LTC3406B - 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406B
1.5MHz, 600mA
Synchronous Step-Down
Regulator in ThinSOT
U
FEATURES
■
■
■
■
■
■
■
■
■
■
■
■
DESCRIPTIO
The LTC ®3406B is a high efficiency monolithic synchronous buck regulator using a constant frequency, current
mode architecture. The device is available in an adjustable
version and fixed output voltages of 1.5V and 1.8V. Supply
current with no load is 300µA and drops to <1µA in
shutdown. The 2.5V to 5.5V input voltage range makes the
LTC3406B ideally suited for single Li-Ion battery-powered
applications. 100% duty cycle provides low dropout operation, extending battery life in portable systems. PWM
pulse skipping mode operation provides very low output
ripple voltage for noise sensitive applications.
High Efficiency: Up to 96%
600mA Output Current at VIN = 3V
2.5V to 5.5V Input Voltage Range
1.5MHz Constant Frequency Operation
No Schottky Diode Required
Low Dropout Operation: 100% Duty Cycle
Low Quiescent Current: 300µA
0.6V Reference Allows Low Output Voltages
Shutdown Mode Draws < 1µA Supply Current
Current Mode Operation for Excellent Line and
Load Transient Response
Overtemperature Protected
Low Profile (1mm) ThinSOTTM Package
Switching frequency is internally set at 1.5MHz, allowing
the use of small surface mount inductors and capacitors.
The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode. Low
output voltages are easily supported with the 0.6V feedback reference voltage. The LTC3406B is available in a low
profile (1mm) ThinSOT package. Refer to LTC3406 for
applications that require Burst Mode® operation.
U
APPLICATIO S
■
■
■
■
■
Cellular Telephones
Personal Information Appliances
Wireless and DSL Modems
Digital Still Cameras
MP3 Players
Portable Instruments
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
Protected by U.S. Patents, including 6580258, 5481178.
U
■
TYPICAL APPLICATIO
100
VOUT = 1.8V
90
VIN
2.7V
TO 5.5V
4
CIN**
4.7µF
CER
VIN
SW
3
COUT†
10µF
CER
LTC3406B-1.8
1
VOUT
RUN
5
3406B F01a
VOUT
1.8V
600mA
EFFICIENCY (%)
80
2.2µH*
70
VIN = 2.7V
50
40
GND
30
2
20
*MURATA LQH32CN2R2M33
**TAIYO YUDEN JMK212BJ475MG
†
TAIYO YUDEN JMK316BJ106ML
VIN = 3.6V
60
VIN = 4.2V
10
0.1
1
100
10
OUTPUT CURRENT (mA)
1000
3406B F01b
Figure 1a. High Efficiency Step-Down Converter
Figure 1b. Efficiency vs Load Current
3406bfa
1
LTC3406B
U
W W
W
ABSOLUTE
AXI U
RATI GS
(Note 1)
Input Supply Voltage .................................. – 0.3V to 6V
RUN, VFB Voltages ..................................... – 0.3V to VIN
SW Voltage .................................. – 0.3V to (VIN + 0.3V)
P-Channel Switch Source Current (DC) ............. 800mA
N-Channel Switch Sink Current (DC) ................. 800mA
Peak SW Sink and Source Current ........................ 1.3A
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Junction Temperature (Notes 3, 6) ...................... 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
U
U
W
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
RUN 1
5 VFB
LTC3406BES5
GND 2
SW 3
4 VIN
S5 PACKAGE
5-LEAD PLASTIC TSOT-23
RUN 1
S5 PART MARKING
LTE2
5 VOUT
LTC3406BES5-1.5
LTC3406BES5-1.8
GND 2
SW 3
TJMAX = 125°C, θJA = 250°C/ W, θJC = 90°C/ W
ORDER PART
NUMBER
TOP VIEW
4 VIN
S5 PART MARKING
S5 PACKAGE
5-LEAD PLASTIC TSOT-23
LTE3
LTE4
TJMAX = 125°C, θJA = 250°C/ W, θJC = 90°C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C. VIN = 3.6V unless otherwise specified.
