LT1572
100kHz, 1.25A Switching
Regulator with Catch Diode
FEATURES
s
s
s
s
s
s
s
s
s
DESCRIPTIO
Catch Diode Included in Package
Wide Input Voltage Range: 3V to 30V
Low Quiescent Current: 6mA
Internal 1.25A Switch
Very Few External Parts Required
Self-Protected Against Overloads
Operates in Nearly All Switching Topologies
Shutdown Mode Draws Only 50µA Typical Current
Can Be Externally Synchronized
The LT
®
1572 is a 1.25A 100kHz monolithic switching
regulator with on-board switch and catch diode included
in one package. It combines an LT1172 with a 1A Schottky
catch diode. The LT1572 can be operated in all standard
switching configurations, including boost, buck, SEPIC,
flyback, forward, inverting and “Cuk”. All necessary con-
trol, oscillator and protection circuitry is included on the
die with the high efficiency switch. This makes the part
extremely easy to use and provides “bustproof” operation
similar to that obtained with 3-pin linear regulators.
The LT1572 operates with supply voltages from 3V to 30V
and draws only 6mA quiescent current. It can deliver load
power up to 15W with no external power devices. By
utilizing a current mode switching technique, the LT1572
achieves excellent response to load and line transients.
The LT1572 has many unique features not found on the
more difficult to use control chips presently available. It
uses adaptive anti-sat switch drive to allow very wide
ranging load currents with no loss in efficiency. An exter-
nally activated shutdown mode reduces total supply cur-
rent to 50µA typical for standby operation. External syn-
chronizing of switching frequency is possible, with a range
of 120kHz to 160kHz.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATI
s
s
s
S
s
3.3V-to-5V and 5V-to-12V Boost Converters
Negative-to-Positive Converter
SEPIC Converter (Input Can Be Greater or
Less Than Output)
Battery Charger
TYPICAL APPLICATI
V
IN
4.5V
TO 10V
L1*
50µH
9
V
IN
11
5V-to-12V Boost Converter
100
Boost Converter Efficiency
BOOST CONVERTER
V
IN
= 5V
V
OUT
= 12V
2 15
EFFICIENCY (%)
V
SW
ANODE
†
3
CATHODE
†
14
*COILTRONICS CTX50-2
**AVX TPS OR SPRAGUE 593D
†
ALWAYS CONNECT BOTH ANODE (2, 15)
AND CATHODE (3, 14) PINS
12V
0.25A
90
80
+
LT1572
C3
100µF
10V
FB
E1
10
E2
12
GND
4, 13
V
C
5
R3
1k
C1
1µF
R1
10.7k
1%
70
+
R2
1.24k
1%
C2**
100µF
16V
60
50
0
50
LT1572 • TA01
U
150
200
100
L0AD CURRENT (mA)
250
LT1572 • TA01
UO
UO
1
LT1572
ABSOLUTE
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
NC 1
ANODE* 2
CATHODE* 3
GND 4
V
C
5
FB 6
NC 7
NC 8
16 NC
15 ANODE*
14 CATHODE*
13 GND
12 E2
11 V
SW
10 E1
9 V
IN
Supply Voltage (Note 4).......................................... 40V
Switch Output Voltage (Note 4) .............................. 60V
Feedback Pin Voltage (Transient, 1ms) ................
