LINER LT1936HMS8E 1.4a, 500khz step-down switching regulator Datasheet

LT1936
1.4A, 500kHz Step-Down
Switching Regulator
FEATURES
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DESCRIPTION
Wide Input Range: 3.6V to 36V
Short-Circuit Protected Over Full Input Range
1.9A Guaranteed Minimum Switch Current
5V at 1.4A from 10V to 36V Input
3.3V at 1.4A from 7V to 36V Input
5V at 1.2A from 6.3V to 36V Input
3.3V at 1.2A from 4.5V to 36V Input
Output Adjustable Down to 1.20V
500kHz Fixed Frequency Operation
Soft-Start
Uses Small Ceramic Capacitors
Internal or External Compensation
Low Shutdown Current: <2μA
Thermally Enhanced 8-Lead MSOP Package
The LT®1936 is a current mode PWM step-down DC/DC
converter with an internal 1.9A power switch, packaged
in a tiny, thermally enhanced 8-lead MSOP. The wide input range of 3.6V to 36V makes the LT1936 suitable for
regulating power from a wide variety of sources, including
automotive batteries, 24V industrial supplies and unregulated wall adapters. Its high operating frequency allows the
use of small, low cost inductors and ceramic capacitors,
resulting in low, predictable output ripple.
Cycle-by-cycle current limit, frequency foldback and
thermal shutdown provide protection against shorted
outputs, and soft-start eliminates input current surge
during start-up. Transient response can be optimized by
using external compensation components, or board space
can be minimized by using internal compensation. The
low current (<2μA) shutdown mode enables easy power
management in battery-powered systems.
APPLICATIONS
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Automotive Battery Regulation
Industrial Control Supplies
Unregulated Wall Adapters
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
TYPICAL APPLICATION
3.3V Step-Down Converter
Efficiency
95
VIN
4.5V TO 36V
4.7μF
BOOST
SHDN
SW
10μH
LT1936
VOUT
3.3V
1.2A
17.4k
COMP
VC
FB
GND
10k
VOUT = 5V
90
0.22μF
22μF
EFFICIENCY (%)
ON OFF
VIN
VIN = 12V
85
VOUT = 3.3V
80
75
70
1936 TA01a
65
0
0.5
1
LOAD CURRENT (A)
1.5
1936 TA01b
1936fd
1
LT1936
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
VIN Voltage ................................................. –0.4V to 36V
BOOST Voltage .........................................................43V
BOOST Above SW Voltage ........................................20V
SHDN Voltage ............................................ –0.4V to 36V
FB, VC, COMP Voltage .................................................6V
Operating Temperature Range (Note 2)
LT1936E ............................................... –40°C to 85°C
LT1936I .............................................. –40°C to 125°C
LT1936H ............................................ –40°C to 150°C
Maximum Junction Temperature
LT1936E, LT1936I ............................................. 125°C
LT1936H ........................................................... 150°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
TOP VIEW
BOOST
VIN
SW
GND
1
2
3
4
9
8
7
6
5
COMP
VC
FB
SHDN
MS8E PACKAGE
8-LEAD PLASTIC MSOP
θJA = 40°C/W, θJC = 10°C/W
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT1936EMS8E#PBF
LT1936EMS8E#TRPBF
LTBMT
8-Lead Plastic MSOP
–40°C to 85°C
LT1936IMS8E#PBF
LT1936IMS8E#TRPBF
LTBRV
8-Lead Plastic MSOP
–40°C to 125°C
LT1936HMS8E#PBF
LT1936HMS8E#TRPBF
LTBWB
8-Lead Plastic MSOP
–40°C to 150°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT1936EMS8E
LT1936EMS8E#TR
LTBMT
8-Lead Plastic MSOP
–40°C to 85°C
LT1936IMS8E
LT1936IMS8E#TR
LTBRV
8-Lead Plastic MSOP
–40°C to 125°C
LT1936HMS8E
LT1936HMS8E#TR
LTBWB
8-Lead Plastic MSOP
–40°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VBOOST = 17V, unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
Undervoltage Lockout
Quiescent Current
VFB = 1.5V
Quiescent Current in Shutdown
VSHDN = 0V
●
FB Voltage
●
●
1.175
TYP
MAX
UNITS
3.45
3.6
V
1.8
2.5
mA
0.1
2
μA
1.200
1.215
V
50
50
200
300
nA
nA
FB Pin Bias Current (Note 4)
VFB = 1.20V, E and I Grades
H Grade
FB Voltage Line Regulation
VIN = 5V to 36V
0.01
%/V
Error Amp gm
VC = 0.5V, IVC = ±5μA
250
μS
Error Amp Voltage Gain
VC = 0.8V, 1.2V
150
1936fd
2
LT1936
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VBOOST = 17V, unless otherwise noted. (Note 2)
VC Clamp
1.8
V
VC Switch Threshold
0.7
V
Internal Compensation R
50
kΩ
150
pF
Internal Compensation C
VCOMP = 1V
COMP Pin Leakage
VCOMP = 1.8V, E and I Grades
H Grade
Switching Frequency
VFB = 1.1V
VFB = 0V
●
●
●
Maximum Duty Cycle
Switch Current Limit
(Note 3)
Switch VCESAT
ISW = 1.2A
400
500
40
87
92
1.9
1
2
μA
μA
600
kHz
kHz
%
2.2
2.6
A
410
520
mV
2
μA
2
2.2
V
Switch Leakage Current
Minimum BOOST Voltage Above SW
ISW = 1.2A
BOOST Pin Current
ISW = 1.2A
28
50
mA
BOOST Pin Leakage
VSW = 0V
0.1
1
μA
SHDN Input Voltage High
2.3
V
SHDN Input Voltage Low
SHDN Pin Current
VSHDN = 2.3V (Note 5)
VSHDN = 12V (Note 5)
VSHDN = 0V
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LT1936E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
34
140
0.01
0.3
V
50
240
0.1
μA
μA
μA
with statistical process controls. The LT1936I specifications are
guaranteed over the –40°C to 125°C temperature range. The LT1936H
specifications are guaranteed over the –40°C to 150°C temperature range.
