1
LTC1574
LTC1574-3.3/LTC1574-5
High Efficiency Step-Down
DC/DC Converters
with Internal Schottky Diode
The LTC
1574 is a family of easy-to-use current mode
DC/DC converters ideally suited for 9V to 5V, 5V to 3.3V
and inverting operation. With an internal 0.9
switch (at
a supply voltage of 12V) and a low forward drop Schottky
diode (0.450V typ at 200mA, T
A
= 25
C), the LTC1574
requires only three external components to construct a
complete high efficiency DC/DC converter.
Under no load condition, the LTC1574 draws only 130
A.
In shutdown, it draws a mere 2
A making this converter
ideal for battery-powered applications. In dropout, the
internal P-channel MOSFET switch is turned on continu-
ously allowing the user to maximize the life of the battery
source.
The maximum inductor current of the LTC1574 family is
pin selectable to either 340mA or 600mA, optimizing
efficiency for a wide range of applications. Operation up to
200kHz permits the use of small surface mount inductors
and capacitors.
For applications requiring higher output current or ultra-
high efficiency, see the LTC1148 and LTC1265 data sheets.
For detailed applications information, see the LTC1174
data sheet.
DESCRIPTIO
N
U
FEATURES
High Efficiency Step-Down Converter
TYPICAL APPLICATIO
N
U
+
V
IN
100
H
22
F*
35V
+
100
F*
10V
1574 TA01
5V
175mA
5
2, 4, 13, 15
7
10
3, 14
12
11
6
V
IN
5.5V to
16V
GND
LTC1574-5
LB
IN
LB
OUT
I
PGM
SHDN
V
OUT
SW
* AVX TPSD226K035
** AVX TPSD107K010
COILTRONICS CTX100-4
LTC1574-5 Efficiency
LOAD CURRENT (mA)
1
EFFICIENCY (%)
100
95
90
85
80
75
70
10
100
1574 TA02
200
V
IN
= 6V
V
IN
= 9V
L = 100
H
V
OUT
= 5V
I
PGM
= 0V
APPLICATIO
N
S
U
and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.
s
Inverting Converters
s
Step-Down Converters
s
Memory Backup Supply
s
Portable Instruments
s
Battery-Powered Equipment
s
Distributed Power Systems
s
High Efficiency: Up to 94%
s
Usable in Noise-Sensitive Products
s
Peak Inductor Current Independent of Inductor Value
s
Short-Circuit Protection
s
Internal Low Forward Drop Schottky Diode
s
Only Three External Components Required
s
Wide V
IN
Range: 4V to 18.5V (Absolute Maximum)
s
Low Dropout Operation
s
Low-Battery Detector
s
Pin Selectable Current Limit
s
Internal 0.9
Power Switch: V
IN
= 12V
s
Standby Current: 130
A
s
Active Low Micropower Shutdown
2
LTC1574
LTC1574-3.3/LTC1574-5
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
FB
Feedback Current into Pin 10
LTC1574
1
A
V
FB
Feedback Voltage
LTC1574
q
1.20
1.25
1.30
V
V
OUT
Regulated Output Voltage
LTC1574-3.3
q
3.14
3.30
3.46
V
LTC1574-5
q
4.75
5.00
5.25
V
V
OUT
Output Voltage Line
V
IN
= 6V to 12V, I
LOAD
= 100mA, I
PGM
= V
IN
(Note 2)
10
70
mV
Regulation
Output Voltage Load
LTC1574-3.3 (Note 2)
20mA < I
LOAD
< 175mA, I
PGM
= 0V
5
70
mV
Regulation
20mA < I
LOAD
< 400mA, I
PGM
= V
IN
45
70
mV
LTC1574-5 (Note 2)
20mA < I
LOAD
< 175mA, I
PGM
= 0V
5
70
mV
20mA < I
LOAD
< 400mA, I
PGM
= V
IN
50
70
mV
I
Q
Input DC Supply Current (Note 3)
Active Mode
4V < V
IN
< 16V, I
PGM
= 0V
450
600
A
Sleep Mode
4V < V
IN
< 16V
130
180
A
Shutdown (Note 4)
V
SHUTDOWN
= 0V, 4V < V
IN
< 16V
2
25
A
V
LBTRIP
Low-Battery Trip Point
1.25
1.4
V
I
LBIN
Current into Pin 12
0.5
A
I
LBOUT
Current Sunk by Pin 11
V
LBOUT
= 0.4V, V
LBIN
= 0V
0.5
1.0
1.5
mA
V
LBOUT
= 5V, V
LBIN
= 10V
1.0
A
V
HYST
Comparator Hysteresis
7.5
15
30
mV
I
PEAK
Current Limit
I
PGM
= V
IN
, V
OUT
= 0V
q
0.54
0.60
0.78
A
I
PGM
= 0V, V
OUT
= 0V
q
0.27
0.34
0.50
A
R
ON
ON Resistance of Switch
q
0.9
1.55
t
OFF
Switch Off Time
V
OUT
at Regulated Value
3
4
5
s
V
IH
Shutdown Pin High
Minimum Voltage at Pin 7 for Device to Be Active
1.2
V
V
IL
Shutdown Pin Low
Maximum Voltage at Pin 7 for Device to Be in Shutdown
0.75
V
I
IH
Shutdown Pin Input Current
V
SHUTDOWN
= 16V
2
A
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
ELECTRICAL CHARACTERISTICS
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
ORDER PART
NUMBER
LTC1574CS
LTC1574CS-3.3
LTC1574CS-5
Consult factory for Industrial and Military grade parts.
