LINER LT1572 100khz, 1.25a switching regulator with catch diode Datasheet

LT1572
100kHz, 1.25A Switching
Regulator with Catch Diode
U
DESCRIPTIO
FEATURES
■
■
■
■
■
■
■
■
■
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
UO
APPLICATI
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S
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 control, 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.
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
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 externally activated shutdown mode reduces total supply current to 50µA typical for standby operation. External synchronizing of switching frequency is possible, with a range
of 120kHz to 160kHz.
, LTC and LT are registered trademarks of Linear Technology Corporation.
UO
TYPICAL APPLICATI
Boost Converter Efficiency
5V-to-12V Boost Converter
100
L1*
50µH
9
11
2 15
†
VSW ANODE
VIN
3
CATHODE† 14
+
LT1572
C3
100µF
10V
FB
E1
10
E2
12
GND
4, 13
+
VC
5
R3
1k
C1
1µF
12V
0.25A
R1
10.7k
1%
R2
1.24k
1%
C2**
100µF
16V
80
70
60
50
LT1572 • TA01
BOOST CONVERTER
VIN = 5V
VOUT = 12V
90
EFFICIENCY (%)
VIN
4.5V
TO 10V
*COILTRONICS CTX50-2
**AVX TPS OR SPRAGUE 593D
†
ALWAYS CONNECT BOTH ANODE (2, 15)
AND CATHODE (3, 14) PINS
0
50
150
200
100
L0AD CURRENT (mA)
250
LT1572 • TA01
1
LT1572
U
W W
W
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 × IL + Vf
VIN (max, output shorted) = 15V +
t×f
buck and inverting mode
R = Inductor DC resistance
IL = 2.5A
Vf = Output catch diode forward voltage at IL
t = 0.6µs, f = 100kHz switching frequency
ELECTRICAL CHARACTERISTICS
CONDITIONS
VREF
Measured at Feedback Pin
VC = 0.8V
IB
Feedback Input Current
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
NC 1
ANODE* 2
CATHODE* 3
GND 4
16 NC
15 ANODE*
LT1572CS
14 CATHODE*
13 GND
VC 5
12 E2
FB 6
11 VSW
NC 7
10 E1
NC 8
9 VIN
S PACKAGE
16-LEAD PLASTIC SO
*ALWAYS CONNECT BOTH ANODE
AND BOTH CATHODE PINS
TJMAX (REGULATOR) = 100°C
TJMAX (DIODE) = 125°C
SEE THERMAL MANAGEMENT SECTION FOR θJA
Consult factory for Industrial and Military grade parts.
Maximum input voltage can be increased by increasing R or Vf.
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.
VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise noted.
SYMBOL PARAMETER
Reference Voltage
U
RATI GS
W
AXI U
●
MIN
TYP
MAX
UNITS
1.224
1.214
1.244
1.244
1.264
1.274
V
V
350
750
1100
nA
nA
VFB = VREF
●
gm
AV
Error Amplifier
Transconductance
∆IC = ±25µA
Error Amplifier Source or
Sink Current
VC = 1.5V
Error Amplifier Clamp
Voltage
Hi Clamp, VFB = 1V
Lo Clamp, VFB = 1.5V
Reference Voltage Line
Regulation
3V ≤ VIN ≤ 40V
VC = 0.8V
Error Amplifier Voltage Gain
0.9V ≤ VC ≤ 1.4V
Minimum Input Voltage (Note 3)
IQ
2
Supply Current
U
ABSOLUTE
3000
2400
4400
●
6000
7000
µmho
µmho
150
120
200
●
350
400
µA
µA
2.30
0.52
V
V
0.03
%/V
1.80
0.25
●
500
●
3V ≤ VIN ≤ 40V, VC = 0.6V
0.38
800
V/V
2.6
3.0
6
9
V
mA
LT1572
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER
Control Pin Threshold
VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise noted.
