LTC1574
LTC1574-3.3/LTC1574-5
High Efficiency Step-Down
DC/DC Converters
with Internal Schottky Diode
FEATURES
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DESCRIPTIO
High Efficiency: Up to 94%
Usable in Noise-Sensitive Products
Peak Inductor Current Independent of Inductor Value
Short-Circuit Protection
Internal Low Forward Drop Schottky Diode
Only Three External Components Required
Wide V
IN
Range: 4V to 18.5V (Absolute Maximum)
Low Dropout Operation
Low-Battery Detector
Pin Selectable Current Limit
Internal 0.9Ω Power Switch: V
IN
< 11V
Standby Current: 130µA
Active Low Micropower Shutdown
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 0.9Ω 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 130µA.
In shutdown, it draws a mere 2µA 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 or LTC1265 data sheets.
For detailed applications information, see the LTC1174
data sheet.
APPLICATIO S
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Inverting Converters
Step-Down Converters
Memory Backup Supply
Portable Instruments
Battery-Powered Equipment
Distributed Power Systems
and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
V
IN
5.5V to
16V
12
11
6
High Efficiency Step-Down Converter
LTC1574-5 Efficiency
100
95
L = 100µH
V
OUT
= 5V
I
PGM
= 0V
V
IN
= 6V
5
V
IN
LB
IN
LTC1574-5
LB
OUT
I
PGM
GND
2, 4, 13, 15
V
OUT
SW
SHDN
7
10
3, 14
100µH
†
+
EFFICIENCY (%)
22µF*
35V
90
V
IN
= 9V
85
80
75
70
+
5V
175mA
100µF*
10V
1574 TA01
* AVX TPSD226K035
** AVX TPSD107K010
†
COILTRONICS CTX100-4
1
U
10
LOAD CURRENT (mA)
100 200
1574 TA02
U
U
1
LTC1574
LTC1574-3.3/LTC1574-5
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
NC 1
GND 2
SW 3
GND 4
V
IN
5
I
PGM
6
SHDN 7
NC 8
16 NC
15 GND
14 SW
13 GND
12 LB
IN
11 LB
OUT
10 V
OUT
(V
FB
*)
9
NC
(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 2) ............................ 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC1574CS
LTC1574CS-3.3
LTC1574CS-5
S PACKAGE
16-LEAD PLASTIC SO
*ADJUSTABLE OUTPUT VERSION
T
JMAX
= 125°C,
θ
JA
= 110°C/W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER
I
FB
V
FB
V
OUT
∆V
OUT
Feedback Current into Pin 10
Feedback Voltage
Regulated Output Voltage
Output Voltage Line
Regulation
Output Voltage Load
Regulation
CONDITIONS
LTC1574
LTC1574
LTC1574-3.3
LTC1574-5
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 9V, SHDN = V
IN
, I
PGM
= 0V, unless otherwise specified.
MIN
TYP
MAX
1
q
q
q
UNITS
µA
V
V
V
mV
mV
mV
mV
mV
µA
µA
µA
V
µA
mA
µA
mV
A
A
Ω
µs
V
1.20
3.14
4.75
1.25
3.30
5.00
10
–5
– 45
–5
– 50
450
130
2
1.25
1.30
3.46
5.25
70
– 70
– 70
– 70
– 70
600
180
25
1.4
0.5
V
IN
= 6V to 12V, I
LOAD
= 100mA, I
PGM
= V
IN
(Note 3)
LTC1574-3.3 (Note 3)
LTC1574-5 (Note 3)
20mA < I
LOAD
< 175mA, I
PGM
= 0V
20mA < I
LOAD
< 400mA, I
PGM
= V
IN
20mA < I
LOAD
< 175mA, I
PGM
= 0V
20mA < I
LOAD
< 400mA, I
PGM
= V
IN
I
Q
Input DC Supply Current (Note 4)
Active Mode
Sleep Mode
Shutdown (Note 5)
Low-Battery Trip Point
Current into Pin 12
Current Sunk by Pin 11
Comparator Hysteresis
Current Limit
ON Resistance of Switch
Switch Off Time
SHDN Pin High
SHDN Pin Low
4V < V
IN
< 16V, I
PGM
= 0V
4V < V
IN
< 16V
SHDN = 0V, 4V < V
IN
< 16V
V
LBTRIP
I
LBIN
I
LBOUT
V
HYST
I
PEAK
R
ON
t
OFF
V
IH
V
IL
V
LBOUT
= 0.4V, V
LBIN
= 0V
V
LBOUT
= 5V, V
LBIN
= 10V
I
PGM
= V
IN
, V
OUT
= 0V
I
PGM
= 0V, V
OUT
= 0V
V
OUT
at Regulated Value
Minimum Voltage at Pin 7 for Device to Be Active
Maximum Voltage at Pin 7 for Device to Be in Shutdown
q
q
q
0.5
7.5
0.54
0.27
1.0
15
0.60
0.34
0.9
1.5
1.0
30
0.83
0.53
1.55
5
3
1.2
4
0.75
2
U
V
W
U
U
W W
W
LTC1574
LTC1574-3.3/LTC1574-5
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER
I
IH
I
IL
V
F
I
R
SHDN Pin Input Current
SHDN Pin Input Current
Schottky Diode Forward Voltage
Schottky Reverse Current
CONDITIONS
SHDN = 16V
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 9V, SHDN = V
IN
, I
PGM
= 0V, unless otherwise specified.