Note 3: Current limit guaranteed by design and/or correlation to static test.
Slope compensation reduces current limit at higher duty cycle.
Note 4: Current flows out of pin.
Note 5: Current flows into pin.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, VOUT = 5V
Efficiency, VOUT = 3.3V
100
Switch Current Limit
3.0
100
2.5
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 24V
80
70
VIN = 5V
90
VOUT = 5V
TA = 25°C
D1 = DFLS140L
L1 = 15μH, TOKO D63CB
60
CURRENT LIMIT (A)
VIN = 12V
90
VIN = 12V
80
VIN = 24V
70
VOUT = 3.3V
TA = 25°C
D1 = DFLS140L
L1 = 10μH, TOKO D63CB
60
0
0.5
1.0
LOAD CURRENT (A)
1.5
1936 G01
TYP
2.0
MIN
1.5
1.0
0.5
0
0
0.5
1.0
LOAD CURRENT (A)
1.5
1936 G02
0
20
40
60
DUTY CYCLE (%)
80
100
1936 G03
1936fd
3
LT1936
TYPICAL PERFORMANCE CHARACTERISTICS
Maximum Load Current
1.8
VOUT = 5V
VOUT = 3.3V
1.6
LOAD CURRENT (A)
LOAD CURRENT (A)
1.6
L = 15μH
1.4
L = 10μH
1.2
L = 10μH
1.4
L = 6.8μH
1.2
1.0
1.0
0
5
10
15
20
INPUT VOLTAGE (V)
25
0
30
5
10
15
20
INPUT VOLTAGE (V)
1936 G04
25
Feedback Voltage
200
100
0.5
1.0
SWITCH CURRENT (A)
Switching Frequency
UVLO (V)
3.4
3.2
1.190
3.0
–50 –25
1.185
25 50 75 100 125 150
TEMPERATURE (°C)
1.5
600
3.6
1.195
0
1936 G06
SWITCHING FREQUENCY (kHz)
1.205
0
TA = –45°C
300
Undervoltage Lockout
1.200
TA = 85°C
TA = 25°C
400
0
30
3.8
–50 –25
500
1936 G05
1.210
FEEDBACK VOLTAGE (V)
Switch Voltage Drop
600
SWITCH VOLTAGE DROP (mV)
Maximum Load Current
1.8
0
550
500
450
400
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
0
25 50 75 100 125 150
TEMPERATURE (°C)
1936 G08
1936 G09
1936 G07
Frequency Foldback
SHDN Pin Current
Soft-Start
3.0
500
400
300
200
200
TA = 25°C
DC = 30%
TA = 25°C
2.5
SHDN PIN CURRENT (μA)
TA = 25°C
600
SWITCH CURRENT LIMIT (A)
SWITCHING FREQUENCY (kHz)
700
2.0
1.5
1.0
150
100
50
0.5
100
0
0
0.5
1.0
FB PIN VOLTAGE (V)
1.5
1936 G10
0
0
0
1
2
3
SHDN PIN VOLTAGE (V)
4
1936 G11
0
12
4
8
SHDN PIN VOLTAGE (V)
16
1936 G12
1936fd
4
LT1936
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Input Voltage
5.0
VOUT = 5V
TA = 25°C
L = 15μH
6.5
Switch Current Limit
3.0
VOUT = 3.3V
TA = 25°C
L = 10μH
SWITCH CURRENT LIMIT (A)
Minimum Input Voltage
7.0
INPUT VOLTAGE (V)
6.0
5.5
5.0
4.0
3.5
2.5
2.0
1.5
1.0
0.5
4.5
4.0
3.0
10
100
LOAD CURRENT (mA)
1000
1
10
100
LOAD CURRENT (mA)
1936 G13
VSW
10V/DIV
VSW
10V/DIV
IL
500mA/DIV
IL
500mA/DIV
VOUT
20mV/DIV
VOUT
20mV/DIV
1μs/DIV
1936 G16
VIN = 12V
VOUT = 3.3V
IOUT = 50mA
L = 10μH
COUT = 22μF
VC Voltages
0
25 50 75 100 125 150
TEMPERATURE (°C)
1936 G15
1936 G17
1μs/DIV
Error Amp Output Current
2.5
60
TA = 25°C
VC = 0.5V
40
2.0
CURRENT LIMIT CLAMP
1.5
1.0
SWITCHING THRESHOLD
0.5
0
–50 –25
0
–50 –25
Switching Waveforms,
Discontinuous Mode
Switching Waveforms
VIN = 12V
VOUT = 3.3V
IOUT = 1A
L = 10μH
COUT = 22μF
1000
1936 G14
VC PIN CURRENT (μA)
1
VC VOLTAGE (V)
INPUT VOLTAGE (V)
4.5
20
0
–20
–40
0
25 50 75 100 125 150
TEMPERATURE (°C)
1936 G18
–60
0
1
FB PIN VOLTAGE (V)
2
1936 G19
1936fd
5
LT1936
PIN FUNCTIONS
BOOST (Pin 1): The BOOST pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
FB (Pin 6): The LT1936 regulates its feedback pin to
1.200V. Connect the feedback resistor divider tap to this
pin. Set the output voltage according to VOUT = 1.200V
(1 + R1/R2). A good value for R2 is 10k.