CONDITIONS
Duty Cycle = 0
●
Normal/Flyback Threshold
on Feedback Pin
VFB
MIN
TYP
MAX
UNITS
0.8
0.6
0.9
1.08
1.25
V
V
0.4
0.45
0.54
V
15.0
14.0
16.3
17.6
18.0
V
V
4.5
6.8
9
V
0.01
0.03
%/V
150
300
500
µmho
70
70
µA
µA
Flyback Reference Voltage
(Note 3)
IFB = 50µA
Change in Flyback Reference
Voltage
0.05 ≤ IFB ≤ 1mA
Flyback Reference Voltage
Line Regulation (Note 3)
IFB = 50µA
7V ≤ VIN ≤ VMAX
Flyback Amplifier
Transconductance (gm)
∆IC = ±10µA
Flyback Amplifier Source
and Sink Current
VC = 0.6V, Source
IFB = 50µA, Sink
●
●
15
25
32
40
BV
Output Switch Breakdown
Voltage (Note 4)
3V ≤ VIN ≤ 40V, ISW = 1.5mA
●
60
80
VSAT
Output Switch
“On” Resistance (Note 1)
●
0.60
●
Control Voltage to Switch
Current Transconductance
ILIM
Switch Current Limit
∆IIN
∆ISW
Supply Current Increase
During Switch On-Time
f
Switching Frequency
DCMAX
V
1.00
2
Duty Cycle = 50%, TJ ≥ 25°C
Duty Cycle = 50%, TJ < 25°C
Duty Cycle = 80% (Note 2)
Maximum Switch Duty Cycle
Shutdown Mode
Supply Current
3V ≤ VIN ≤ 40V
VC = 0.05V
Shutdown Mode
Threshold Voltage
3V ≤ VIN ≤ 40V
●
●
●
1.25
1.25
1.00
Ω
A/V
3.0
3.5
2.5
A
A
A
25
35
mA/A
88
85
100
●
112
115
kHz
kHz
●
80
90
95
%
100
250
µA
150
250
300
mV
mV
●
100
50
Flyback Sense Delay Time (Note 3)
µs
1.5
DIODE
PARAMETER
CONDITIONS
Forward Voltage (Note 5)
If = 200mA
If = 500mA
If = 1A
Reverse Leakage (Note 5)
Diode Thermal Resistance
MIN
TYP
MAX
UNITS
0.45
0.52
0.55
0.57
0.65
0.70
V
V
V
VR = 5V, TJ = 25°C
VR = 5V, TJ = 75°C
1
25
5
100
µA
µA
VR = 20V, TJ = 25°C
VR = 20V, TJ = 75°C
3
70
15
300
µA
µA
(Note 6)
90
●
●
●
°C/W
3
LT1572
ELECTRICAL CHARACTERISTICS
VIN = 15V, VC = 0.5V, VFB = VREF, 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 ● 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 VC in hi clamp, VFB = 0.8V. ISW = 1A.