(Voltage Referred to GND Pin)
Input Supply Voltage (Pin 5) .................. 0.3V to 18.5V
Switch Current (Pin 3, 14) ........................................ 1A
Switch Voltage (Pin 3, 14) .......................... V
IN
18.5V
Operating Temperature Range .................... 0
C to 70
C
Junction Temperature (Note 1) ............................ 125
C
Storage Temperature Range ................. 65
C to 150
C
Lead Temperature (Soldering, 10 sec) .................. 300
C
T
A
= 25
C, V
IN
= 9V, V
SHUTDOWN
= V
IN
, I
PGM
= 0V, unless otherwise specified.
TOP VIEW
S PACKAGE
16-LEAD PLASTIC SO
*ADJUSTABLE OUTPUT VERSION
T
JMAX
= 125
C,
JA
= 110
C/W
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
NC
GND
SW
GND
V
IN
I
PGM
SHDN
NC
NC
GND
SW
GND
LB
IN
LB
OUT
V
OUT
(V
FB
*)
NC
3
LTC1574
LTC1574-3.3/LTC1574-5
ELECTRICAL CHARACTERISTICS
T
A
= 25
C, V
IN
= 9V, V
SHUTDOWN
= V
IN
, I
PGM
= 0V, unless otherwise specified.
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
IL
Shutdown Pin Input Current
0
V
SHUTDOWN
0.8V
0.5
A
V
F
Schottky Diode Forward Voltage
Forward Current = 200mA
0.450
0.570
V
I
R
Schottky Reverse Current
Reverse Voltage = 5V
10
25
A
Reverse Voltage = 18.5V
100
250
A
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1: T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formulas:
T
J
= T
A
+ (P
D
110
C/W)
Note 2: Guaranteed by Design.
Note 3: Does not include Schottky reverse current. Dynamic supply
current is higher due to the gate charge being delivered at the switching
frequency.
Note 4: Current into Pin 5 only, measured without electrolytic input
capacitor.
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
Efficiency vs Input Voltage
Efficiency vs Load Current
Efficiency vs Load Current
LOAD CURRENT (mA)
1
EFFICIENCY (%)
10
100
1574 TPC01
500
100
90
80
70
60
50
L = 50
H
V
OUT
= 3.3V
I
PGM
= V
IN
COIL = CTX50-4
V
IN
= 5V
V
IN
= 9V
LOAD CURRENT (mA)
1
EFFICIENCY (%)
10
100
1574 TPC02
400
100
95
90
85
80
75
70
L = 50
H
V
OUT
= 5V
I
PGM
= V
IN
COIL = CTX50-4
V
IN
= 6V
V
IN
= 9V
INPUT VOLTAGE (V)
5
EFFICIENCY (%)
7
9
10
14
1574 TPC03
6
8
11
12
13
95
94
93
92
91
90
89
V
OUT
= 5V
L = 100
H
COIL = CTX100-4
I
LOAD
= 300mA
I
PGM
= V
IN
I
LOAD
= 100mA
I
PGM
= 0V
Switch Resistance vs
Input Voltage
Switch Leakage Current
vs Temperature
Efficiency Using Different Types
of Inductor Core Material
LOAD CURRENT (mA)
1
EFFICIENCY (%)
10
100
500
100
90
80
70
60
50
V
IN
= 5V
V
OUT
= 3.3V
I
PGM
= V
IN
CTX50-4
CTX50-4P
1574 TPC04
TEMPERATURE (
C)
0
LEAKAGE CURRENT (nA)
180
160
140
120
100
80
60
40
20
0
40
80
100
1574 TPC05
20
60
V
IN
= 13.5V
INPUT VOLTAGE (V)
4
RDS
(ON)
(
)
12
1574 TPC06
6
8
10
14
16
18
20
T
A
= 25
C
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
4
LTC1574
LTC1574-3.3/LTC1574-5
PI
N
FU
N
CTIO
N
S
U
U
U
V
OUT
or V
FB
(Pin 10): For the LTC1574, this pin connects
to the main voltage comparator input. On the LTC1574-5
and LTC1574-3.3, this pin goes to an internal resistive
divider which sets the output voltage.