SYMBOL
PARAMETER
IVFB
Feedback Current
VFB
Regulated Feedback Voltage
CONDITIONS
MIN
●
LTC3406B (Note 4) TA = 25°C
LTC3406B (Note 4) 0°C ≤ TA ≤ 85°C
LTC3406B (Note 4) –40°C ≤ TA ≤ 85°C
●
∆VFB
Reference Voltage Line Regulation
VIN = 2.5V to 5.5V (Note 4)
●
VOUT
Regulated Output Voltage
LTC3406B-1.5
LTC3406B-1.8
●
●
∆VOVL
Output Overvoltage Lockout
∆VOVL = VOVL – VFB, LTC3406B
∆VOVL = VOVL – VOUT, LTC3406B-1.5/LTC3406B-1.8
∆VOUT
Output Voltage Line Regulation
VIN = 2.5V to 5.5V
IPK
Peak Inductor Current
VIN = 3V, VFB = 0.5V or VOUT = 90%,
Duty Cycle < 35%
VLOADREG
Output Voltage Load Regulation
VIN
Input Voltage Range
IS
Input DC Bias Current
Shutdown
TYP
MAX
UNITS
±30
nA
0.6120
0.6135
0.6150
V
V
V
0.5880
0.5865
0.5850
0.6
0.6
0.6
0.04
0.4
1.455
1.746
1.500
1.800
1.545
1.854
20
2.5
50
7.8
80
13
mV
%
0.04
0.4
%
1
1.25
A
●
0.75
0.5
●
2.5
(Note 5)
VFB = 0.5V or VOUT = 90%
VRUN = 0V, VIN = 4.2V
●
1.2
%/V
V
V
%/V
5.5
V
300
0.1
400
1
µA
µA
1.5
210
1.8
MHz
kHz
fOSC
Oscillator Frequency
VFB = 0.6V or VOUT = 100%
VFB = 0V or VOUT = 0V
RPFET
RDS(ON) of P-Channel FET
ISW = 100mA
0.4
0.5
Ω
RNFET
RDS(ON) of N-Channel FET
ISW = –100mA
0.35
0.45
Ω
ILSW
SW Leakage
VRUN = 0V, VSW = 0V or 5V, VIN = 5V
±0.01
±1
µA
3406bfa
2
LTC3406B
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C. VIN = 3.6V unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
VRUN
RUN Threshold
●
IRUN
RUN Leakage Current
●
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LTC3406BE 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
with statistical process controls.
Note 3: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
LTC3406B: TJ = TA + (PD)(250°C/W)
MIN
TYP
MAX
UNITS
0.3
1
1.5
V
±0.01
±1
µA
Note 4: The LTC3406B is tested in a proprietary test mode that connects
VFB to the output of the error amplifier.
Note 5: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 6: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure1a Except for the Resistive Divider Resistor Values)
Efficiency vs Input Voltage
100
100
TA = 25°C
95
VOUT = 1.2V
TA = 25°C
100
IOUT = 10mA
75
70
VIN = 3.6V
60
VIN = 4.2V
50
40
70
55
20
20
50
10
0.1
6
3406B G01
Reference Voltage vs
Temperature
VIN = 4.2V
REFERENCE VOLTAGE (V)
VIN = 2.7V
50
40
30
10
0.1
1.60
0.604
0.599
0.594
1000
3406B G04
1.55
1.50
1.45
1.40
0.589
VIN = 3.6V
1
100
10
OUTPUT CURRENT (mA)
VIN = 3.6V
1.65
0.609
60
20
1.70
VIN = 3.6V
80
70
Oscillator Frequency vs
Temperature
FREQUENCY (MHz)
90
3406B GO3
0.614
VOUT = 2.5V
TA = 25°C
1000
1
100
10
OUTPUT CURRENT (mA)
3406B GO2
Efficiency vs Output Current
100
10
0.1
1000
1
100
10
OUTPUT CURRENT (mA)
VIN = 4.2V
40
30
5
4
INPUT VOLTAGE (V)
VIN = 3.6V
50
30
3
VIN = 2.7V
60
60
2
VOUT = 1.5V
TA = 25°C
80
70
EFFICIENCY (%)
80
65
EFFICIENCY (%)
90
VIN = 2.7V
80
IOUT = 600mA
85
EFFICIENCY (%)
EFFICIENCY (%)
90
IOUT = 100mA
90
Efficiency vs Output Current
Efficiency vs Output Current
0.584
–50 –25
1.35
50
25
75
0
TEMPERATURE (°C)
100
125
3406B G05
1.30
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3406B G06
3406bfa
3
LTC3406B
