LINER LTC3620EDCPBF

LTC3620
Ultralow Power 15mA
Synchronous Step-Down
Switching Regulator
DESCRIPTION
FEATURES
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The LTC®3620 is a high efficiency, synchronous buck
regulator, suitable for very low power, very small footprint
applications powered by a single Li-Ion battery.
High Efficiency: Up to 95%
Maximum Current Output: 15mA
Externally Programmable Frequency Clamp with
Internal 50kHz Default Minimizes Audio Noise
18μA IQ Current
2.9V to 5.5V Input Voltage Range
Low-Battery Detection
0.6V Reference Allows Low Output Voltages
Shutdown Mode Draws <1μA Supply Current
2.8V Undervoltage Lockout
Unique Low Noise Control Architecture
Internal Power MOSFETs
No Schottky Diodes Required
Internal Soft-Start
Tiny 2mm × 2mm 8-Lead DFN Package
The internal synchronous switches increase efficiency
and eliminate the need for external Schottky diodes. Low
output voltages are easily supported by the 0.6V feedback
reference voltage. The LTC3620-1 option is internally
programmed to provide a 1.1V output.
The LTC3620 uses a unique variable frequency architecture
to minimize power loss and achieve high efficiency. The
switching frequency is proportional to the load current, and
an internal frequency clamp forces a minimum switching
frequency at light loads to minimize noise in the audio
range. The user can program the frequency of this clamp
by applying an external clock to the FMIN/MODE pin.
The battery status output, LOBATB, indicates when the
input voltage drops below 3V. To help prevent damage to
the battery, an undervoltage lockout (UVLO) circuit shuts
down the part if the input voltage falls below 2.8V.
APPLICATIONS
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Hearing Aids
Wireless Headsets
Li-Ion Cell Applications
Button Cell Replacement
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 7528587.
The LTC3620 is available in a low profile, 2mm × 2mm
8-lead DFN package.
TYPICAL APPLICATION
Output Voltage Ripple
vs Load Current
High Efficiency Low Power
Step-Down Converter
25
VOUT = 1.1V
FMIN/MODE = 0V
L = 22μH
20
1μF
CER
LOBATB
SW
LTC3620
FMIN/MODE VFB
GND
22μH
22pF
432k
523k
VOUT
1.1V
$VOUT (mVP-P)
RUN
VIN = 5.5V
15
VIN = 3.6V
10
2.5
70
2.0
60
50
40
20
5
EFFICIENCY
80
30
1μF
CER
3.0
1.5
VIN = 3V
FMIN/MODE = 0V
VOUT = 1.1V
VOUT = 1.8V
VOUT = 2.5V
10
1.0
LOSS
POWER LOSS (mW)
VIN
90
EFFICIENCY (%)
VIN
2.9V TO 5.5V
Efficiency vs Load Current
100
0.5
3620 TA01a
0
0
10
5
OUTPUT CURRENT (mA)
15
3620 TA01b
0
0.1
0
1
LOAD CURRENT (mA)
10
3620 TA01c
3620f
1
LTC3620
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Notes 1, 2)
Input Supply Voltage .................................... –0.3V to 6V
RUN Voltage ................................. –0.3V to (VIN + 0.3V)
VFB Voltage ................................... –0.3V to (VIN + 0.3V)
LOBATB Voltage ........................................... –0.3V to 6V
FMIN/MODE Voltage ..................... –0.3V to (VIN + 0.3V)
SW Voltage .................................. –0.3V to (VIN + 0.3V)
P-channel Switch Source Current (DC) ..................50mA
N-channel Switch Sink Current (DC) ......................50mA
Operating Junction Temperature Range
(Notes 2, 4) ............................................–40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
TOP VIEW
8 VIN
SW 1
GND 2
FMIN/MODE 3
9
LOBATB 4
7 RUN
6 VFB
5 NC
DC PACKAGE
8-LEAD (2mm s 2mm) PLASTIC DFN
TJMAX = 125°C, θJA = 88.5°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
LTC3620EDC#PBF
LTC3620EDC#TRPBF
LFJJ
8-Lead (2mm × 2mm) Plastic DFN
–40°C to 85°C
LTC3620EDC-1#PBF
LTC3620EDC-1#TRPBF
LFJK
8-Lead (2mm × 2mm) Plastic DFN
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
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 l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V unless otherwise noted.