±15V
Operating Junction Temperature Range
Operating .............................................. 0°C to 100°C
Short Circuit ......................................... 0°C to 125°C
Storage Temperature Range ............... – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
DIODE
Average Forward Current .......................................... 1A
Peak Repetitive Forward Current .............................. 2A
Peak Non-Repetitive Forward Current....................... 3A
Peak Repetitive Reverse Voltage............................. 20V
Continuous (Average) Reverse Voltage .................. 15V
Operating Junction Temperature ......................... 125°C
Note 1:
Minimum effective switch “on” time for the LT1572 (in current
limit only) is
≈
0.6µs. This limits the maximum safe input voltage during
an output shorted condition. Buck mode and inverting mode input voltage
during an output shorted condition is limited to:
R
×
I
L
+ V
f
V
IN
(max, output shorted) = 15V +
t
×
f
buck and inverting mode
R = Inductor DC resistance
I
L
= 2.5A
V
f
= Output catch diode forward voltage at I
L
t = 0.6µs, f = 100kHz switching frequency
ORDER PART
NUMBER
LT1572CS
S PACKAGE
16-LEAD PLASTIC SO
*ALWAYS CONNECT BOTH ANODE
AND BOTH CATHODE PINS
T
JMAX
(REGULATOR) = 100°C
T
JMAX
(DIODE) = 125°C
SEE THERMAL MANAGEMENT SECTION FOR
θ
JA
Consult factory for Industrial and Military grade parts.
Maximum input voltage can be increased by increasing R or V
f
.
External current limiting such as that shown in AN19, Figure 39, will
provide protection up to the full supply voltage rating. C1 in Figure 39
should be reduced to 200pF.
Transformer designs will tolerate much higher input voltages because
leakage inductance limits rate of rise of current in the switch. These
designs must be evaluated individually to assure that current limit is well
controlled up to maximum input voltage.
Boost mode designs are never protected against output shorts because
the external catch diode and inductor connect input to output.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER
V
REF
I
B
g
m
Reference Voltage
Feedback Input Current
Error Amplifier
Transconductance
Error Amplifier Source or
Sink Current
Error Amplifier Clamp
Voltage
Reference Voltage Line
Regulation
A
V
I
Q
Error Amplifier Voltage Gain
Minimum Input Voltage (Note 3)
Supply Current
CONDITIONS
V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, output pin open, unless otherwise noted.
MIN
q
q
TYP
1.244
1.244
350
MAX
1.264
1.274
750
1100
6000
7000
350
400
2.30
0.52
0.03
UNITS
V
V
nA
nA
µmho
µmho
µA
µA
V
V
%/V
V/V
Measured at Feedback Pin
V
C
= 0.8V
V
FB
= V
REF
∆I
C
=
±25µA
1.224
1.214
q
3000
2400
150
120
1.80
0.25
4400
200
V
C
= 1.5V
q
Hi Clamp, V
FB
= 1V
Lo Clamp, V
FB
= 1.5V
3V
≤
V
IN
≤
40V
V
C
= 0.8V
0.9V
≤
V
C
≤
1.4V
q
q
0.38
500
800
2.6
6
3.0
9
3V
≤
V
IN
≤
40V, V
C
= 0.6V
2
U
V
mA
W
U
U
W W
W
LT1572
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER
Control Pin Threshold
Normal/Flyback Threshold
on Feedback Pin
V
FB
Flyback Reference Voltage
(Note 3)
Change in Flyback Reference
Voltage
Flyback Reference Voltage
Line Regulation (Note 3)
Flyback Amplifier
Transconductance (g
m
)
Flyback Amplifier Source
and Sink Current
BV
V
SAT
Output Switch Breakdown
Voltage (Note 4)
Output Switch
“On” Resistance (Note 1)
Control Voltage to Switch
Current Transconductance
I
LIM
Switch Current Limit
I
FB
= 50µA
CONDITIONS
Duty Cycle = 0
V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, output pin open, unless otherwise noted.