Note 2: For duty cycles (DC) between 50% and 80%, minimum
guaranteed switch current is given by ILIM = 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.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Current Limit vs Duty Cycle
Minimum Input Voltage
4
Switch Saturation Voltage
2.9
1.6
SWITCH CURRENT (A)
3
–55°C
2
25°C
125°C
1
0
0
2.7
2.6
SWITCH CURRENT = 0A
2.5
2.4
2.3
–75 –50 –25
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
2
1
TJ = –55°C
TJ = 25°C
–2
–5
10
30
40
20
INPUT VOLTAGE (V)
50
60
1572 G04
4
0.6
0.4
0.2
0
0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
SWITCH CURRENT (A)
1572 G03
Feedback Bias Current vs
Temperature
800
1.248
700
1.246
1.244
1.242
1.240
1.238
1.234
–75 –50 –25
600
500
400
300
200
100
1.236
0
–55°C
1.250
–3
–4
25°C
0.8
0
FEEDBACK BIAS CURRENT (nA)
REFERENCE VOLTAGE (V)
REFERENCE VOLTAGE CHANGE (mV)
4
TJ = 150°C
100°C
1.0
Reference Voltage vs Temperature
5
0
150°C
1.2
1572 G02
Line Regulation
–1
1.4
0 25 50 75 100 125 150
TEMPERATURE (°C)
1572 G01
3
SWITCH SATURATION VOLTAGE (V)
MINIMUM INPUT VOLTAGE (V)
SWITCH CURRENT = 1.25A
2.8
25 50 75 100 125 150
TEMPERATURE (°C)
0
1572 G05
0
–75 –50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1572 G06
LT1572
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TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Supply Voltage
(Shutdown Mode)
Driver Current* vs Switch Current
160
TJ = 25°C
140
TJ = 25°C
14
35
NOTE THAT THIS CURRENT DOES NOT
INCLUDE DRIVER CURRENT, WHICH IS
A FUNCTION OF LOAD CURRENT AND
DUTY CYCLE.
13
100
VC = 50mV
80
60
40
30
SUPPLY CURRENT (mA)
120
DRIVER CURRENT (mA)
SUPPLY CURRENT (µA)
Supply Current vs Input Voltage*
15
40
25
TJ = –55°C
20
15
TJ = ≥ 25°C
10
12
11
10
90% DUTY CYCLE
9
50% DUTY CYCLE
8
10% DUTY CYCLE
7
20
5
VC = 0V
10
20
30
SUPPLY VOLTAGE (V)
0
40
1572 G07
Shutdown Mode Supply Current
4500
TRANSCONDUCTANCE (µmho)
180
140
TJ = 150°C
120
100
80
60
–55°C ≤ TJ ≤ 125°C
40
0.50
0.75
1.00
SWITCH CURRENT (A)
20
gm =
3500
3000
2500
2000
1500
6
VSUPPLY = 60V
VSUPPLY = 3V
5
4
3
2
1
–75 –50 –25
0
TJ = 25°C
–100
–200
VFB = 0.8V (CURRENT OUT OF VC PIN)
–300
–400
0 25 50 75 100 125 150
TEMPERATURE (°C)
0
1572 G13
1.5
2.0
1.0
VC PIN VOLTAGE (V)
1000
450
900
800
400
350
300
250
2.5
Switch “Off” Characteristics
500
–55°C
25°C
150°C
200
150
700
600
500 VSUPPLY
= 3V
400
VSUPPLY
= 15V
300
100
200
50
100
VSUPPLY
= 40V
0
0
0 25 50 75 100 125 150
TEMPERATURE (°C)
0.5
1572 G12
SWITCH CURRENT (µA)
FEEDBACK VOLTAGE (mV)
IDLE SUPPLY CURRENT (mA)
8
7
100
Feedback Pin Clamp Voltage
9
VFB = 1.5V (CURRENT INTO VC PIN)
200
1000
Idle Supply Current
vs Temperature
VC = 0.6V
1572 G09
VC Pin Characteristics
1572 G11
11
60
50
* UNDER VERY LOW OUTPUT CURRENT CONDITIONS,
DUTY CYCLE FOR MOST CIRCUITS WILL APPROACH
10% OR LESS.
∆I (VC PIN)
∆V (FB PIN)
1572 G10
10
30
40
20
INPUT VOLTAGE (V)
4000
0
–75 –50 –25
10 20 30 40 50 60 70 80 90 100
VC PIN VOLTAGE (mV)
10
300
500
0
0
0
1.25
1572 G08
Error Amplifier Transconductance
5000
160
0.25
* AVERAGE POWER SUPPLY CURRENT IS
FOUND BY MULTIPLYING DRIVER CURRENT BY
DUTY CYCLE, THEN ADDING QUIESCENT CURRENT.