VIN (Pin 2): The VIN pin supplies current to the LT1936’s
internal regulator and to the internal power switch. This
pin must be locally bypassed.
VC (Pin 7): The VC pin is used to compensate the LT1936
control loop by tying an external RC network from this pin
to ground. The COMP pin provides access to an internal
RC network that can be used instead of the external
components.
SW (Pin 3): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
COMP (Pin 8): To use the internal compensation network,
tie the COMP pin to the VC pin. Otherwise, tie COMP to
ground or leave it floating.
GND (Pin 4): Tie the GND pin to a local ground plane
below the LT1936 and the circuit components. Return the
feedback divider to this pin.
Exposed Pad (Pin 9): The Exposed Pad must be soldered
to the PCB and electrically connected to ground. Use a
large ground plane and thermal vias to optimize thermal
performance.
SHDN (Pin 5): The SHDN pin is used to put the LT1936 in
shutdown mode. Tie to ground to shut down the LT1936.
Tie to 2.3V or more for normal operation. If the shutdown
feature is not used, tie this pin to the VIN pin. SHDN also
provides a soft-start function; see the Applications Information. Do not drive SHDN more than 5V above VIN.
BLOCK DIAGRAM
2
VIN
VIN
C2
INT REG
AND
UVLO
BOOST
∑
ON OFF
SLOPE
COMP
R3
5
R
Q
S
Q
D2
1
SHDN
C4
C3
DRIVER
Q1
SW
OSC
L1
VOUT
3
D1
FREQUENCY
FOLDBACK
C1
R1
FB
VC
RC
50k
7
VC
8
COMP
6
gm
R2
CC
150pF
1.200V
4
GND
1936 BD
R4
C5
1936fd
6
LT1936
OPERATION
(Refer to Block Diagram)
The LT1936 is a constant frequency, current mode stepdown regulator. A 500kHz oscillator enables an RS flip-flop,
turning on the internal 1.9A power switch Q1. An amplifier and comparator monitor the current flowing between
the VIN and SW pins, turning the switch off when this
current reaches a level determined by the voltage at VC.
An error amplifier measures the output voltage through
an external resistor divider tied to the FB pin and servos
the VC pin. If the error amplifier’s output increases, more
current is delivered to the output; if it decreases, less
current is delivered. An active clamp (not shown) on the
VC pin provides current limit. The VC pin is also clamped
to the voltage on the SHDN pin; soft-start is implemented
by generating a voltage ramp at the SHDN pin using an
external resistor and capacitor.
An internal regulator provides power to the control circuitry.
This regulator includes an undervoltage lockout to prevent
switching when VIN is less than ~3.45V. The SHDN pin is
used to place the LT1936 in shutdown, disconnecting the
output and reducing the input current to less than 2μA.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate
the internal bipolar NPN power switch for efficient operation.
The oscillator reduces the LT1936’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during startup
and overload.
1936fd
7
LT1936
APPLICATIONS INFORMATION
FB Resistor Network
Inductor Selection and Maximum Output Current
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resistors according to:
A good first choice for the inductor value is
V
R1=R2 OUT – 1
1.200 R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Input Voltage Range
The input voltage range for LT1936 applications depends
on the output voltage and the Absolute Maximum Ratings
of the VIN and BOOST pins.
The minimum input voltage is determined by either the
LT1936’s minimum operating voltage of ~3.45V or by its
maximum duty cycle. The duty cycle is the fraction of
time that the internal switch is on and is determined by
the input and output voltages:
DC =
VOUT + VD
VIN – VSW + VD
where VD is the forward voltage drop of the catch diode
(~0.5V) and VSW is the voltage drop of the internal switch
(~0.5V at maximum load). This leads to a minimum input
voltage of:
VIN(MIN) =
VOUT + VD
– VD + VSW
DCMAX
with DCMAX = 0.87.
The maximum input voltage is determined by the absolute
maximum ratings of the VIN and BOOST pins and by the
minimum duty cycle DCMIN = 0.08:
VIN(MAX ) =
VOUT + VD
– VD + VSW
DCMIN
Note that this is a restriction on the operating input voltage;
the circuit will tolerate transient inputs up to the absolute
maximum ratings of the VIN and BOOST pins.