LB
OUT
(Pin 11): Open drain of an N-Channel Pull-Down.
This pin will sink current when (Pin 12) LB
IN
goes below
1.25V.
LB
IN
(Pin 12): The () Input of the Low-Battery Voltage
Comparator. The (+) input is connected to a reference
voltage of 1.25V.
NC (Pins 1, 8, 9, 16): No Connection.
GND (Pins 2, 4, 13, 15): Ground.
SW (Pins 3, 14): Drain of P-Channel MOSFET Switch and
Cathode of Schottky Diode.
V
IN
(Pin 5): Input Supply Voltage. It must be decoupled
close to ground (Pin 4).
I
PGM
(Pin 6): This pin selects the current limit of the
P-channel switch. With I
PGM
= V
IN
, the current trip point is
600mA and with I
PGM
= 0V, the current trip point is
reduced to 340mA.
SHDN (Pin 7): Pulling this pin to ground keeps the internal
switch off and puts the LTC1574 in micropower shutdown.
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
Operating Frequency and Inductor
Since the LTC1574 utilizes a constant off-time architecture,
its operating frequency is dependent on the value of V
IN
. The
frequency of operation can be expressed as:
f
t
V
V
V
V
Hz
OFF
IN
OUT
IN
D
=
-
+
( )
1
where t
OFF
= 4
s and V
D
is the voltage drop across the
internal Schottky diode. Note that the operating frequency
is a function of the input and output voltage.
Although the size of the inductor does not affect the fre-
quency or inductor peak current, it does affect the ripple
current. The peak-to-peak ripple current is given by:
I
V
V
L
RIPPLE
OUT
D
=
+
( )
-
4 10
6
A
P-P
By choosing a smaller inductor, a low ESR (Effective Series
Resistance) output filter capacitor has to be used. Core loss
will increase due to higher ripple current.
Short-Circuit Protection
The LTC1574 is protected from output short circuits by its
internal current limit. Depending on the condition of the
Low-Battery Detector
The low-battery indicator senses the input voltage through
an external resistive divider. This divided voltage connects
to the "" input of a voltage comparator (Pin 12) which is
compared with a 1.25V reference voltage. With the current
I
PGM
pin, the limit is either set to 340mA or 600mA. In
addition, the off-time of the switch is increased to allow the
inductor current to decay far enough to prevent any current
build-up (see Figure 1).
I
PGM
= V
IN
I
PGM
= 0
GND
L = 100
H
V
IN
= 13.5V
100mA/DIV
20
s/DIV
1574 F01
Figure 1. Inductor Current with Output Shorted
5
LTC1574
LTC1574-3.3/LTC1574-5
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
going into Pin 12 being negligible, the following expres-
sion is used for setting the trip limit:
V
R
R
LBTRIP
=
+
1 25 1
4
3
.
difference between the absolute maximum voltage rating
and the output voltage. A maximum of 12V is specified in
Figure 4, giving the circuit 1.5V of headroom for V
IN
. Note
that the circuit can operate from a minimum of 4V,
making it ideal for a four NiCd cell application. For a
higher output current circuit, please refer to the Typical
Applications section.
LTC1574
+
1.25V
REFERENCE
R4
R3
12
V
IN
1574 F02
Figure 2. Low-Battery Comparator
LTC1574 Adjustable Applications
The LTC1574 develops a 1.25V reference voltage between
the feedback terminal (Pin 10) and ground (see Figure 3).
By selecting resistor R1, a constant current is caused to
flow through R1 and R2 to set the overall output voltage.