200
0% DUTY CYCLE
5
VC PIN CURRENT (µA)
0
SUPPLY CURRENT (µA)
6
0
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
0
10 20 30 40 50 60 70 80 90 100
SWITCH VOLTAGE (V)
1572 G15
5
LT1572
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Shutdown Thresholds
– 350
250
– 250
200
– 200
VOLTAGE
150
– 150
100
–100
VC VOLTAGE IS REDUCED UNTIL
REGULATOR CURRENT DROPS
BELOW 300µA
0
–75 –50 –25
22
– 50
0
25 50 75 100 125 150
TEMPERATURE (°C)
0
1.8
TIME (µs)
– 300
23
2.0
VC PIN CURRENT (µA)
VC PIN VOLTAGE (mV)
CURRENT (OUT OF VC PIN)
300
50
2.2
– 400
FLYBACK VOLTAGE (V)
400
350
1.6
1.4
1.2
1.0
–75 –50 –25 0 25 50 75 100 125 150
JUNCTION TEMPERATURE (°C)
–30
90
3000
120
2000
1000
150
0
180
–1000
210
10M
1572 G19
6
FEEDBACK PIN VOLTAGE (mV)
60
gm
1M
100k
FREQUENCY (Hz)
19
RFB = 1k
18
17
RFB = 10k
15
–75 –50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1572 G18
500
–24
490
–22
480
–20
–18
470
460
FEEDBACK PIN VOLTAGE
(AT THRESHOLD)
–16
–14
450
–12
440
430
FEEDBACK PIN CURRENT
(AT THRESHOLD)
–10
420
–8
410
–6
400
–50 –25
0
–4
25 50 75 100 125 150
TEMPERATURE (°C)
1572 G20
FEEDBACK PIN CURRENT (µA)
30
5000
PHASE (DEG)
TRANSCONDUCTANCE (µmho)
0
θ
10k
20
Normal/Flyback Mode Threshold on
Feedback Pin
7000
4000
RFB = 500Ω
1572 G17
Transconductance of Error
Amplifier
6000
21
16
1572 G16
1k
Isolated Mode Flyback Reference
Voltage
Flyback Blanking Time
LT1572
W
BLOCK DIAGRA
VIN
16V
SWITCH
OUT
ANODE
2.3V
REG
FLYBACK
ERROR
AMP
100kHz
OSC
LOGIC
CATHODE
LT1172
5A, 75V
SWITCH
DRIVER
ANTISAT
MODE
SELECT
COMP
–
FB
ERROR
AMP
VC
+
+
SHUTDOWN
CIRCUIT
1.24V
REF
0.15V
CURRENT
AMP
GAIN ≈ 6
0.16Ω
–
E1*
*ALWAYS CONNECT E1 TO GROUND
E2
1572 BD
U
OPERATIO
The LT1572 is a current mode switcher. This means that
switch duty cycle is directly controlled by switch current
rather than by output voltage. Referring to the block
diagram, the switch is turned “on” at the start of each
oscillator cycle. It is turned “off” when switch current
reaches a predetermined level. Control of output voltage is
obtained by using the output of a voltage sensing error
amplifier to set current trip level. This technique has
several advantages. First, it has immediate response to
input voltage variations, unlike ordinary switchers which
have notoriously poor line transient response. Second, it
reduces the 90° phase shift at mid-frequencies in the
energy storage inductor. This greatly simplifies closedloop frequency compensation under widely varying input
voltage or output load conditions. Finally, it allows simple
pulse-by-pulse current limiting to provide maximum switch
protection under output overload or short conditions.
A low dropout internal regulator provides a 2.3V supply for
all internal circuitry on the LT1572. This low dropout
design allows input voltage to vary from 3V to 40V with
virtually no change in device performance. A 100kHz
oscillator is the basic clock for all internal timing. It turns
“on” the output switch via the logic and driver circuitry.