SYMBOL
PARAMETER
VIN
Input Voltage Range
VFB
Regulated Feedback Voltage (Note 3)
CONDITIONS
MIN
l
LTC3620
LTC3620
LTC3620-1
LTC3620-1
l
l
TYP
2.9
0.594
0.588
1.089
1.078
MAX
UNITS
5.5
V
0.6
0.6
1.1
1.1
0.606
0.612
1.111
1.122
V
V
V
V
0.15
%/V
ΔVFB
Reference Voltage Line Regulation
VIN = 3V to 5.5V (Note 3)
0.05
VLOADREG
Output Voltage Load Regulation
(Note 3)
0.5
IQ
Quiescent Current, No Switching
VFB = 0.65V, FMIN/MODE = VIN
18
25
μA
IQSD
Quiescent Current in Shutdown
RUN = 0V
0.01
1
μA
RUN = VIN, VIN = 2.5V
0.5
μA
35
mA
50
kHz
IQU
Quiescent Current in UVLO Condition
IPK
Peak Inductor Current
fSW
Minimum Switching Frequency (Internal) VFB = 0.65V, FIN/MODE = 0
VRUN
RUN Input Voltage High
l
40
0.8
V
RUN Input Voltage Low
IRUN
RUN Leakage Current
%
±0.01
0.3
V
±1
μA
3620f
2
LTC3620
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V unless otherwise noted.
SYMBOL
PARAMETER
VFMIN
FMIN/MODE Input Voltage High
CONDITIONS
MIN
TYP
MAX
UNITS
0.9
V
FMIN/MODE Input Voltage Low
20
0.7
V
300
kHz
±1
μA
fEXT
FMIN/MODE Input Frequency
IFMIN/MODE
FMIN/MODE Pin Leakage Current
ISW
Switch Leakage Current
VRUN = 0V, VSW = 0V or 5.5V, VIN = 5.5V
IFB
VFB Pin Current
LTC3620, VFB = 0.6V
LTC3620-1, VFB = 1.1V
VUVLO
Undervoltage Lockout (UVLO)
VIN Decreasing
VLOBATB
LOBATB Threshold
VIN Decreasing
RLOBATB
LOBATB Pull-Down On-Resistance
15
Ω
VHLOBATB
LOBATB Hysteresis
100
mV
RPFET
RDS(ON) of P-channel FET (Note 5)
ISW = 50mA, VIN = 3.6V
2.0
Ω
RNFET
RDS(ON) of N-channel FET (Note 5)
ISW = –50mA, VIN = 3.6V
1.0
Ω
±0.01
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: TJ is calculated from the ambient temperature, TA, and power
dissipation, PD, according to the following formula:
TJ = TA + (PD)(88.5°C/W)
Note 3: The LTC3620 is tested in a proprietary test mode that connects VFB
to the output of the error amplifier.
±0.01
±1
μA
0
1.2
±30
2.0
nA
μA
2.7
2.8
2.9
V
2.93
3.0
3.08
V
Note 4: The LTC3620E is guaranteed to meet performance specifications
from 0°C to 85°C junction temperature. Specifications over the –40°C
to 85°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls.
Note 5: The DFN switch-on resistance is guaranteed by correlation to
wafer level measurements.