MIN
q
TYP
0.9
0.45
16.3
6.8
0.01
MAX
1.08
1.25
0.54
17.6
18.0
9
0.03
500
70
70
UNITS
V
V
V
V
V
V
%/V
µmho
µA
µA
V
0.8
0.6
0.4
15.0
14.0
4.5
q
0.05
≤
I
FB
≤
1mA
I
FB
= 50µA
7V
≤
V
IN
≤
V
MAX
∆I
C
=
±10µA
V
C
= 0.6V, Source
I
FB
= 50µA, Sink
3V
≤
V
IN
≤
40V, I
SW
= 1.5mA
q
q
q
q
150
15
25
60
300
32
40
80
0.60
2
1.00
Ω
A/V
Duty Cycle = 50%, T
J
≥
25°C
Duty Cycle = 50%, T
J
<
25°C
Duty Cycle = 80% (Note 2)
q
q
q
1.25
1.25
1.00
25
88
85
80
100
90
100
100
50
150
1.5
3.0
3.5
2.5
35
112
115
95
250
250
300
A
A
A
mA/A
kHz
kHz
%
µA
mV
mV
µs
∆I
IN
∆I
SW
f
DC
MAX
Supply Current Increase
During Switch On-Time
Switching Frequency
q
Maximum Switch Duty Cycle
Shutdown Mode
Supply Current
Shutdown Mode
Threshold Voltage
Flyback Sense Delay Time (Note 3)
3V
≤
V
IN
≤
40V
V
C
= 0.05V
3V
≤
V
IN
≤
40V
q
q
DIODE
PARAMETER
Forward Voltage (Note 5)
CONDITIONS
I
f
= 200mA
I
f
= 500mA
I
f
= 1A
V
R
= 5V, T
J
= 25°C
V
R
= 5V, T
J
= 75°C
V
R
= 20V, T
J
= 25°C
V
R
= 20V, T
J
= 75°C
Diode Thermal Resistance
(Note 6)
q
q
q
MIN
TYP
0.45
0.52
0.55
1
25
3
70
90
MAX
0.57
0.65
0.70
5
100
15
300
UNITS
V
V
V
µA
µA
µA
µA
°C/W
Reverse Leakage (Note 5)
3
LT1572
ELECTRICAL CHARACTERISTICS
V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, output pin open, unless otherwise noted.
Note 5:
See graphs for guaranteed forward voltage and reverse leakage
current over temperature. Parameters are 100% tested at 25°C and
guaranteed at other temperatures by design and QA sampling.
Note 6:
Package soldered to FR4 board with
≥1oz
copper and an internal
or backside plane underneath the package to aid thermal transfer. Diode is
partly thermally coupled to regulator section. See Application Information
section for details on thermal calculations.
The
q
denotes the specifications which apply over the full operating
temperature range, 0°C to 100°C for the regulator chip and 0°C to 125°C
for the diode.
Note 1:
Measured with V
C
in hi clamp, V
FB
= 0.8V. I
SW
= 1A.
Note 2:
For duty cycles (DC) between 50% and 80%, minimum
guaranteed switch current is given by I
LIM
= 0.833 (2 – DC).
Note 3:
Minimum input voltage for isolated flyback mode is 7V.
Note 4:
Because the catch diode has a peak repetitive reverse voltage of
20V, diode breakdown may be the limiting factor on input voltage or
switch voltage in many applications.