Special adaptive anti-sat circuitry detects onset of saturation in the power switch and adjusts driver current instantaneously to limit switch saturation. This minimizes driver
dissipation and provides very rapid turn-off of the switch.
A 1.2V bandgap reference biases the positive input of the
error amplifier. The negative input is brought out for
output voltage sensing. This feedback pin has a second
function; when pulled low with an external resistor, it
programs the LT1572 to disconnect the main error amplifier output and connects the output of the flyback amplifier
7
LT1572
U
OPERATIO
to the comparator input. The LT1572 will then regulate the
value of the flyback pulse with respect to the supply
voltage.1 This flyback pulse is directly proportional to
output voltage in the traditional transformer coupled flyback
topology regulator. By regulating the amplitude of the
flyback pulse, the output voltage can be regulated with no
direct connection between input and output. The output is
fully floating up to the breakdown voltage of the transformer windings. Multiple floating outputs are easily obtained with additional windings. A special delay network
inside the LT1572 ignores the leakage inductance spike at
the leading edge of the flyback pulse to improve output
regulation.
Other Application Help
The error signal developed at the comparator input is
brought out externally. This pin (VC) has four different
functions. It is used for frequency compensation, current
limit adjustment, soft starting, and total regulator shutdown. During normal regulator operation this pin sits at a
voltage between 0.9V (low output current) and 2.0V (high
output current). The error amplifiers are current output
(gm) types, so this voltage can be externally clamped for
adjusting current limit. Likewise, a capacitor coupled
external clamp will provide soft start. Switch duty cycle
goes to zero if the VC pin is pulled to ground through a
diode, placing the LT1572 in an idle mode. Pulling the VC
pin below 0.15V causes total regulator shutdown, with
only 50µA supply current for shutdown circuitry biasing.
See AN19 for full application details.
Thermal Management
More circuits and application help for the LT1572 can be
found in the LT1172 data sheet, both in loose form and in
the 1994 Linear Databook Volume III. Extensive additional
help is contained in Application Note 19. All application
circuits using the LT1172 can also use the LT1572 as long
as the 20V maximum reverse voltage of the diode is not
exceeded. A CAD program called SwitcherCAD is also
available. This program can be used with the LT1572 by
simply treating the LT1572 as an LT1172 and ignoring the
predicted die temperature results obtained from
SwitcherCAD itself.
Thermal management is particularly important with the
LT1572 because both switch and diode power dissipation
increase rapidly at low input voltage when using the
popular boost topology. Regulator and diode die temperature must be calculated separately because they are not
connected to an isothermal plane inside the package.
Diode plus regulator thermal resistance is approximately
70°C/W when the LT1572 is soldered to 1oz copper traces
over an internal or backside copper plane using FR4 board
material. However, individual calculation of die temperature must take thermal coupling into account. To accomplish this, thermal resistance is broken into two sections,
a common (coupled) section and a second uncoupled
section. Die temperatures are calculated from:
E1 and E2 Pins
TREG = TA + PREG (90°C/W) + PDIODE (45°C/W)
The LT1572 has the emitters of the power transistor
brought out separately from the ground pin. This eliminates errors due to ground pin voltage drops and allows
the user to reduce switch current limit 2:1 by leaving the
second emitter (E2) disconnected. The first emitter (E1)
should always be connected to the ground pin. Note that
switch “on” resistance doubles when E2 is left open, so
efficiency will suffer somewhat when switch currents
exceed 300mA. Also, note that chip dissipation will actually increase with E2 open during normal load operation,
even though dissipation in current limit mode will decrease.
TDIODE = TA + PDIODE (90°C/W) + PREG (45°C/W)
1See note under block diagram.
8
TA = ambient temperature
TREG = regulator die temperature
TDIODE = diode die temperature
PREG = total regulator power dissipation
PDIODE = diode power dissipation
The following formulas can be used as a rough guide to
calculate LT1572 power dissipation. For more details, the
reader is referred to Application Note 19 (AN19), “Efficiency Calculations” section.