3620f
3
LTC3620
TYPICAL PERFORMANCE CHARACTERISTICS
Switching Frequency
vs Load Current, FMIN/MODE
0.5
100
10
0.01
TA = 25°C
VIN = 3.6V
VOUT = 1.1V
1
0.1
LOAD CURRENT (mA)
620
VIN = 3.6V
VOUT = 1.1V
FMIN/MODE = 0V
TA = 25°C
0.3
615
0.1
–0.1
590
585
–0.5
10 20
10
5
LOAD CURRENT (mA)
0
28
QUIESCENT CURRENT (μA)
VFB (V)
1.105
1.100
1.095
1.090
1.085
50
0
TEMPERATURE (°C)
100
VOUT = 1.1V
FMIN/MODE = VIN
2.84
26
22
VIN = 5V
20
VIN = 3.6V
18
16
2.82
2.81
2.80
2.79
2.78
14
2.77
12
2.76
50
0
TEMPERATURE (°C)
100
130
2.75
–50
50
0
TEMPERATURE (°C)
3620 G05
3.05
40
VOUT = 1.1V
FMIN/MODE = 0V
130
Switching Waveforms at 250μA
Load, FMIN/MODE = 0V
VOUT = 1.1V
L = 22μH
VOUT (AC)
20mV/DIV
39
3.03
100
3620 G06
Peak Inductor Current
vs Temperature
LOBATB Threshold
vs Temperature
VIN = 5.5V
3.02
38
IPEAK (mA)
3.01
3.00
2.99
2.97
VSW
2V/DIV
37
36
2.98
VIN = 3.6V
IL
25mA/DIV
35
VOUT = 1.1V
VIN = 3.6V
TA = 25°C
2.96
2.95
–50
VOUT = 1.1V
FMIN/MODE = 0V
2.83
24
3620 G04
LOBATB THRESHOLD (V)
UVLO Threshold vs Temperature
2.85
10
–50
130
130
100
3620 G03
UVLO THRESHOLD (V)
VIN = 3.6V
FMIN/MODE = 0V
1.110
50
0
TEMPERATURE (°C)
3620 G02
Quiescent Current vs Temperature
1.115
3.04
580
–50
15
30
1.080
–50
600
–0.3
LTC3620-1 Feedback Voltage
vs Temperature
1.120
605
595
3620 G01
1.125
VIN = 3.6V
VOUT = 1.1V
FMIN/MODE = 0V
610
VFB (mV)
200kHz, EXTERNAL
FMIN/MODE = 0V
FMIN/MODE = VIN
VOUT VOLTAGE CHANGE (%)
SWITCHING FREQUENCY (kHz)
1000
LTC3620 Feedback Voltage
vs Temperature
Load Regulation
50
0
TEMPERATURE (°C)
100
130
3620 G07
34
–50
0
50
TEMPERATURE (°C)
100
4μs/DIV
3620 G09
130
3620 G08
3620f
4
LTC3620
TYPICAL PERFORMANCE CHARACTERISTICS
Switching Waveforms at 1mA
Load, FMIN/MODE = 0V
Switching Waveforms at 12mA
Load, FMIN/MODE = 0V
VOUT (AC)
20mV/DIV
VOUT (AC)
20mV/DIV
VSW
2mV/DIV
VSW
2V/DIV
IL
25mA/DIV
IL
25mA/DIV
VOUT = 1.1V
VIN = 3.6V
TA = 25°C
4μs/DIV
3620 G10
Switching Waveforms at 250μA
Load, FMIN/MODE = 200kHz Clock
VFMIN/MODE
1V/DIV
VOUT (AC)
20mV/DIV
VSW
2V/DIV
IL
25mA/DIV
VOUT = 1.1V
VIN = 3.6V
TA = 25°C
Switching Waveforms at 1mA
Load, FMIN/MODE = 200kHz Clock
400ns/DIV
3620 G11
Switching Waveforms at 12mA
Load, FMIN/MODE = 200kHz
VFMIN/MODE
1V/DIV
VFMIN/MODE
1V/DIV
VOUT (AC)
20mV/DIV
VOUT (AC)
20mV/DIV
VSW
2V/DIV
VSW
2V/DIV
VOUT = 1.1V
VIN = 3.6V