The regulated output voltage is determined by:
V
R
R
OUT
=
+
1 25 1
2
1
.
For most applications, a 30k resistor is suggested for R1.
To prevent stray pickup, a 100pF capacitor is suggested
across R1 located close to the LTC1574.
LTC1574
V
FB
R2
R1
10
V
OUT
1574 F03
100pF
Figure 3. LTC1574 Adjustable Configuration
Inverting Applications
The LTC1574 can easily be set up for a negative output
voltage. If 5V is desired, the LTC1574-5 is ideal for this
application as it requires the least components. Figure 4
shows the schematic for this application. Note that the
output voltage is now taken off the GND pins. Therefore,
the maximum input voltage is now determined by the
+
+
12
LTC1574-5
LB
IN
LB
OUT
I
PGM
GND
V
IN
SHDN
V
OUT
SW
11
6
5
7
10
3, 14
2, 4, 13, 15
50
H**
V
OUT
5V
45mA
2
47
F*
16V
1574 F04
*
**
AVX TPSD476K016
COILTRONICS CTX50-4
INPUT VOLTAGE
4V TO 12V
0.1
F
+
2
47
F*
16V
Figure 4. Positive-to-Negative 5V Converter
Figure 5. Low Noise 5V to 3.3V Regulator
V
IN
L1**
100
H
100
F*
10V
1574 F05
5
2, 4, 13, 15
56k
33k
7
3, 14
10
12
11
6
V
IN
5V
V
OUT
3.3V
425mA
GND
LTC1574
LB
IN
LB
OUT
I
PGM
SHDN
SW
V
FB
* AVX TPSD107K010
** COILTRONICS CTX100-4
+
100
F*
10V
C2
6.8nF
+
Low Noise Regulators
In some applications it is important not to introduce any
switching noise within the audio frequency range. Due to
the nature of the LTC1574 during Burst Mode
TM
operation,
there is a possibility that the regulator will introduce audio
noise at some load currents. To circumvent this problem,
a feed-forward capacitor can be used to shift the noise
spectrum up and out of the audio band. Figure 5 shows the
low noise connection with C2 being the feed-forward
capacitor. The peak-to-peak output ripple is reduced to
30mV over the entire load range. A toroidal surface mount
Burst Mode is a trademark of Linear Technology Corporation
6
LTC1574
LTC1574-3.3/LTC1574-5
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
I
V
V
L
RIPPLE
OUT
D
=
+
( )
-
4 10
6
A
P-P
Solving for L in the above equation and with V
D
= 0.5V,
L = 44
H. The next higher standard value of L is 50
H
(example: Coiltronics CTX50-4). The operating frequency,
ignoring voltage across diode V
D
is:
f
V
V
kHz
OUT
IN
-
=
2 5 10 1
111
5
.
With the value of L determined, the requirements for C
IN
and C
OUT
are calculated. For C
IN
, its RMS current rating
should be at least:
I
I
V
V
V
V
A
mA
RMS
OUT
OUT
IN
OUT
IN
RMS
=
-
(
)
[
]
(
)
=
1 2
174
/
For C
OUT
, the RMS current rating should be at least:
I
I
A
RMS
PEAK
RMS
( )
=
2
300mA
TYPICAL APPLICATIO
N
S
U
Low Noise, High Efficiency 3.3V Regulator
V
IN
5
100pF
2, 4, 13, 15
6
12
11
7
10
3, 14
50
H
0.1
F
6.8nF
22
F*
25V
2
1574 TA03
V
OUT
3.3V
450mA
56k
33k
V
IN
4V TO 12.5V
GND
LTC1574
I
PGM
LB
IN
LB
OUT
SHDN
V
FB
SW
* AVX TPSD226K025
** AVX TPSD107K010
COILTRONICS CTX50-4
+
+
100
F**
10V
2
inductor L1 is chosen for its excellent self-shielding prop-
erties. Open magnetic structures such as drum and rod
cores are to be avoided since they inject high flux levels
into their surroundings. This can become a major source
of noise in any converter circuit.
Design Example
As a design example, assume V
IN
= 9V (nominal),
V
OUT
= 5V and I
OUT
= 350mA maximum. The LTC1574-5
is used for this application with I
PGM
(Pin 6) connected to
V
IN
. The minimum value of L is determined by assuming
the LTC1574-5 is operating in continuous mode.