LT1572
U
OPERATIO
Average supply current (including driver current) is:
IIN ≈ 6mA + ISW (0.004 + DC/40)
ISW = switch current
DC = switch duty cycle
Switch power dissipation is given by:
PSW = (ISW)2 × RSW × DC
RSW = LT1572 switch “on” resistance (1Ω maximum)
Total power dissipation is the sum of supply current times
input voltage plus switch power:
PREG = IIN × VIN + PSW
In a typical example, using a boost converter to generate
12V at 0.12A from a 5V input, duty cycle is approximately
60%, and switch current is about 0.65A, yielding:
IIN = 6mA + 0.65(0.004 + DC/40) = 18mA
PSW = (0.65)2 × 1Ω × 0.6 = 0.25W
PREG = 5V × 0.018A + 0.25 = 0.34W
Approximate diode power dissipation for boost and buck
converters is shown below. For other topologies or more
accurate results, see Application Note 19 or use
SwitcherCAD.
Boost: PDIODE = IOUT × Vf
Buck: PDIODE = IOUT × Vf × (VIN – VOUT)/VIN
Vf = diode forward voltage at a current equal to IOUT for a
buck converter and IOUT × VOUT/VIN for a boost converter.
In most applications, full load current is used to calculate
die temperature. However, if overload conditions must
also be accounted for, three approaches are possible.
First, if loss of regulated output is acceptable under
overload conditions, the internal thermal limit of the
LT1572 will protect the die in most applications by shutting off switch current. Thermal limit is not a tested
parameter, however, and should be considered only for
noncritical applications with temporary overloads.
The second approach for lower current applications is to
leave the second switch emitter (E2) open. This increases
switch “on” resistance by 2:1, but reduces switch current
limit by 2:1 also, resulting in a net 2:1 reduction in I2R
switch dissipation under current limit conditions.
The third approach is to clamp the VC pin to a voltage less
than its internal clamp level of 2V. The LT1172 switch
current limit is zero at approximately 1V on the VC pin and
2A at 2V on the VC pin. Peak switch current can be
externally clamped between these two levels with a diode.
See AN19 for details.
Diode Characteristics
The catch diode used in the LT1572 is a power Schottky
diode with a very low storage time and low forward
voltage. This gives good efficiency in switching regulator
applications, but some thought must be given to maximum operating voltage and high temperature reverse
leakage. Peak repetitive reverse voltage rating on the diode
is 20V. In a boost converter, maximum diode reverse
voltage is equal to regulated output voltage, so this limits
maximum output voltage to 20V. In a negative-to-positive
converter, maximum diode voltage will be equal to the
sum of output voltage plus input voltage. Use the equations in Application Note 19 or SwitcherCAD or calculate
maximum diode voltage for other topologies.
Diode reverse leakage increases rapidly with temperature.
This leakage is not high enough to significantly impact
efficiency or diode power dissipation, but it can be of
concern in shutdown mode if the diode is connected in
such a way that the leakage adds to regulator shutdown
current. Use the graphs of diode leakage versus voltage
and temperature to ensure proper high temperature system performance.
The LT1572 diode is internally bonded to more than two
package pins to reduce internal bond wire currents. All
pins must be used to prevent excessive current in the
individual internal bond wires. This is important in low
load current applications because the LT1572 will draw
high surge currents during start-up (to charge the output
capacitor) even with no output load current.