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure1a Except for the Resistive Divider Resistor Values)
Oscillator Frequency vs
Supply Voltage
1.844
TA = 25°C
1.5
1.4
1.3
1.2
2
3
4
5
SUPPLY VOLTAGE (V)
6
1.824
0.5
1.814
1.804
1.784
0.1
0.2
0.1
340
320
300
280
260
240
220
MAIN SWITCH
SYNCHRONOUS SWITCH
50
25
75
0
TEMPERATURE (°C)
DYNAMIC SUPPLY CURRENT (µA)
DYNAMIC SUPPLY CURRENT (µA)
RDS(ON) (Ω)
0.3
360
200
100
125
3
2
5
4
SUPPLY VOLTAGE (V)
6
3406B G10
Switch Leakage vs Temperature
100
SWITCH LEAKAGE (pA)
SWITCH LEAKAGE (nA)
250
150
100
MAIN SWITCH
50
25
75
0
TEMPERATURE (°C)
240
220
200
–50 –25
125
3406B G13
50
25
75
0
TEMPERATURE (°C)
80
60
125
3406B G12
VOUT
10mV/DIV
AC COUPLED
MAIN
SWITCH
IL
200mA/DIV
40
0
100
SW
2V/DIV
SYNCHRONOUS
SWITCH
20
100
260
Discontinuous Operation
VIN = 3.6V
VOUT = 1.8V
ILOAD = 50mA
SYNCHRONOUS SWITCH
0
–50 –25
280
RUN = 0V
TA = 25°C
VIN = 5.5V
RUN = 0V
200
VIN = 3.6V
VOUT = 1.8V
ILOAD = 0A
300
Switch Leakage vs Input Voltage
120
50
320
3406B G11
300
7
6
Dynamic Supply Current vs
Temperature
VOUT = 1.8V
ILOAD = 0A
TA = 25°C
380
0.4
5
4
2
3
INPUT VOLTAGE (V)
3406B G09
340
400
VIN = 2.7V
0.5
1
3406B G08
0.7
0
–50 –25
0
100 200 300 400 500 600 700 800 900
LOAD CURRENT (mA)
Dynamic Supply Current vs
Supply Voltage
VIN = 3.6V
SYNCHRONOUS
SWITCH
0
0
RDS(ON) vs Temperature
VIN = 4.2V
0.3
0.2
1.774
MAIN
SWITCH
0.4
1.794
3406B G07
0.6
TA = 25°C
0.6
RDS(ON) (Ω)
1.6
0.7
VIN = 3.6V
TA = 25°C
1.834
1.7
OUTPUT VOLTAGE (V)
OSCILLATOR FREQUENCY (MHz)
1.8
RDS(ON) vs Input Voltage
Output Voltage vs Load Current
0
1
2
3
4
INPUT VOLTAGE (V)
5
1µs/DIV
3406B G15
6
3406B G14
3406bfa
4
LTC3406B
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a Except for the Resistive Divider Resistor Values)
Start-Up from Shutdown
Load Step
RUN
5V/DIV
Load Step
VOUT
100mV/DIV
AC COUPLED
VOUT
1V/DIV
VOUT
100mV/DIV
AC COUPLED
IL
500mA/DIV
IL
500mA/DIV
IL
500mA/DIV
ILOAD
500mA/DIV
ILOAD
500mA/DIV
VIN = 3.6V
40µs/DIV
VOUT = 1.8V
ILOAD = 600mA (LOAD: 3Ω RESISTOR)
3406B G16
3406B G17
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
ILOAD = 0mA TO 600mA
Load Step
3406B G18
Load Step
VOUT
100mV/DIV
AC COUPLED
VOUT
100mV/DIV
AC COUPLED
IL
500mA/DIV
IL
500mA/DIV
ILOAD
500mA/DIV
ILOAD
500mA/DIV
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
ILOAD = 100mA TO 600mA
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
ILOAD = 50mA TO 600mA
3406B G19
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
ILOAD = 200mA TO 600mA
3406B G20
U
U
U
PI FU CTIO S
RUN (Pin 1): Run Control Input. Forcing this pin above
1.5V enables the part. Forcing this pin below 0.3V shuts
down the device. In shutdown, all functions are disabled
drawing <1µA supply current. Do not leave RUN floating.
GND (Pin 2): Ground Pin.
SW (Pin 3): Switch Node Connection to Inductor. This pin
connects to the drains of the internal main and synchronous power MOSFET switches.
VIN (Pin 4): Main Supply Pin. Must be closely decoupled
to GND, Pin 2, with a 2.2µF or greater ceramic capacitor.
VFB (Pin 5) (LTC3406B): Feedback Pin. Receives the
feedback voltage from an external resistive divider across
the output.