L = 2.2 (VOUT + VD)
where VD is the voltage drop of the catch diode (~0.4V)
and L is in μH. With this value the maximum output current will be above 1.2A at all duty cycles and greater than
1.4A for duty cycles less than 50% (VIN > 2 VOUT). The
inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should be
about 30% higher. For robust operation in fault conditions
(start-up or short circuit) and high input voltage (>30V),
the saturation current should be above 2.6A. To keep the
efficiency high, the series resistance (DCR) should be less
than 0.1Ω, and the core material should be intended for
high frequency applications. Table 1 lists several vendors
and suitable types.
Table 1. Inductor Vendors
VENDOR
URL
PART SERIES
TYPE
Murata
www.murata.com
LQH55D
Open
TDK
www.component.tdk.com
SLF7045
SLF10145
Shielded
Shielded
Toko
www.toko.com
D62CB
D63CB
D75C
D75F
Shielded
Shielded
Shielded
Open
Sumida
www.sumida.com
CR54
CDRH74
CDRH6D38
CR75
Open
Shielded
Shielded
Open
Of course, such a simple design guide will not always
result in the optimum inductor for your application. A
larger value provides a slightly higher maximum load
current and will reduce the output voltage ripple. If your
load is lower than 1.2A, then you can decrease the value
of the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one
with a lower DCR resulting in higher efficiency. Be aware
that if the inductance differs from the simple rule above,
then the maximum load current will depend on input voltage. There are several graphs in the Typical Performance
Characteristics section of this data sheet that show the
maximum load current as a function of input voltage and
inductor value for several popular output voltages. Low
1936fd
8
LT1936
APPLICATIONS INFORMATION
inductance may result in discontinuous mode operation,
which is okay but further reduces maximum load current.
For details of maximum output current and discontinuous
mode operation, see Linear Technology Application Note
44. Finally, for duty cycles greater than 50% (VOUT/VIN
> 0.5), there is a minimum inductance required to avoid
subharmonic oscillations. Choosing L greater than 1.6
(VOUT + VD) μH prevents subharmonic oscillations at all
duty cycles.
Catch Diode
A 1A Schottky diode is recommended for the catch diode,
D1. The diode must have a reverse voltage rating equal
to or greater than the maximum input voltage. The ON
Semiconductor MBRM140 is a good choice. It is rated
for 1A DC at a case temperature of 110°C and 1.5A at a
case temperature of 95°C. Diode Incorporated’s DFLS140L
is rated for 1.1A average current; the DFLS240L is rated
for 2A average current. The average diode current in an
LT1936 application is approximately IOUT (1 – DC).
Input Capacitor
Bypass the input of the LT1936 circuit with a 4.7μF or
higher value ceramic capacitor of X7R or X5R type. Y5V
types have poor performance over temperature and applied voltage, and should not be used. A 4.7μF ceramic
is adequate to bypass the LT1936 and will easily handle
the ripple current. However, if the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT1936 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT1936 and the catch diode; see the
PCB Layout section. A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT1936. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT1936 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT1936’s
voltage rating. This situation is easily avoided; see the Hot
Plugging Safety section.
For space sensitive applications, a 2.2μF ceramic capacitor can be used for local bypassing of the LT1936 input.
However, the lower input capacitance will result in increased input current ripple and input voltage ripple, and
may couple noise into other circuitry. Also, the increased
voltage ripple will raise the minimum operating voltage
of the LT1936 to ~3.7V.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT1936 to produce the DC output. In this role it
determines the output ripple, and low impedance at the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT1936’s control loop.
Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance.
A good value is:
COUT =
150
VOUT
where COUT is in μF. Use X5R or X7R types. This choice
will provide low output ripple and good transient response.
Transient performance can be improved with a high value
capacitor if the compensation network is also adjusted to
maintain the loop bandwidth.
A lower value of output capacitor can be used, but transient
performance will suffer. With an external compensation
network, the loop gain can be lowered to compensate for the
lower capacitor value. When using the internal compensation network, the lowest value for stable operation is:
COUT >
66
VOUT
1936fd
9
LT1936
APPLICATIONS INFORMATION
Table 2. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES
COMMENTS
Panasonic
(714) 373-7366
www.panasonic.com
Ceramic,
Polymer,
Tantalum
EEF Series
Murata
(408) 749-9714
(404) 436-1300
AVX
Taiyo Yuden
(864) 963-6300
www.kemet.com
Ceramic,
Tantalum
www.sanyovideo.com
Ceramic,
Polymer,
Tantalum
www.murata.com
Ceramic
www.avxcorp.com
Ceramic,
Tantalum
www.taiyo-yuden.com
Ceramic
This is the minimum output capacitance required, not
the nominal capacitor value. For example, a 3.3V output
requires 20μF of output capacitance. If a small 22μF, 6.3V
ceramic capacitor is used, the circuit may be unstable because the effective capacitance is lower than the nominal
capacitance when biased at 3.3V. Look carefully at the
capacitor’s data sheet to find out what the actual capacitance is under operating conditions (applied voltage and
temperature). A physically larger capacitor, or one with a
higher voltage rating, may be required.
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose one
that is intended for use in switching regulators. The ESR
should be specified by the supplier, and should be 0.05Ω
or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR. Table 2 lists
several capacitor vendors.