TA = 25°C
2μs/DIV
3620 G13
Start-Up Waveforms
IL
25mA/DIV
VOUT = 1.1V
VIN = 3.6V
TA = 25°C
Transient Response, 250μA to 3mA
Step, FMIN/MODE = 0V
3620 G12
2μs/DIV
VOUT
200mV/DIV
IL
25mA/DIV
IL
25mA/DIV
VOUT = 1.1V
VIN = 3.6V
TA = 25°C
400ns/DIV
3620 G14
3620 G15
VOUT = 1.1V
200μs/DIV
VIN = 3.6V
IOUT = 0mA
FMIN/MODE = 0V
TA = 25°C
Transient Response, 1mA to 10mA
Step, FMIN/MODE = 0V
PFET RDS(ON) vs Temperature
2.3
ISW = 35mA
2.1
VOUT (AC)
20mV/DIV
1.9
RDS(ON) (Ω)
VOUT (AC)
10mV/DIV
ILOAD
5mA/DIV
ILOAD
5mA/DIV
VIN = 3.6V
1.7
1.5
VIN = 5V
1.3
1.1
0.9
VIN = 3.6V
VOUT = 1.1V
TA = 25°C
4ms/DIV
3620 G16
VIN = 3.6V
VOUT = 1.1V
TA = 25°C
4ms/DIV
3620 G17
0.7
0.5
–50
50
0
TEMPERATURE (°C)
100
130
3620 G18
3620f
5
LTC3620
TYPICAL PERFORMANCE CHARACTERISTICS
NFET RDS(ON) vs Temperature
100
90
90
1.3
80
80
1.2
70
70
1.1
EFFICIENCY (%)
1.4
ISW = 35mA
VIN = 3.6V
1.0
0.9
VIN = 5V
0.8
60
50
40
0.7
20
0.6
10
0.5
–50
50
0
TEMPERATURE (°C)
TA = 25°C
VIN = 3V
FMIN/MODE = 0V
VOUT = 1.1V
VOUT = 1.8V
VOUT = 2.5V
30
100
0
0.1
130
1
LOAD CURRENT (mA)
EFFICIENCY (%)
100
1.5
RDS(ON) (Ω)
Efficiency vs Load Current,
FMIN/MODE Frequency
Efficiency vs Load Current, VOUT
60
50
40
20
0
0.1
Efficiency vs Load Current, VIN
Efficiency vs fMIN, 1mA Load
Internal fMIN vs Temperature
54
80
79
53
70
78
TA = 25°C
VOUT = 1.1V
FMIN/MODE = VIN
20
VIN = 3V
VIN = 3.6V
VIN = 5.5V
10
0
0.1
1
LOAD CURRENT (mA)
INTERNAL fMIN (kHz)
80
EFFICIENCY (%)
90
EFFICIENCY (%)
55
30
77
76
75
74
TA = 25°C
VIN = 3.6V
VOUT = 1.1V
FMIN/MODE = EXTERNAL CLOCK
73
72
71
0
10
100
VIN = 3.6V
50
VIN = 5V
49
48
46
300
45
–50
50
0
TEMPERATURE (°C)
Spectral Content, 500μA Load
100
3620 G24
3620 G23
Spectral Content, 5mA Load
–60
–40
52.5kHz
–81.4dBm
–80
POWER RATIO (dBm)
POWER RATIO (dBm)
52
51
47
200
fMIN (kHz)
3620 G22
–100
–120
–140
–160
10
3620 G21
81
40
1
LOAD CURRENT (mA)
3620 G20
100
50
FMIN = 20kHz
FMIN = 100kHz
FMIN = 200kHz
10
10
3620 G19
60
TA = 25°C
VIN = 3V
VOUT = 1.1V
30
12.5kHz
VOUT = 1.1V
VIN = 3.6V
FMIN/MODE = 0V
TA = 25°C
92.5kHz
8kHz/DIV
3620 G25
–60
355.6kHz
–80.2dBm
–80
–100
–120
–140
1kHz
39.9kHz/DIV
VOUT = 1.1V
VIN = 3.6V
FMIN/MODE = 0V
TA = 25°C
400kHz
3620 G26
3620f
6
LTC3620
PIN FUNCTIONS
SW (Pin 1): Switch Node Connection to Inductor. This pin
connects to the internal power MOSFET Switches.