VOUT (Pin 5) (LTC3406B-1.5/LTC3406B-1.8): Output Voltage Feedback Pin. An internal resistive divider divides the
output voltage down for comparison to the internal reference voltage.
3406bfa
5
LTC3406B
W
FU CTIO AL DIAGRA
U
U
SLOPE
COMP
OSC
OSC
4 VIN
FREQ
SHIFT
–
VFB /VOUT
+
5
LTC3406B-1.5
R1 + R2 = 550k
0.6V
R1
FB
LTC3406B-1.8
R1 + R2 = 540k
+
ICOMP
R2
S
Q
R
Q
RS LATCH
VIN
–
0.6V REF
0.6V + ∆VOVL
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTISHOOTTHRU
3 SW
+
+
1
OV
OVDET
RUN
5Ω
+
–
– EA
SHUTDOWN
IRCMP
2 GND
–
3406B BD
U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
Pulse Skipping Mode Operation
The LTC3406B uses a constant frequency, current mode
step-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET) switches
are internal. During normal operation, the internal top
power MOSFET is turned on each cycle when the oscillator
sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor
current at which ICOMP resets the RS latch, is controlled by
the output of error amplifier EA. When the load current
increases, it causes a slight decrease in the feedback
voltage, FB, relative to the 0.6V reference, which in turn,
causes the EA amplifier’s output voltage to increase until
the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is
turned on until either the inductor current starts to reverse,
as indicated by the current reversal comparator IRCMP, or
the beginning of the next clock cycle. The comparator
OVDET guards against transient overshoots >7.8% by
turning the main switch off and keeping it off until the fault
is removed.
At light loads, the inductor current may reach zero or reverse on each pulse. The bottom MOSFET is turned off by
the current reversal comparator, IRCMP, and the switch
voltage will ring. This is discontinuous mode operation,
and is normal behavior for the switching regulator. At very
light loads, the LTC3406B will automatically skip pulses in
pulse skipping mode operation to maintain output regulation. Refer to LTC3406 data sheet if Burst Mode operation
is preferred.
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator is reduced to about 210kHz, 1/7 the nominal
frequency. This frequency foldback ensures that the inductor current has more time to decay, thereby preventing
runaway. The oscillator’s frequency will progressively
increase to 1.5MHz when VFB or VOUT rises above 0V.
3406bfa
6
LTC3406B
U
OPERATIO (Refer to Functional Diagram)
Dropout Operation
Slope Compensation and Inductor Peak Current
As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward the
maximum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one cycle
until it reaches 100% duty cycle. The output voltage will then
be determined by the input voltage minus the voltage drop
across the P-channel MOSFET and the inductor.
Slope compensation provides stability in constant frequency architectures by preventing subharmonic oscillations at high duty cycles. It is accomplished internally by
adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this
results in a reduction of maximum inductor peak current
for duty cycles > 40%. However, the LTC3406B uses a
patent-pending scheme that counteracts this compensating ramp, which allows the maximum inductor peak
current to remain unaffected throughout all duty cycles.
Low Supply Operation
The LTC3406B will operate with input supply voltages as
low as 2.5V, but the maximum allowable output current is
reduced at this low voltage. Figure 2 shows the reduction
in the maximum output current as a function of input
voltage for various output voltages.
1200
MAXIMUM OUTPUT CURRENT (mA)
An important detail to remember is that at low input supply
voltages, the RDS(ON) of the P-channel switch increases
(see Typical Performance Characteristics). Therefore, the
user should calculate the power dissipation when the
LTC3406B is used at 100% duty cycle with low input
voltage (See Thermal Considerations in the Applications
Information section).
1000
800
600
VOUT = 1.8V
VOUT = 2.5V
VOUT = 1.5V
400
200
0
2.5
3.0
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
5.0
5.5
3406B F02
Figure 2. Maximum Output Current vs Input Voltage
3406bfa
7
LTC3406B
U
W
U U
APPLICATIO S I FOR ATIO
The basic LTC3406B application circuit is shown in Figure
1. External component selection is driven by the load
requirement and begins with the selection of L followed by
CIN and COUT.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE
(µH)
DCR
(Ω MAX)
Sumida
CDRH3D16
1.5
2.2
3.3
4.7
0.043
0.075
0.110
0.162
1.55
1.20
1.10
0.90
3.8 × 3.8 × 1.8
Sumida
CMD4D06
2.2
3.3
4.7
0.116
0.174
0.216
0.950
0.770
0.750
3.5 × 4.3 × 0.8
Panasonic
ELT5KT
3.3
4.7
0.17
0.20
1.00
0.95
4.5 × 5.4 × 1.2
Murata
LQH3C
1.0
2.2
4.7
0.060
0.097
0.150
1.00
0.79
0.65
2.5 × 3.2 × 2.0
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1µH to 4.7µH. Its value is chosen based on the
desired ripple current. Large value inductors lower ripple
current and small value inductors result in higher ripple
currents. Higher VIN or VOUT also increases the ripple
current as shown in equation 1. A reasonable starting point
for setting ripple current is ∆IL = 240mA (40% of 600mA).