POSCAP
TPS Series
This capacitor (CF) is not part of the loop compensation
but is used to filter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR. An alternative to using
external compensation components is to use the internal
RC network by tying the COMP pin to the VC pin. This reduces component count but does not provide the optimum
transient response when the output capacitor value is high,
and the circuit may not be stable when the output capacitor
value is low. If the internal compensation network is not
used, tie COMP to ground or leave it floating.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
LT1936
CURRENT MODE
POWER STAGE
gm = 2mho
Frequency Compensation
SW
ERROR
AMPLIFIER
OUTPUT
R1
FB
1.2V
gm =
250μmho
The LT1936 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT1936 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size.
Frequency compensation is provided by the components
tied to the VC pin, as shown in Figure 1. Generally a capacitor (CC) and a resistor (RC) in series to ground are used.
In addition, there may be lower value capacitor in parallel.
T494, T495
+
Sanyo
(864) 963-6300
–
Kemet
CPL
ESR
C1
+
600k
C1
50k
VC
CF
RC
150pF
COMP
POLYMER
OR
TANTALUM
GND
CERAMIC
R2
CC
1936 F01
Figure 1. Model for Loop Response
1936fd
10
LT1936
APPLICATIONS INFORMATION
complicated and the best values depend on the application
and in particular the type of output capacitor. A practical
approach is to start with one of the circuits in this data
sheet that is similar to your application and tune the compensation network to optimize the performance. Stability
should then be checked across all operating conditions,
including load current, input voltage and temperature. The
LT1375 data sheet contains a more thorough discussion of
loop compensation and describes how to test the stability
using a transient load.
Figure 1 shows an equivalent circuit for the LT1936 control
loop. The error amplifier is a transconductance amplifier
with finite output impedance. The power section, consisting
of the modulator, power switch and inductor, is modeled
as a transconductance amplifier generating an output
current proportional to the voltage at the VC pin. Note that
the output capacitor integrates this current, and that the
capacitor on the VC pin (CC) integrates the error amplifier
output current, resulting in two poles in the loop. In most
cases a zero is required and comes from either the output
capacitor ESR or from a resistor RC in series with CC.
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
is much lower than the switching frequency. A phase lead
capacitor (CPL) across the feedback divider may improve
the transient response.
Figure 2 compares the transient response across several
output capacitor choices and compensation schemes.
In each case the load current is stepped from 200mA to
800mA and back to 200mA.
COUT = 22μF
(AVX 1210ZD226MAT)
(2a)
VOUT
100mV/DIV
COMP
VC
COUT = 22μF ×2
(2b)
VOUT
100mV/DIV
COMP
VC
COUT = 150μF
(4TPC150M)
(2c)
VOUT
100mV/DIV
COMP
VC
COUT = 150μF
(4TPC150M)
(2d)
VOUT
100mV/DIV
COMP
VC
220k
100pF
IOUT
500mA/DIV
800mA
200mA
50μs/DIV
1936 F02
Figure 2. Transient Load Response of the LT1936 with Different Output
Capacitors as the Load Current is Stepped from 200mA to 800mA. VOUT = 3.3V
1936fd
11
LT1936
APPLICATIONS INFORMATION
BOOST Pin Considerations
Capacitor C3 and diode D2 are used to generate a boost
voltage that is higher than the input voltage. In most cases
a 0.22μF capacitor and fast switching diode (such as the
1N4148 or 1N914) will work well. Figure 3 shows two
ways to arrange the boost circuit. The BOOST pin must
be at least 2.3V above the SW pin for best efficiency. For
outputs of 3V and above, the standard circuit (Figure 3a)
is best. For outputs between 2.8V and 3V, use a 0.47μF
capacitor and a Schottky diode. For lower output voltages
the boost diode can be tied to the input (Figure 3b), or to
another supply greater than 2.8V. The circuit in Figure 3a is
more efficient because the BOOST pin current comes from
a lower voltage. You must also be sure that the maximum
voltage rating of the BOOST pin is not exceeded.
A 2.5V output presents a special case. This is a popular
output voltage, and the advantage of connecting the
boost circuit to the output is that the circuit will accept a
36V maximum input voltage rather than 20V (due to the
BOOST pin rating). However, 2.5V is marginally adequate
to support the boosted drive stage at low ambient temperatures. Therefore, special care and some restrictions
on operation are necessary when powering the BOOST pin
from a 2.5V output. Minimize the voltage loss in the boost
D2
C3
BOOST
LT1936
VIN
VIN
VOUT
SW
GND
VBOOST – VSW ≅ VOUT
MAX VBOOST ≅ VIN + VOUT
(3a)
D2
C3
BOOST
LT1936
VIN
VIN
SW
VOUT
GND
1933 F03
VBOOST – VSW ≅ VIN
MAX VBOOST ≅ 2VIN
(3b)
Figure 3. Two Circuits for Generating the Boost Voltage
circuit by using a 1μF boost capacitor and a good, low drop
Schottky diode (such as the ON Semi MBR0540). Because
the required boost voltage increases at low temperatures,
the circuit will supply only 1A of output current when the
ambient temperature is –45°C, increasing to 1.2A at 0°C.