MIN
TYP
MAX
2
0.5
0.450
10
100
0.570
25
250
UNITS
µA
µA
V
µA
µA
0
≤
SHDN
≤
0.8V
Forward Current = 200mA
Reverse Voltage = 5V
Reverse Voltage = 18.5V
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
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 3:
Guaranteed by design.
Note 4:
Does not include Schottky reverse current. Dynamic supply
current is higher due to the gate charge being delivered at the switching
frequency.
Note 5:
Current into Pin 5 only, measured without electrolytic input
capacitor.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
100
V
IN
= 5V
100
95
90
EFFICIENCY (%)
EFFICIENCY (%)
80
V
IN
= 9V
V
IN
= 9V
85
80
75
70
L = 50µH
V
OUT
= 5V
I
PGM
= V
IN
COIL = CTX50-4
1
10
100
LOAD CURRENT (mA)
400
1574 • TPC02
EFFICIENCY (%)
70
L = 50µH
V
OUT
= 3.3V
I
PGM
= V
IN
COIL = CTX50-4
1
10
100
LOAD CURRENT (mA)
500
1574 • TPC01
60
50
Efficiency Using Different Types
of Inductor Core Material
100
CTX50-4
LEAKAGE CURRENT (nA)
CTX50-4P
80
180
160
90
EFFICIENCY (%)
RDS
(ON)
(Ω)
70
60
V
IN
= 5V
V
OUT
= 3.3V
I
PGM
= V
IN
1
10
100
LOAD CURRENT (mA)
500
1574 • TPC04
50
U W
Efficiency vs Load Current
95
94
V
IN
= 6V
Efficiency vs Input Voltage
V
OUT
= 5V
L = 100µH
COIL = CTX100-4
90
93
92
91
90
89
5
6
7
8
9 10 11 12
INPUT VOLTAGE (V)
13
14
I
LOAD
= 100mA
I
PGM
= 0V
I
LOAD
= 300mA
I
PGM
= V
IN
1574 • TPC03
Switch Leakage Current
vs Temperature
V
IN
= 13.5V
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0
20
40
60
TEMPERATURE (°C)
80
100
1574 • TPC05
Switch Resistance vs
Input Voltage
T
A
= 25°C
140
120
100
80
60
40
20
0
0.7
4
6
8
10 12 14 16
INPUT VOLTAGE (V)
18
20
1574 • TPC06
3
LTC1574
LTC1574-3.3/LTC1574-5
PI FU CTIO S
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.
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.
APPLICATIO S I FOR ATIO
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
=
1
V
IN
−
V
OUT
t
OFF
V
IN
+
V
D
(
Hz
)
I
PGM
= V
IN
100mA/DIV
where t
OFF
= 4µs 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:
V
+
V
D
I
RIPPLE
=
4 • 10
−
6
OUT
L
(
A
P-P
)
When choosing a small inductor, core loss will increase due
to higher ripple current. Therefore, a low ESR output
capacitor has to be used.
Short-Circuit Protection
The LTC1574 is protected from output short circuits by its
internal current limit. Depending on the condition of the
4
U
W
U U
U
U
U
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
= 0
GND
L = 100µH
V
IN
= 13.5V
20µs/DIV
1574 • F01
Figure 1. Inductor Current with Output Shorted
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
LTC1574
LTC1574-3.3/LTC1574-5
APPLICATIO S I FOR ATIO
R4
V
LBTRIP
=
1.25
1
+
R3
V
IN
R4
12
R3
LTC1574
going into Pin 12 being negligible, the following expres-
sion is used for setting the trip limit:
–
+
1.25V
REFERENCE
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:
R2
V
OUT
=
1.25
1
+
R1
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.
V
OUT
LTC1574
V
FB
10
100pF
1574 • F03
R2
R1
Figure 3. LTC1574 Adjustable Configuration
12
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
U
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.
INPUT VOLTAGE
4V TO 12V
5
V
IN
12
11
6
LB
IN
LTC1574-5
7
SHDN
V
OUT
SW
GND
2, 4, 13, 15
* AVX TPSD476K016
** COILTRONICS CTX50-4
10
3, 14
50µH**
0.1µF
W
U U
+
47µF*
16V
×
2
LB
OUT
I
PGM
+
47µF*
16V
×
2
V
OUT
–5V
45mA
1574 • F04
Figure 4. Positive-to-Negative 5V Converter
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
V
IN
5V
5
V
IN
LTC1574
LB
IN
LB
OUT
I
PGM
GND
2, 4, 13, 15
SHDN
SW
V
FB
7
3, 14
10
L1**
100µH
+
100µF*
10V
11
6
56k
C2
6.8nF
V
OUT
3.3V
425mA
100µF*
10V
+
33k
* AVX TPSD107K010
** COILTRONICS CTX100-4
1574 • F05
Figure 5. Low Noise 5V to 3.3V Regulator
5
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