∆IL =
⎛ V ⎞
VOUT ⎜ 1 − OUT ⎟
VIN ⎠
⎝
f L
1
( )( )
(1)
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720mA rated
inductor should be enough for most applications (600mA
+ 120mA). For better efficiency, choose a low DC-resistance inductor.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron core inductors with
similar electrical characteristics. The choice of which style
inductor to use often depends more on the price vs size
requirements and any radiated field/EMI requirements
than on what the LTC3406B requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3406B applications.
MAX DC
SIZE
CURRENT (A) W × L × H (mm3)
CIN and COUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle VOUT/VIN. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
CIN required IRMS ≅ IOMAX
[V ( V
OUT
IN − VOUT
1/ 2
)]
VIN
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations
do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based on
2000 hours of life. This makes it advisable to further derate
the capacitor, or choose a capacitor rated at a higher
temperature than required. Always consult the manufacturer if there is any question.
The selection of COUT is driven by the required effective
series resistance (ESR).
3406bfa
8
LTC3406B
U
W
U U
APPLICATIO S I FOR ATIO
Typically, once the ESR requirement for COUT has been
met, the RMS current rating generally far exceeds the
IRIPPLE(P-P) requirement. The output ripple ∆VOUT is determined by:
⎛
1 ⎞
∆VOUT ≅ ∆IL ⎜ ESR +
⎟
8fC OUT ⎠
⎝
where f = operating frequency, COUT = output capacitance
and ∆IL = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ∆IL increases with input voltage.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalum. These are
specially constructed and tested for low ESR so they give
the lowest ESR for a given volume. Other capacitor types
include Sanyo POSCAP, Kemet T510 and T495 series, and
Sprague 593D and 595D series. Consult the manufacturer
for other specific recommendations.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. Because the
LTC3406B’s control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size.
However, care must be taken when ceramic capacitors are
used at the input and the output. When a ceramic capacitor
is used at the input and the power is supplied by a wall
adapter through long wires, a load step at the output can
induce ringing at the input, VIN. At best, this ringing can
couple to the output and be mistaken as loop instability. At
worst, a sudden inrush of current through the long wires
can potentially cause a voltage spike at VIN, large enough
to damage the part.
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size.
Output Voltage Programming (LTC3406B Only)
In the adjustable version, the output voltage is set by a
resistive divider according to the following formula:
⎛ R2⎞
VOUT = 0.6V ⎜ 1 + ⎟
⎝ R1⎠
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 3.
0.6V ≤ VOUT ≤ 5.5V
R2
VFB
LTC3406B
R1
GND
3406B F03
Figure 3. Setting the LTC3406B Output Voltage
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
often useful to analyze individual losses to determine what
is limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
3406bfa
9
LTC3406B
U
W
U U
APPLICATIO S I FOR ATIO
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses in LTC3406B circuits: VIN quiescent current and I2R
losses. The VIN quiescent current loss dominates the
efficiency loss at very low load currents whereas the I2R
loss dominates the efficiency loss at medium to high load
currents. In a typical efficiency plot, the efficiency curve at
very low load currents can be misleading since the actual
power lost is of no consequence as illustrated in Figure 4.
VIN = 3.6V
POWER LOSS (W)
0.1
VOUT = 2.5V
Other losses including CIN and COUT ESR dissipative
losses and inductor core losses generally account for less
than 2% total additional loss.
VOUT = 1.8V
VOUT = 1.2V
0.001
VOUT = 1.5V
0.0001
0.1
RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)
The RDS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Charateristics
curves. Thus, to obtain I2R losses, simply add RSW to
RL and multiply the result by the square of the average
output current.