Also, the minimum input voltage to start the boost circuit
is higher at low temperature. See the Typical Applications
section for a 2.5V schematic and performance curves.
The minimum operating voltage of an LT1936 application
is limited by the undervoltage lockout (~3.45V) and by
the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by the
boost circuit. If the input voltage is ramped slowly, or the
LT1936 is turned on with its SHDN pin when the output
is already in regulation, then the boost capacitor may not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on input
and output voltages, and on the arrangement of the boost
circuit. The minimum load generally goes to zero once the
circuit has started. Figure 4 shows a plot of minimum load
to start and to run as a function of input voltage. In many
cases the discharged output capacitor will present a load
to the switcher, which will allow it to start. The plots show
the worst-case situation where VIN is ramping very slowly.
For lower start-up voltage, the boost diode can be tied to
VIN; however, this restricts the input range to one-half of
the absolute maximum rating of the BOOST pin.
At light loads, the inductor current becomes discontinuous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
300mV above VOUT. At higher load currents, the inductor
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT1936, requiring a higher
input voltage to maintain regulation.
Soft-Start
The SHDN pin can be used to soft-start the LT1936, reducing
the maximum input current during start-up. The SHDN pin
is driven through an external RC filter to create a voltage
ramp at this pin. Figure 5 shows the start-up waveforms
with and without the soft-start circuit. By choosing a large
1936fd
12
LT1936
APPLICATIONS INFORMATION
Minimum Input Voltage VOUT = 5V
Minimum Input Voltage VOUT = 3.3V
6.0
VOUT = 3.3V
TA = 25°C
L = 10μH
5.5
5.0
4.5
4.0
3.0
0
TO START
6
TO RUN
5
TO RUN
3.5
VOUT = 5V
TA = 25°C
L = 15μH
7
TO START
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
8
10
100
LOAD CURRENT (mA)
4
1000
1
10
100
LOAD CURRENT (mA)
1000
1936 F04b
1936 F04a
Figure 4. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
RUN
5V/DIV
RUN
SHDN
IIN
500mA/DIV
GND
VOUT
5V/DIV
RUN
1936 F05a
0.5ms/DIV
1936 F05b
RUN
5V/DIV
15k
SHDN
0.22μF
50μs/DIV
IIN
500mA/DIV
GND
VOUT
5V/DIV
Figure 5. To Soft-Start the LT1936, Add a Resistor and Capacitor to the SHDN Pin.
VIN = 12V, VOUT = 3.3V, COUT = 2 × 22μF, RLOAD = 3.3Ω
RC time constant, the peak start-up current can be reduced
to the current that is required to regulate the output, with
no overshoot. Choose the value of the resistor so that it
can supply 60μA when the SHDN pin reaches 2.3V.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate excessively, an LT1936 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT1936 is absent. This may occur in battery charging applications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT1936’s
output. If the VIN pin is allowed to float and the SHDN pin
is held high (either by a logic signal or because it is tied
to VIN), then the LT1936’s internal circuitry will pull its
quiescent current through its SW pin. This is fine if your
system can tolerate a few mA in this state. If you ground
1936fd
13
LT1936
APPLICATIONS INFORMATION
the SHDN pin, the SW pin current will drop to essentially
zero. However, if the VIN pin is grounded while the output
is held high, then parasitic diodes inside the LT1936 can
pull large currents from the output through the SW pin
and the VIN pin. Figure 6 shows a circuit that will run only
when the input voltage is present and that protects against
a shorted or reversed input.
IN
MINIMIZE
LT1936
C2, D1 LOOP
GND
C2
R4
D2
C3
R2
D1
D4
MBRS140
VIN
R1
L1
VIN
BOOST
C1
LT1936
SHDN
GND
VOUT
SW
OUT
VC
COMP GND FB
VIAS
BACKUP
1936 F07
Figure 7. A Good PCB Layout Ensures Low EMI Operation
High Temperature Considerations
1936 F06
Figure 6. Diode D4 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output; It Also Protects the Circuit
from a Reversed Input. The LT1936 Runs Only When the Input
is Present
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 7 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents flow in the LT1936’s VIN and SW pins, the catch
diode (D1) and the input capacitor (C2). The loop formed
by these components should be as small as possible. These
components, along with the inductor and output capacitor,
should be placed on the same side of the circuit board,
and their connections should be made on that layer. Place
a local, unbroken ground plane below these components.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB and VC nodes small so that the ground
traces will shield them from the SW and BOOST nodes.
The Exposed Pad on the bottom of the package must be
soldered to ground so that the pad acts as a heat sink. To
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near
the LT1936 to additional ground planes within the circuit
board and on the bottom side.
The die temperature of the LT1936 must be lower than the
maximum rating of 125°C (150°C for the H grade). This is
generally not a concern unless the ambient temperature
is above 85°C. For higher temperatures, care should be
taken in the layout of the circuit to ensure good heat sinking of the LT1936. The maximum load current should be
derated as the ambient temperature approaches 125°C
(150°C for the H grade).
The die temperature is calculated by multiplying the LT1936
power dissipation by the thermal resistance from junction
to ambient. Power dissipation within the LT1936 can be
estimated by calculating the total power loss from an
efficiency measurement and subtracting the catch diode
loss. The resulting temperature rise at full load is nearly
independent of input voltage. Thermal resistance depends
on the layout of the circuit board, but values from 40°C/W
to 60°C/W are typical.