9
LT1572
U
OPERATIO
Synchronizing with Bipolar Transistor
Synchronizing
The LT1572 can be externally synchronized in the frequency range of 120kHz to 160kHz. This is accomplished
as shown in the accompanying figures. Synchronizing
occurs when the VC pin is pulled to ground with an external
transistor. To avoid disturbing the DC characteristics of
the internal error amplifier, the width of the synchronizing
pulse should be under 0.3µs. C2 sets the pulse width at ≅
0.2µs. The effect of a synchronizing pulse on the LT1572
amplifier offset can be calculated from:
VIN
LT1572
VC
GND
C2
39pF
R3
C1

 KT 
VC 
 q  tS fS IC + R3 


 
∆VOS =
IC
( )( )
KT = 26mV at 25°C
q
tS = pulse width
fS = pulse frequency
IC = VC source current (≈ 200µA)
VC = operating VC voltage (1V to 2V)
R3 = resistor used to set mid-frequency “zero”
in frequency compensation network.
With tS = 0.2µs, fS = 150kHz, VC = 1.5V, and R3 = 2k, offset
voltage shift is ≈ 3.8mV. This is not particularly bothersome, but note that high offsets could result if R3 were
reduced to a much lower value. Also, the synchronizing
transistor must sink higher currents with low values of R3,
so larger drives may have to be used. The transistor must
be capable of pulling the VC pin to within 200mV of ground
to ensure synchronizing.
10
R1
3k
2N2369
R2
2.2k
FROM 5V
LOGIC
1572 OP01
Synchronizing with MOS Transistor
VIN
LT1572
GND
VC
R3
C1
D1
1N4158
C2
100pF
VN2222*
R2
2.2k
D2
1N4158
FROM 5V
LOGIC
*SILICONIX OR EQUIVALENT
1572 OP02
LT1572
UO
TYPICAL APPLICATI
S
Negative Buck Converter
+
* REQUIRED IF INPUT LEADS ≥ 2"
** PULSE ENGINEERING 92114
COILTRONICS 50-2-52
CATHODE
L1**
50µH
VIN
C2
200µF
VSW
+
C3*
100µF
E1
Q1
2N3906
LT1572 ANODE
E2
LOAD
R1
4.64k
R4
12k
–5.2V
0.75A
FB
VC
GND
C1
R2
1.24k
R3
VIN
–7V TO –20V
1572 TA03
Backlight CCFL Supply (see AN55 for details)
INPUT VOLTAGE†
4.5V TO 20V
L2***
1k
L1**
300µH
A
33pF
3kV
LAMP
Q1*
CATHODE
10µF
TANT
+
VIN
VSW
E2
0.02µF
B
GND
VC
+
2µF
D2
1N914
Q2*
LT1572 ANODE
E1
D1
1N914
R3
10k
50k
INTENSITY
ADJUST
R1
560Ω
FB
C6
1µF
* Q1,Q2 = BCP56 OR MPS650/561
** COILTRONICS CTX300-4
*** SUMIDA 6345-020 OR COILTRONICS 110092-1
† A MODIFICATION WILL ALLOW OPERATION DOWN TO 4.5V. CONSULT FACTORY.
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
1572 TA04
11
LT1572
U
PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted.
S Package
16-Lead Plastic SOIC
0.386 – 0.394*
(9.804 – 10.008)
16
15
14
13
12
11
10
9
0.150 – 0.157*
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
1
0.010 – 0.020
× 45°
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
2
3
5
4
6
7
0.053 – 0.069
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
0° – 8° TYP
0.016 – 0.050
0.406 – 1.270
8
0.050
(1.270)
TYP
0.014 – 0.019
(0.355 – 0.483)
SO16 0893
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
RELATED PARTS
PART NUMBER
DESCRIPTION
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100kHz, 1.25A High Efficiency Switching Regulator
LT1572 Without Diode
LT1173
Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
Operates Down to 2V Input
LT1372
500kHz High Efficiency 1.5A Step-Up Switching Regulator
Latest Technology, Uses Tiny Inductors
LTC1574
High Efficiency Step-Down DC/DC Converter
with Internal Schottky Diode
LTC1174 with Diode
12
Linear Technology Corporation
COMMENTS
LT/GP 0595 10K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
 LINEAR TECHNOLOGY CORPORATION 1995