1
0.01
2. I2R losses are calculated from the resistances of the
internal switches, RSW, and external inductor RL. In
continuous mode, the average output current flowing
through inductor L is “chopped” between the main
switch and the synchronous switch. Thus, the series
resistance looking into the SW pin is a function of both
top and bottom MOSFET RDS(ON) and the duty cycle
(DC) as follows:
Thermal Considerations
1
10
100
LOAD CURRENT (mA)
1000
3406B F04
Figure 4. Power Lost vs Load Current
1. The VIN quiescent current is due to two components:
the DC bias current as given in the electrical characteristics and the internal main switch and synchronous
switch gate charge currents. The gate charge current
results from switching the gate capacitance of the
internal power MOSFET switches. Each time the gate is
switched from high to low to high again, a packet of
charge, dQ, moves from VIN to ground. The resulting
dQ/dt is the current out of VIN that is typically larger than
the DC bias current. In continuous mode, IGATECHG =
f(QT + QB) where QT and QB are the gate charges of the
internal top and bottom switches. Both the DC bias and
gate charge losses are proportional to VIN and thus
their effects will be more pronounced at higher supply
voltages.
In most applications the LTC3406B does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3406B is running at high ambient temperature with low supply voltage and high duty cycles, such
as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction
temperature reaches approximately 150°C, both power
switches will be turned off and the SW node will become
high impedance.
To avoid the LTC3406B from exceeding the maximum
junction temperature, the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated exceeds the
maximum junction temperature of the part. The temperature rise is given by:
TR = (PD)(θJA)
where PD is the power dissipated by the regulator and θJA
is the thermal resistance from the junction of the die to the
ambient temperature.
3406bfa
10
LTC3406B
U
W
U U
APPLICATIO S I FOR ATIO
The junction temperature, TJ, is given by:
T J = TA + TR
where TA is the ambient temperature.
As an example, consider the LTC3406B in dropout at an
input voltage of 2.7V, a load current of 600mA and an
ambient temperature of 70°C. From the typical performance graph of switch resistance, the RDS(ON) of the
P-channel switch at 70°C is approximately 0.52Ω. Therefore, power dissipated by the part is:
PD = ILOAD2 • RDS(ON) = 187.2mW
For the SOT-23 package, the θJA is 250°C/ W. Thus, the
junction temperature of the regulator is:
TJ = 70°C + (0.1872)(250) = 116.8°C
which is below the maximum junction temperature of
125°C.
Note that at higher supply voltages, the junction temperature is lower due to reduced switch resistance (RDS(ON)).
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
equal to (∆ILOAD • ESR), where ESR is the effective series
resistance of COUT. ∆ILOAD also begins to charge or
discharge COUT, which generates a feedback error signal.
The regulator loop then acts to return VOUT to its steadystate value. During this recovery time VOUT can be monitored for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
A second, more severe transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with COUT, causing a rapid drop in VOUT. No regulator can
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately (25 • CLOAD).
Thus, a 10µF capacitor charging to 3.3V would require a
250µs rise time, limiting the charging current to about
130mA.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3406B. These items are also illustrated graphically in
Figures 5 and 6. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
trace and the VIN trace should be kept short, direct and
wide.
2. Does the VFB pin connect directly to the feedback
resistors? The resistive divider R1/R2 must be connected between the (+) plate of COUT and ground.
3. Does the (+) plate of CIN connect to VIN as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
4. Keep the switching node, SW, away from the sensitive
VFB node.
5. Keep the (–) plates of CIN and COUT as close as possible.
3406bfa
11
LTC3406B
U
W
U U
APPLICATIO S I FOR ATIO
1
RUN
VFB
5
LTC3406B
2
–
3
+
L1
SW
RUN
LTC3406B-1.8
COUT
VOUT
1
R1
R2
GND
VIN
2
CFWD
4
–
5
COUT
VOUT
CIN
GND VOUT
+
3
L1
SW
VIN
4
CIN
+
VIN
VIN
–
3406B F05b
3406B F05a
BOLD LINES INDICATE HIGH CURRENT PATHS
BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 5a. LTC3406B Layout Diagram
Figure 5b. LTC3406B-1.8 Layout Diagram
VIA TO GND
R1
L1
PIN 1
CFWD
LTC3406B
LTC3406B-1.8
VOUT
SW
L1
COUT
SW
COUT
CIN
CIN
GND
GND
3406B F06b
3406B F06a
Figure 6a. LTC3406B Suggested Layout
Figure 6b. LTC3406B-1.8 Suggested Layout
Design Example
As a design example, assume the LTC3406B is used in a
single lithium-ion battery-powered cellular phone
application. The VIN will be operating from a maximum of
4.2V down to about 2.7V. The load current requirement
is a maximum of 0.6A but most of the time it will be in
standby mode, requiring only 2mA. Efficiency at both low
and high load currents is important. Output voltage is
2.5V. With this information we can calculate L using
equation (1),
L=
⎛ V ⎞
VOUT ⎜ 1 − OUT ⎟
VIN ⎠
⎝
f ∆IL
1
( )( )
VIN
VIA TO VOUT
R2
PIN 1
VOUT
VIA TO VOUT
VIA TO VIN
VIN
VIA TO VIN
(3)
Substituting VOUT = 2.5V, VIN = 4.2V, ∆IL = 240mA and
f = 1.5MHz in equation (3) gives:
L=
⎛ 2.5V ⎞
2.5V
⎜1 −
⎟ = 2.81µH
1.5MHz(240mA) ⎝ 4.2V ⎠
A 2.2µH inductor works well for this application. For best
efficiency choose a 720mA or greater inductor with less
than 0.2Ω series resistance.
