)
1000
500
V
FB
1
×
10
–3
(
)
R
OUT
570k
0
ERROR AMPLIFIER EQUIVALENT CIRCUIT
R
LOAD
= 50Ω
–500
10
100
1k
10k
FREQUENCY (Hz)
100k
–50
1M
1576 G09
4
U W
1576 G04
Shutdown Pin Bias Current
(V
SHDN
= Shutdown Threshold)
180
160
0.8
0.7
0.6
0.5
0.4
Shutdown Thresholds
START-UP
SHUTDOWN
0.3
0.2
0.1
0
0
50
25
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
125
125
0
0
25
50
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
1576 G05
1576 G06
Shutdown Supply Current
25
30
25
20
15
10
5
0
Shutdown Supply Current
V
IN
= 10V
10
5
0
125
0
5
10
INPUT VOLTAGE (V)
15
1576 G08
0
0.1
0.2
0.3
SHUTDOWN VOLTAGE (V)
0.4
1576 G010
1576 G07
Error Amplifier Transconductance
200
1600
1400
1200
1000
800
600
400
200
0
25
75 100
0
50
–50 –25
JUNCTION TEMPERATURE (°C)
200
250
Frequency Foldback
SWITCHING FREQUENCY
PHASE (DEG)
150
C
OUT
2.4pF
50
100
0
50
FEEDBACK PIN CURRENT
125
0
0
1.0
0.5
1.5
FEEDBACK VOLTAGE (V)
2.0
1576 G12
1576 G11
LT1578/LT1578-2.5
TYPICAL PERFORMANCE CHARACTERISTICS
Switching Frequency
240
SWITCH CURRENT LIMIT (A)
2.0
1.5
1.0
0.5
INPUT VOLTAGE (V)
220
FREQUENCY (kHz)
200
180
160
0
25
50
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
Maximum Output Current
at V
OUT
= 5V
1.6
1.4
L = 60µH
L = 30µH
L = 15µH
1.6
1.4
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
1.2
1.0
0.8
0.6
0.4
0.2
0
6
9
12
INPUT VOLTAGE (V)
1578 G15
BOOST Pin Current
30
25
BOOST PIN CURRENT (mA)
20
15
10
5
0
1.0
THRESHOLD VOLTAGE (V)
0
0.25
Kool Mµ is a registered trademark of Magnetics, Inc.
Metglas is a registered trademark of AlliedSignal, Inc.
U W
1576 G13
Switch Current Limit Foldback
3.0
2.5
4.25
4.50
Minimum Input Voltage to Start
with 3.3V Output
4.00
3.75
0
125
0
1.0
0.4
0.6
0.8
0.2
FEEDBACK PIN VOLTAGE (V)
1.2
1578 G19
3.50
1
10
100
LOAD CURRENT (mA)
1000
1576 G14
Maximum Output Current
at V
OUT
= 3.3V
1.6
L = 60µH
1.4
L = 30µH
L = 15µH
1.2
1.0
0.8
0.6
0.4
0.2
0
4
6
8
10
12
14
1578 G16
Maximum Output Current
at V
OUT
= 2.5V
L = 60µH
L = 30µH
L = 15µH
1.2
1.0
0.8
0.6
0.4
0.2
0
15
4
6
8
10
12
14
1578 G17
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
V
C
Pin Shutdown Threshold
0.8
0.6
0.4
0.2
0.50 0.75 1.00
SWITCH CURRENT (A)
1.25
1.50
0
0
25
50
75 100
–50 –25
JUNCTION TEMPERATURE (°C)
125
1576 G20
1576 G21
5
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