NC (Pin 5): No Connect.
VFB (Pin 6): Regulator Feedback Pin. This pin receives
the feedback voltage from the resistive divider across the
output. For the LTC3620-1, this pin must be connected
directly to VOUT . VOUT is internally divided from VOUT to the
reference voltage of 0.6V as seen in the Block Diagram.
GND (Pin 2): Ground Connection for Internal Circuitry and
Power Path Return. Tie directly to local ground plane.
FMIN/MODE (Pin 3): Frequency Clamp Select Input. Driving this pin with a 20kHz to 300kHz external clock sets the
minimum switching frequency. Pulling this pin low sets
the minimum switching frequency to the internally set
50kHz. Pulling this pin high defeats the minimum switching
frequency and allows the part to switch at arbitrarily low
frequencies dependent on the load current.
RUN (Pin 7): Regulator Enable Pin. Apply a voltage greater
than 0.8V to enable the regulator. Do not float this pin.
VIN (Pin 8): Input Supply Pin. Must be locally bypassed.
Exposed Pad (Pin 9): GND. Must be soldered to PCB.
LOBATB (Pin 4): Low-Battery Status Output. This opendrain output pulls low when VIN falls below 3V.
BLOCK DIAGRAM
LOBATB
VIN
VIN
4
8
PEAK INDUCTOR
CURRENT ADJUST
LOBAT
8
3V
RUN
ICMP
7
SHUTDOWN
UVLO
FMIN/MODE
3
SELECT
50kHz
PFD
SW
SWITCH
DRIVER
1
0.6V
VFB
(LTC3620)
6
VFB
(LTC3620-1)
6
EAMP
RCMP
2
3620 BD
GND
3620f
7
LTC3620
OPERATION
The LTC3620 is a variable frequency buck switching regulator with a maximum output current of 15mA. At high
loads the LTC3620 will supply constant peak current pulses
through the output inductor at a frequency dependent on
the load current.
A switching cycle is initiated by a pulse from the error
amplifier, EAMP. The top FET is turned on and remains on
until the peak current threshold is sensed by ICMP (35mA
at full loads). When this occurs, the top FET it is turned off
and the bottom FET is turned on. The bottom FET remains
on until the inductor current drops to 0A, as sensed by the
reverse-current comparator, RCMP. The time interval before
another switching cycle is initiated is adjusted based on
the output voltage error, measured by the EAMP to be the
difference between VFB and the 0.6V reference.
As the load current decreases, the EAMP will decrease
the switching frequency to match the load, until the minimum switching frequency (internally or externally set) is
reached. With the FMIN/MODE pin pulled low, the minimum
frequency is internally set to 50kHz. Further decreasing
the load will cause the phase frequency detector (PFD) to
decrease the peak inductor current in order to maintain
the switching frequency at 50kHz.
The minimum switching frequency can be externally set
by clocking the FMIN/MODE pin at the desired minimum
switching frequency. The load current below which the
SWITCHING FREQUENCY (kHz)
1000
switching frequency will be clamped is dependent on the
externally set frequency and the value of the inductor used.
A higher externally set minimum frequency will result in
a higher load current threshold below which the part will
lock to this minimum frequency. The relationship between
load current and minimum frequency is described by the
following equation:
IMAX(LOCK)
2
VIN ) ( fMIN ) (L ) ( 35mA )
(
=
2VOUT ( VIN – VOUT )
The LTC3620 will switch at this externally set frequency
at load currents below this threshold; though in general,
neither this minimum nor this synchronization will be
maintained during load transients.
At very light loads, the minimum PFET on time will be
reached and the peak inductor current can no longer
be reduced. In this situation, the LTC3620 will resume
decreasing the regulator switching frequency to prevent
the output voltage from climbing uncontrollably.
For those applications which are not sensitive to the spectral
content of the output ripple, the minimum frequency clamp
can be defeated by pulling the FMIN/MODE pin high. In this
mode the inductor current peaks will be held at 35mA and
the switching frequency will decrease without limit.