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Current Limit vs Duty Cycle
4
2.9
SWITCH SATURATION VOLTAGE (V)
MINIMUM INPUT VOLTAGE (V)
SWITCH CURRENT (A)
3
25°C
2
–55°C
125°C
1
0
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1572 G01
Line Regulation
5
REFERENCE VOLTAGE CHANGE (mV)
4
REFERENCE VOLTAGE (V)
3
2
1
0
–1
–2
–3
–4
–5
0
10
T
J
= –55°C
T
J
= 150°C
FEEDBACK BIAS CURRENT (nA)
T
J
= 25°C
30
40
20
INPUT VOLTAGE (V)
4
U W
50
60
1572 G04
Minimum Input Voltage
1.6
Switch Saturation Voltage
1.4
150°C
1.2
1.0
0.8
0.6
0.4
0.2
0
0
0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
SWITCH CURRENT (A)
1572 G03
SWITCH CURRENT = 1.25A
2.8
2.7
2.6
SWITCH CURRENT = 0A
2.5
2.4
2.3
–75 –50 –25
100°C
25°C
–55°C
0 25 50 75 100 125 150
TEMPERATURE (°C)
1572 G02
Reference Voltage vs Temperature
1.250
1.248
1.246
1.244
1.242
1.240
1.238
1.236
1.234
–75 –50 –25
800
700
600
500
400
300
200
100
Feedback Bias Current vs
Temperature
25 50 75 100 125 150
TEMPERATURE (°C)
0
1572 G05
0
–75 –50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
0
1572 G06
LT1572
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Supply Voltage
(Shutdown Mode)
160
140
T
J
= 25°C
35
SUPPLY CURRENT (mA)
DRIVER CURRENT (mA)
SUPPLY CURRENT (µA)
120
100
80
60
40
20
0
0
10
20
30
SUPPLY VOLTAGE (V)
40
1572 G07
V
C
= 50mV
V
C
= 0V
Shutdown Mode Supply Current
200
180
160
SUPPLY CURRENT (µA)
TRANSCONDUCTANCE (µmho)
V
C
PIN CURRENT (µA)
140
120
100
80
60
40
20
0
0
10 20 30 40 50 60 70 80 90 100
V
C
PIN VOLTAGE (mV)
1572 G10
T
J
= 150°C
–55°C
≤
T
J
≤
125°C
Idle Supply Current
vs Temperature
11
10
V
C
= 0.6V
500
450
IDLE SUPPLY CURRENT (mA)
FEEDBACK VOLTAGE (mV)
9
8
7
6
5
4
3
2
1
–75 –50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1572 G13
SWITCH CURRENT (µA)
V
SUPPLY
= 60V
V
SUPPLY
= 3V
U W
Driver Current* vs Switch Current
40
Supply Current vs Input Voltage*
15
14
13
T
J
= 25°C
NOTE THAT THIS CURRENT DOES NOT
INCLUDE DRIVER CURRENT, WHICH IS
A FUNCTION OF LOAD CURRENT AND
DUTY CYCLE.
90% DUTY CYCLE
50% DUTY CYCLE
10% DUTY CYCLE
0% DUTY CYCLE
0
10
30
40
20
INPUT VOLTAGE (V)
50
60
1572 G09
30
25
20
15
10
5
0
0
0.25
0.50
0.75
1.00
SWITCH CURRENT (A)
1.25
1572 G08
12
11
10
9
8
7
6
5
T
J
= –55°C
T
J
=
≥
25°C
* AVERAGE POWER SUPPLY CURRENT IS
FOUND BY MULTIPLYING DRIVER CURRENT BY
DUTY CYCLE, THEN ADDING QUIESCENT CURRENT.
* UNDER VERY LOW OUTPUT CURRENT CONDITIONS,
DUTY CYCLE FOR MOST CIRCUITS WILL APPROACH
10% OR LESS.
Error Amplifier Transconductance
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
–75 –50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1572 G11
V
C
Pin Characteristics
300
200
100
0
T
J
= 25°C
–100
–200
V
FB
= 0.8V (CURRENT OUT OF V
C
PIN)
–300
–400
0
0.5
1.5
2.0
1.0
V
C
PIN VOLTAGE (V)
2.5
1572 G12
∆I
(V
C
PIN)
g
m
=
∆V
(FB PIN)
V
FB
= 1.5V (CURRENT INTO V
C
PIN)
Feedback Pin Clamp Voltage
1000
900
800
–55°C
25°C
150°C
700
600
Switch “Off” Characteristics
400
350
300
250
200
150
100
50
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
FEEDBACK CURRENT (mA)
1572 G14
500 V
SUPPLY
= 3V
400
300
200
100
0
0
10 20 30 40 50 60 70 80 90 100
SWITCH VOLTAGE (V)
1572 G15
V
SUPPLY
= 15V
V
SUPPLY
= 40V
5
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