Die temperature rise was measured on a 4-layer, 5cm ×
6.5cm circuit board in still air at a load current of 1.4A.
For 12V input to 3.3V output the die temperature elevation
above ambient was 26°C; for 24V in to 3.3V out the rise
was 31°C; for 12V in to 5V the rise was 31°C and for 24V
in to 5V the rise was 34°C.
1936fd
14
LT1936
APPLICATIONS INFORMATION
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT1936 circuits. However, these capacitors can cause problems if the LT1936 is plugged into a
live supply (see Linear Technology Application Note 88 for
a complete discussion). The low loss ceramic capacitor
combined with stray inductance in series with the power
source forms an under damped tank circuit, and the voltage
at the VIN pin of the LT1936 can ring to twice the nominal
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
VIN
input voltage, possibly exceeding the LT1936’s rating and
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT1936 into an energized
supply, the input network should be designed to prevent
this overshoot.
Figure 8 shows the waveforms that result when an LT1936
circuit is connected to a 24V supply through six feet of
24-gauge twisted pair. The first plot is the response with
a 4.7μF ceramic capacitor at the input. The input voltage
rings as high as 50V and the input current peaks at 26A.
DANGER
VIN
20V/DIV
RINGING VIN MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT1936
LT1936
+
4.7μF
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
IIN
10A/DIV
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
20μs/DIV
(8a)
LT1936
+
22μF
35V
AI.EI.
VIN
20V/DIV
+
4.7μF
IIN
10A/DIV
(8b)
0.7Ω
LT1936
20μs/DIV
VIN
20V/DIV
+
0.1μF
4.7μF
IIN
10A/DIV
(8c)
20μs/DIV
1936 F08
Figure 8. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT1936 is Connected to a Live Supply
1936fd
15
LT1936
APPLICATIONS INFORMATION
One method of damping the tank circuit is to add another
capacitor with a series resistor to the circuit. In Figure 8b
an aluminum electrolytic capacitor has been added. This
capacitor’s high equivalent series resistance damps the
circuit and eliminates the voltage overshoot. The extra
capacitor improves low frequency ripple filtering and
can slightly improve the efficiency of the circuit, though
it is likely to be the largest component in the circuit. An
alternative solution is shown in Figure 8c. A 0.7Ω resistor
is added in series with the input to eliminate the voltage
overshoot (it also reduces the peak input current). A 0.1μF
capacitor improves high frequency filtering. This solution
is smaller and less expensive than the electrolytic capacitor.
For high input voltages its impact on efficiency is minor,
reducing efficiency by one percent for a 5V output at full
load operating from 24V.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 100
shows how to generate a bipolar output supply using a
buck regulator.
Outputs Greater Than 6V
For outputs greater than 6V, add a resistor of 1k to 2.5k
across the inductor to damp the discontinuous ringing
of the SW node, preventing unintended SW current. The
12V Step-Down Converter circuit in the Typical Applications section shows the location of this resistor. Also note
that for outputs above 6V, the input voltage range will be
limited by the maximum rating of the BOOST pin. The 12V
circuit shows how to overcome this limitation using an
additional Zener diode.
TYPICAL APPLICATIONS
3.3V Step-Down Converter
D2
VIN
4.5V TO 36V
ON OFF
C1
4.7μF
VIN
BOOST
SHDN
SW
LT1936
COMP
VC
C3
0.22μF
L1
10μH
D1
R1
17.4k
VOUT
3.3V
1.2A
FB
GND
R2
10k
C2
47μF
1936 TA03
1936fd
16
LT1936
TYPICAL APPLICATIONS
5V Step-Down Converter
D2
VIN
6.3V TO 36V
ON OFF
C1
4.7μF
VIN
BOOST
SHDN
SW
LT1936
COMP
VC
C3
0.22μF
L1
15μH
D1
R1
31.6k
VOUT
5V
1.2A
FB
GND
R2
10k
C2
22μF
1936 TA04
1.8V Step-Down Converter
Efficiency, 1.8V Output
90
D2
BOOST
SHDN
SW
C1
4.7μF
L1
4.7μH
D1
LT1936
COMP
C3
0.22μF
R1
10k
VOUT
1.8V
1.3A
FB
VC
GND
C2
47μF
×2
R2
20k
2.0
VOUT = 1.8V
TA = 25°C
VIN = 5V
80
1.5
VIN = 12V
70
1.0
60
0.5
POWER LOSS (W)
ON OFF
VIN
EFFICIENCY (%)
VIN
3.6V TO 20V
POWER LOSS