CIN will require an RMS current rating of at least 0.3A ≅
ILOAD(MAX)/2 at temperature and COUT will require an ESR
of less than 0.25Ω. In most cases, a ceramic capacitor will
satisfy this requirement.
3406bfa
12
LTC3406B
U
W
U U
APPLICATIO S I FOR ATIO
For the feedback resistors, choose R1 = 316k. R2 can
then be calculated from equation (2) to be:
Figure 7 shows the complete circuit along with its efficiency curve.
100
⎛V
⎞
R2 = ⎜ OUT − 1⎟ R1 = 1000k
⎝ 0.6
⎠
4
CIN†
4.7µF
CER
VIN
SW
2.2µH*
3
VOUT
2.5V
22pF
COUT**
10µF
CER
LTC3406B
1
VFB
RUN
VIN = 4.2V
80
5
EFFICIENCY (%)
VIN
2.7V
TO 4.2V
VOUT = 2.5V
90
50
40
VIN = 3.6V
20
316k
2
60
30
1M
GND
VIN = 2.7V
70
10
0.1
3406B F07a
*MURATA LQH32CN2R2M33
** TAIYO YUDEN JHK316BJ106ML
†
TAIYO YUDEN JMK212BJ475MG
1000
1
100
10
OUTPUT CURRENT (mA)
3406B F07b
Figure 7b
Figure 7a
U
TYPICAL APPLICATIO S
Efficiency vs Output Current
Single Li-Ion 1.5V/600mA Regulator for
High Efficiency and Small Footprint
4
CIN**
4.7µF
CER
VIN
SW
3
2.2µH*
LTC3406B-1.5
1
RUN
VOUT
GND
5
VOUT = 1.5V
90
COUT1†
10µF
CER
3406B TA05
VIN = 2.7V
80
VOUT
1.5V
EFFICIENCY (%)
VIN
2.7V
TO 4.2V
100
70
VIN = 3.6V
60
VIN = 4.2V
50
40
2
*MURATA LQH32CN2R2M33
**TAIYO YUDEN CERAMIC JMK212BJ475MG
†
TAIYO YUDEN CERAMIC JMK316BJ106ML
30
20
10
0.1
1
100
10
OUTPUT CURRENT (mA)
1000
3406B TA06
Load Step
Load Step
VOUT
100mV/DIV
AC COUPLED
VOUT
100mV/DIV
AC COUPLED
IL
500mA/DIV
IL
500mA/DIV
ILOAD
500mA/DIV
ILOAD
500mA/DIV
VIN = 3.6V
20µs/DIV
VOUT = 1.5V
ILOAD = 0mA TO 600mA
3406B TA07
VIN = 3.6V
20µs/DIV
VOUT = 1.5V
ILOAD = 200mA TO 600mA
3406B TA08
3406bfa
13
LTC3406B
U
TYPICAL APPLICATIO S
Single Li-Ion 1.2V/600mA Regulator for
High Efficiency and Small Footprint
Efficiency vs Output Current
100
4
CIN†
4.7µF
CER
VIN
SW
3
2.2µH*
VOUT
1.2V
22pF
COUT**
10µF
CER
LTC3406B
1
VFB
RUN
5
GND
2
301k
*MURATA LQH32CN2R2M33
** TAIYO YUDEN JHK316BJ106ML
†
TAIYO YUDEN JMK212BJ475MG
3406B TA09
301k
VOUT = 1.2V
90
VIN = 2.7V
80
EFFICIENCY (%)
VIN
2.7V
TO 4.2V
70
VIN = 3.6V
60
50
VIN = 4.2V
40
30
20
10
0.1
1000
1
100
10
OUTPUT CURRENT (mA)
3406B TA10
Load Step
Load Step
VOUT
100mV/DIV
AC COUPLED
VOUT
100mV/DIV
AC COUPLED
IL
500mA/DIV
IL
500mA/DIV
ILOAD
500mA/DIV
ILOAD
500mA/DIV
VIN = 3.6V
20µs/DIV
VOUT = 1.2V
ILOAD = 0mA TO 600mA
3406B TA11
VIN = 3.6V
20µs/DIV
VOUT = 1.2V
ILOAD = 200mA TO 600mA
3406B TA12
3406bfa
14
LTC3406B
U
PACKAGE DESCRIPTIO
S5 Package
5-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1635)
0.62
MAX
0.95
REF
2.90 BSC
(NOTE 4)
1.22 REF
1.4 MIN
3.85 MAX 2.62 REF
2.80 BSC
1.50 – 1.75
(NOTE 4)
PIN ONE
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.30 – 0.45 TYP
5 PLCS (NOTE 3)
0.95 BSC
0.80 – 0.90