200kHz, EXTERNAL
FMIN/MODE = 0V
FMIN/MODE = VIN
100
10
0.01
TA = 25°C
VIN = 3.6V
VOUT = 1.1V
0.1
1
LOAD CURRENT (mA)
10 20
3620 F01
Figure 1. Switching Frequency vs Load Current, FMIN/MODE
3620f
8
LTC3620
APPLICATIONS INFORMATION
There are a number of different values, sizes and brands
of inductors that will work well with this part. Table 1 has a
number of recommended inductors, though there are many
other manufacturers and devices that may also be suitable.
Consult each manufacturer for more detailed information
and for their entire selection of related parts.
Table 1: Representative Surface Mount Inductors
VENDOR
PART
NUMBER
VALUE
(μH)
MAX DC
CURRENT
DCR (Ω)
(mA)
W×L×H
(mm3)
Taiyo
Yuden
CBMF1608T 22 ±10% 1.3 Max
70
0.8 × 1.6 × 0.8
Murata
LQH2MC_02 18 ±20% 1.8 ±30%
22 ±20% 2.1 ±30%
190
185
1.6 × 2 × 0.9
Würth
744028220 22 ±30% 1.48 Max
Electronics
270
2.8 × 2.8 × 1.1
Coilcraft
380
320
2.95 × 2.95 × 0.9
LPS3010
18 ±20% 1.0 Max
22 ±20% 1.2 Max
There is a trade-off between physical size and efficiency;
The inductors in Table 1 are shown because of their small
footprints, choose larger sized inductors with less core
loss and lower DCR to maximize efficiency.
The ideal inductor value will vary depending on which
characteristics are most critical to the designer. Use the
equations and recommendations in the next sections to help
you find the correct inductance value for your design.
Avoiding Audio Range Switching
In order to best avoid switching in the audio range at the
lowest possible load current, the minimum frequency
should be set as low as is acceptable, and the inductor
value should be minimized. For a 1.1V output the smallest
recommended inductor value is 15μH.
The part is optimized to get 35mA peaks for VIN = 3.6V and
VOUT = 1.1V with an 18μH inductor. If the falling slope is
too steep the NFET will continue to conduct shortly after
the inductor current reaches zero, causing a small reverse
current. This means the net power delivered with every
pulse will decrease. To mitigate this problem the inductor
can be resized. Table 2 shows recommended inductors and
output capacitors for commonly used output voltages.
Table 2. Recommended Inductor and Output Capacitor Sizes for
Different VOUT
VOUT (V)
0.9
1.1
1.1 (LTC3620-1)
1.8
2.5
L (μH)
15
22
22
33
47
COUT (μF)
2.2
1
2.2
2.2
4.7
Because the rising dI/dt decreases for increased VOUT
and increased L, the inductor current peaks will decrease,
causing the maximum load current to decrease as well.
Figure 2 shows typical maximum load current versus
output voltage.
20
19
MAXIMUM LOAD CURRENT (mA)
Choosing an Inductor
TA = 25°C
18
17
16
15
14
13
12
11
10
0.6
1.6
1.1
2.1
OUTPUT VOLTAGE (V)
2.6
3620 F02
Figure 2. Maximum Output Current vs VOUT , VIN = 3.6V
Adjusting for VOUT
Output Voltage Ripple
The inductor current peak and zero crossing are dependent
on the dI/dt. The equations for the rising and falling slopes
are as follows:
The quantity of charge transferred from VIN to VOUT per
switching cycle is directly proportional to the inductor
value. Consequently, the output voltage ripple is directly
proportional to the inductor value, and the switching
frequency for a given load is inversely proportional to the
inductor value. For a given load current, higher switching
frequency will typically lower the efficiency because of the
Rising dI/dt = (VIN-VOUT)/L
Falling dI/dt = VOUT/L
3620f
9
LTC3620
APPLICATIONS INFORMATION
increase in switching losses internal to the part. This can
be partially offset by using inductors with lower loss.