D1: DFLS140L
D2: 1N4148
L1: TOKO D63CB
1936 TA05a
50
0
0
1.5
0.5
1
LOAD CURRENT (A)
1936 TA05b
1.2V Step-Down Converter
Efficiency, 1.2V Output
D2
VIN
BOOST
SHDN
SW
C1
4.7μF
C3
0.22μF
D1
LT1936
COMP
VC
L1
3.3μH
75
VOUT
1.2V
1.3A
FB
GND
100k
C2
47μF
×2
VIN = 5V
1.5
VIN = 12V
1.0
70
65
60
0.5
55
D1: DFLS140L
D2: 1N4148
L1: TOKO D63CB
2.0
VOUT = 1.2V
TA = 25°C
POWER LOSS (W)
ON OFF
80
EFFICIENCY (%)
VIN
3.6V TO 20V
POWER LOSS
1936 TA06a
50
0
0.5
1
LOAD CURRENT (A)
0
1.5
1936 TA06b
1936fd
17
LT1936
TYPICAL APPLICATIONS
2.5V Step-Down Converter
D2
VIN
3.6V TO 36V
ON OFF
VIN
BOOST
SHDN
SW
C1
4.7μF
LT1936
COMP
C3
1μF
L1
6.2μH
D1
R1
11k
VOUT
2.5V
1.2A
TA > 0°C
FB
VC
GND
R2
10k
C2
47μF
D1: DFLS140L
D2: MBRO540
L1: TOKO D63CB
1936 TA07a
Efficiency, 2.5V Output
100
Minimum Input Voltage
5.5
VOUT = 2.5V
TA = 25°C
5.0
INPUT VOLTAGE (V)
90
EFFICIENCY (%)
VOUT = 2.5V
VIN = 5V
80
VIN = 12V
TO START
TA = –45°C
4.5
TO START
TA = 25°C
4.0
TO RUN
TA = –45°C
70
3.5
TO RUN
TA = 25°C
60
0
3.0
1.5
0.5
1.0
LOAD CURRENT (A)
100
10
LOAD CURRENT (mA)
1
1936 TA07b
1000
1936 TA07c
12V Step-Down Converter
D2
D3
6.8V
VIN
14.5V TO 36V
ON OFF
VIN
BOOST
SHDN
SW
C1
2.2μF
VC
D1: MBRM140
D2: 1N4148
D3: CMDZ5235B
L1
22μH
1.8k
D1
LT1936
COMP
C3
0.22μF
VOUT
12V
1.2A
FB
GND
R2
20k
R1
182k
C2
22μF
1936 TA08
1936fd
18
LT1936
PACKAGE DESCRIPTION
MS8E Package
8-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1662 Rev E)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.06 ± 0.102
(.081 ± .004)
1
0.889 ± 0.127
(.035 ± .005)
2.794 ± 0.102
(.110 ± .004)
0.29
REF
1.83 ± 0.102
(.072 ± .004)
0.05 REF
5.23
(.206)
MIN
2.083 ± 0.102 3.20 – 3.45
(.082 ± .004) (.126 – .136)
DETAIL “B”
8
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
0.65
(.0256)
BSC
0.42 ± 0.038
(.0165 ± .0015)
TYP
8
7 6 5
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
0.52
(.0205)
REF
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
DETAIL “A”
0° – 6° TYP
GAUGE PLANE
1
0.53 ± 0.152
(.021 ± .006)
DETAIL “A”
2 3
4
1.10
(.043)
MAX
0.86
(.034)
REF
0.18
(.007)
SEATING
PLANE
0.22 – 0.38
(.009 – .015)
TYP
0.65
(.0256)
BSC
0.1016 ± 0.0508
(.004 ± .002)
MSOP (MS8E) 0908 REV E
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
1936fd
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.
19
LT1936
TYPICAL APPLICATION
2.5V Step-Down Converter
Minimum Input Voltage
5.5
D2
ON OFF
VIN
BOOST
SHDN
SW
C1
4.7μF
C3
0.22μF
D1
LT1936
COMP
VC
L1
8.2μH
R1
11k
FB
GND
VOUT = 2.5V
5.0
VOUT
2.5V
1.3A
R2
10k
C2
47μF
INPUT VOLTAGE (V)
VIN
3.6V TO 20V
CONNECTING THE BOOST CIRCUIT TO THE
INPUT LOWERS THE MINIMUM INPUT
VOLTAGE TO RUN AND TO START TO LESS
THAN 3.7V AT ALL LOADS
4.5
4.0
3.5
D1: DFLS140L
D2: 1N4148
L1: TOKO D63CB
1936 TA09a
3.0
1
100
10
LOAD CURRENT (mA)
1000
1936 TA09b
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25V, Dual 1.4A (IOUT), 1.1MHz, High Efficiency Step-Down
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60V, 1.2A (IOUT), 500kHz, High Efficiency Step-Down
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60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down
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80V, 50mA, Low Noise Linear Regulator
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Dual 600mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40μA, ISD < 1μA,
10-Lead MSE Package
LTC3412
2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC
Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60μA, ISD < 1μA,
16-Lead TSSOPE Package
LTC3414
4A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter
VIN: 2.3V to 5.5V, VOUT(MIN) = 0.8V, IQ = 64μA, ISD < 1μA,
20-Lead TSSOPE Package
LT3430/LT3431
60V, 2.75A (IOUT), 200kHz/500kHz, High Efficiency
Step-Down DC/DC Converters
VIN: 5.5V to 60V, VOUT(MIN) = 1.20V, IQ = 2.5mA, ISD = 30μA,
16-Lead TSSOPE Package
Burst Mode is a registered trademark of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation.
1936fd
20 Linear Technology Corporation
LT 1108 REV D • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
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© LINEAR TECHNOLOGY CORPORATION 2006