0.20 BSC
0.01 – 0.10
1.00 MAX
DATUM ‘A’
0.30 – 0.50 REF
0.09 – 0.20
(NOTE 3)
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
1.90 BSC
S5 TSOT-23 0302
3406bfa
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.
15
LTC3406B
U
TYPICAL APPLICATIO
5V Input to 3.3V/0.6A Regulator
VIN
5V
4
†
CIN
4.7µF
CER
VIN
SW
3
2.2µH*
COUT**
10µF
CER
LTC3406B
1
VFB
RUN
VOUT
3.3V
22pF
5
GND
2
1M
*MURATA LQH32CN2R2M33
** TAIYO YUDEN JHK316BJ106ML
†
TAIYO YUDEN JMK212BJ475MG
3406B TA13
221k
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1616
500mA (IOUT), 1.4MHz, High Efficiency Step-Down
DC/DC Converter
90% Efficiency, VIN = 3.6V to 25V, VOUT = 1.25V, IQ = 1.9mA,
ISD = <1µA, ThinSOT Package
LT1676
450mA (IOUT), 100kHz, High Efficiency Step-Down
DC/DC Converter
90% Efficiency, VIN = 7.4V to 60V, VOUT = 1.24V, IQ = 3.2mA,
ISD = 2.5µA, S8 Package
LTC1701/LT1701B
750mA (IOUT), 1MHz, High Efficiency Step-Down
DC/DC Converter
90% Efficiency, VIN = 2.5V to 5V, VOUT = 1.25V, IQ = 135µA,
ISD = <1µA, ThinSOT Package
LT1776
500mA (IOUT), 200kHz, High Efficiency Step-Down
DC/DC Converter
90% Efficiency, VIN = 7.4V to 40V, VOUT = 1.24V, IQ = 3.2mA,
ISD = 30µA, N8, S8 Packages
LTC1877
600mA (IOUT), 550kHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.7V to 10V, VOUT = 0.8V, IQ = 10µA,
ISD = <1µA, MS8 Package
LTC1878
600mA (IOUT), 550kHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 10µA,
ISD = <1µA, MS8 Package
LTC1879
1.2A (IOUT), 550kHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.7V to 10V, VOUT = 0.8V, IQ = 15µA,
ISD = <1µA, TSSOP-16 Package
LTC3403
600mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter with Bypass Transistor
96% Efficiency, VIN = 2.5V to 5.5V, VOUT = Dynamically Adjustable,
IQ = 20µA, ISD = <1µA, DFN Package
LTC3404
600mA (IOUT), 1.4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 10µA,
ISD = <1µA, MS8 Package
LTC3405/LTC3405A
300mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter
96% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 20µA,
ISD = <1µA, ThinSOT Package
LTC3406
600mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter
96% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.6V, IQ = 20µA,
ISD = <1µA, ThinSOT Package
LTC3411
1.25A (IOUT), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60µA,
ISD = <1µA, MS Package
LTC3412
2.5A (IOUT), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60µA,
ISD = <1µA, TSSOP-16E Package
LTC3440
600mA (IOUT), 2MHz, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 2.5V, IQ = 25µA,
ISD = <1µA, MS Package
3406bfa
16
Linear Technology Corporation
LT/TP 0604 1K REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2002