The peak-to-peak output voltage ripple can be approximated by:
ΔV =
(I )(L)( V
PK
2
IN
)
2 (COUT ) ( VOUT ) ( VIN – VOUT )
The output ripple is a strong function of the peak inductor
current, IPK. When the LTC3620 is locked to the minimum
switching frequency, IPK is decreased to maintain regulation. Consequently, ΔVOUT is reduced in and below the
lock range.
Efficiency
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.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses in the LTC3620’s circuits: VIN quiescent current
and I2R losses. VIN quiescent current loss dominates the
efficiency loss at 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 little consequence, as illustrated on the
front page of this data sheet.
The quiescent current is due to two components: the DC
bias current, IQ, 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 and
proportional to frequency. Both the DC bias and gate charge
losses are proportional to VIN and thus their effects will
be more pronounced at higher supply voltages.
The RDS(ON) for both the top and bottom MOSFETS can
be obtained from the Typical Performance Characteristics
curves. The I2R losses per pulse will be proportional to
the peak current squared times the sum of the switch
resistance and the inductor resistance:
Loss IPK 2
IR
=
R
Pulse
3 EFF
2
where REFF = RL + RPFET D + RNFET (1 – D), and D is the
ratio of the top switch on-time to the total time of the pulse.
Additional losses incurred from the inductor DC resistance
and core loss may be significant in smaller inductors.
Capacitor Selection
Higher value, lower cost, ceramic capacitors are now
widely available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them
ideal for switching regulator applications. Because the
LTC3620’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.
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.
The output voltage ripple is inversely proportional to the
output capacitor. The larger the capacitor, the smaller the
ripple, and vice versa. However, the transient response
time is directly proportional to COUT , so a larger COUT
means slower response time.
To maintain stability and an acceptable output voltage
ripple, values for COUT should range from 1μF to 5μF.
3620f
10
LTC3620
APPLICATIONS INFORMATION
the frequency clamp loop in returning the peak inductor
current to its maximum.
Setting Output Voltage
The output voltage is set by tying VFB to a resistive divider
using the following formula (refer to Figure 3):
Thermal Considerations
0.6V (R1+ R2)
R2
The fixed output version, the LTC3620-1, includes an
internal resistive divider, eliminating the need for external
resistors. The resistor divider is chosen such that the VFB
input current is approximately 1μA. For this version, the
VFB pin should be connected directly to VOUT .
The LTC3620 requires the package backplane metal to be
well soldered to the PC board. This gives the DFN package
exceptional thermal properties, making it difficult in normal
operation to exceed the maximum junction temperature
of the part. In most applications the LTC3620 does not
dissipate much heat due to its high efficiency and low
current. In applications where the LTC3620 is running at
high ambient temperatures and high load currents, the heat
dissipated may exceed the maximum junction temperature
of the part if it is not well thermally grounded.
Maximum Load Current and Maximum Frequency
Design Example
The maximum current that the LTC3620 can provide is
calculated to be just slightly less than half the maximum
peak current.
This example designs a 1.1V output using a Li-Ion battery with voltages between 2.8V to 4.2V, and an average
of 3.6V. The internally provided 50kHz clock will be used
for the minimum switching frequency, so the FMIN/MODE
pin will be pulled low. For a 1.1V output, an 18μH inductor
should be used (refer to Table 2).
VOUT =
R1 and R2 should be large to minimize standing load
current and improve efficiency.
The inductor value will determine how much energy is
delivered to the output for each switching cycle, and thus
the duration of each pulse and the maximum frequency.
Larger inductors will have slower ramp rates, longer pulses,
and thus lower maximum frequencies. Conversely, smaller
inductors will result in higher maximum frequencies.
COUT can be chosen from Table 2 or can be based on a
desired maximum output voltage ripple, ΔVOUT . For this
case let’s use a maximum ΔVOUT equal to 1% of VOUT ,
or 11mV.