INDUCTOR CURRENT
TIME
I
PEAK
I
V
AVG CURRENT = I
OUT
=
= 350mA
I
PEAK
+ I
V
2
1574 F06
Figure 6. Continuous Inductor Current
With I
OUT
= 350mA and I
PEAK
= 0.6A (I
PGM
= V
IN
), I
V
= 0.1A.
The peak-to-peak ripple inductor current, I
RIPPLE
, is 0.5A
and is also equal to:
7
LTC1574
LTC1574-3.3/LTC1574-5
Low Dropout 5V Step-Down Regulator with Low-Battery Detection
Positive to 5V Converter
High Efficiency 3.3V Regulator
V
IN
5
2, 4, 13, 15
6
11
12
7
10
3, 14
L1
100
H
0.1
F
47
F**
16V
2
1574 TA04
V
OUT
5V
365mA
*LOW-
BATTERY
INDICATOR
4.7k
162k
47.5k
V
IN
5.5V to 12.5V
GND
LTC1574-5
I
PGM
LB
OUT
LB
IN
SHDN
V
OUT
SW
+
47
F**
16V
2
+
* LOW-BATTERY INDICATOR IS
SET UP TO TRIP AT V
IN
= 5.5V
** AVX TPSD476K016
SELECTION
MANUFACTURER
PART NO.
TYPE
COILTRONICS
CTX100-4
SURFACE MOUNT
SUMIDA
CD75-101
SURFACE MOUNT
GOWANDA
GA10-103K
THROUGH HOLE
V
IN
5
2, 4, 13, 15
6
11
12
7
10
3, 14
L1
50
H
0.1
F
1574 TA06
V
OUT
5V
*LOW-
BATTERY
INDICATOR
4.7k
280k
43k
V
IN
4V TO 12.5V
GND
LTC1574-5
I
PGM
LB
OUT
LB
IN
SHDN
V
OUT
SW
+
100
F***
10V
+
* LOW-BATTERY INDICATOR IS
SET TO TRIP AT V
IN
= 4.4V
** AVX TPSD106K035
*** AVX TPSD107K010
SELECTION
MANUFACTURER PART NO.
TYPE
COILTRONICS
CTX50-3
SURFACE MOUNT
COILCRAFT
DT3316-473 SURFACE MOUNT
SUMIDA
CD54-470
SURFACE MOUNT
GOWANDA
GA10-472K THROUGH HOLE
10
F**
35V
2
V
IN
(V) I
OUT
(mA)
4
110
6
140
8
170
10
200
12.5
235
TYPICAL APPLICATIO
N
S
U
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.
V
IN
5
2, 4, 13, 15
6
12
11
7
10
3, 14
50
H
0.1
F
22
F*
25V
2
1574 TA05
V
OUT
3.3V
425mA
V
IN
4V TO 12.5V
GND
LTC1574-3.3
I
PGM
LB
IN
LB
OUT
SHDN
V
OUT
SW
* AVX TPSD226K025
** AVX TPSD476K016
COILTRONICS CTX50-4
+
47
F*
16V
2
+
8
LTC1574
LTC1574-3.3/LTC1574-5
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900
q
FAX
: (408) 434-0507
q
TELEX
: 499-3977
LT/GP 0795 6K PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1995
Dimension in inches (millimeters) unless otherwise noted.
S Package
16-Lead Plastic SOIC
PACKAGE DESCRIPTIO
N
U
0.016 0.050
0.406 1.270
0.010 0.020
(0.254 0.508)
45
0
8
TYP
0.008 0.010
(0.203 0.254)
1
2
3
4
5
6
7
8
0.150 0.157**
(3.810 3.988)
16
15
14
13
0.386 0.394*
(9.804 10.008)
0.228 0.244
(5.791 6.197)
12
11
10
9
SO16 0695
0.053 0.069
(1.346 1.752)
0.014 0.019
(0.355 0.483)
0.004 0.010
(0.101 0.254)
0.050
(1.270)
TYP
DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
*
**
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT
1076
Step-Down Switching Regulator
2A Monolithic Bipolar Switcher for V
IN
to 60V
LTC1174
High Efficiency Step-Down/Inverting DC/DC Converter
Same as LTC1574 Without Schottky Diode in SO-8 Package
LTC1265
1.2A, High Efficiency Step-Down DC/DC Converter
Current Mode with 0.3
Switch for Higher Current
LT1375/LT1376
1.5A, 500kHz Step-Down Switching Regulator
High Frequency, Synchronizable in SO-8 Package
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