When using a frequency clamp, large abrupt increasing
load steps from levels below the locking range to levels
near the maximum output may result in a large drop in
the output voltage. This is due to the low bandwidth of
(35mA )(22µH)(3.6V )
2
COUT =
2ΔVOUT (1.1V ) ( 3.6V – 1.1V )
VIN
2.9V TO 5.5V
= 1.6µF ≈ 1.5µF
LOBATB
VIN
RUN
1M
LOBATB
L
LTC3620
SW
1μF
CER
22pF
FMIN/MODE VFB
GND
VOUT
1.1V
R1
COUT
R2
3620 F03
Figure 3. Design Example Schematic
3620f
11
LTC3620
APPLICATIONS INFORMATION
The best way to select the feedback resistors is to select
a target combined resistance, and try different standard
1% resistor sizes to see which combination will give the
least error. For this example a target combined resistance
of around 1M will be used. By checking R1 values between
422k and 475k, and calculating R2 using the formula:
R2 =
(0.6V )R1
VOUT – 0.6V
it can be found that a value of R2 = 523k and R1 = 432k
minimizes the error in this range.
The error can be checked by solving for VOUT and finding the percent error from the desired 1.1V. Using these
resistor values will result in VOUT = 1.096V, and an error
of around 0.4%. Using different target resistor sums is
acceptable, but a smaller sum will decrease efficiency at
lower loads, and a larger sum will increase noise sensitivity at the VFB pin.
+
COUT
Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3620:
1. The power traces consisting of GND, SW and VIN should
be kept short, direct and wide.
2. The VFB pin should connect directly to the respective
feedback resistors, which should also have short, direct
paths to VOUT and GND respectively.
3. Keep COUT and CIN as close to the LTC3620 as
possible.
4. All parts connecting to ground should have their ground
terminals in close proximity to the LTC3620 GND
connection.
5. Keep the SW node and external clock, if used, away from
the sensitive VFB node. Also, minimize the length and
area of all traces connected to the SW pin, and always
use a ground plane under the switching regulator to
minimize interplane coupling.
+
A larger capacitor could be used to reduce this number.
Keep in mind that while a larger output capacitor will
decrease voltage ripple, it will also increase the transient
settling time. The optimal range for COUT should be between 1μF and 5μF.
COUT
VIN
CIN
L
L
SW 1
GND 2
FMIN/MODE
LOBATB
VIN
CIN
•
•
3
4
• • •
• • •
• • •
8 VIN
SW 1
7 RUN
GND 2
6 VFB
FMIN/MODE
5 NC
LOBATB
R2
R1
CFF*
LTC3620 Layout Diagram
3
4
7 RUN
6 VFB
5 NC
VOUT
VOUT
*CFF = 22pF FEEDFORWARD CAPACITOR
•
•
8 VIN
• • •
• • •
• • •
3620 F05
3620 F04
LTC3620-1 Layout Diagram
3620f
12
LTC3620
TYPICAL APPLICATIONS
High Efficiency Low Power Step-Down Converter, FMIN/MODE = 0
VIN
2.9V TO 5.5V
LOBATB
VIN
RUN
1M
LOBATB
22μH
1μF
CER
VOUT
1.1V
SW
LTC3620
FMIN/MODE VFB
GND
22pF
432k
523k
1μF
CER
3620 TA02a
Efficiency vs VIN
100
100
90
90
80
80
70
60
50
40
30
20
10
0
0.1
TA = 25°C
VOUT = 1.1V
FMIN/MODE = 0V
VIN = 3V
VIN = 3.6V
VIN = 5.5V
1
LOAD CURRENT (mA)
10
3620 TA02b
EFFICIENCY (%)
EFFICIENCY (%)
Efficiency vs Load Current
70
60
50
40
TA = 25°C
VOUT = 1.1V
IOUT = 500μA
IOUT = 1mA
IOUT = 10mA
30
20
10
0
2.5
3.5
4.5
VIN (V)
5.5
6.5
3620 TA02c
3620f
13
LTC3620
TYPICAL APPLICATIONS
High Efficiency Low Power Step-Down Converter,
Externally Programmed fMIN
VIN
2.9V TO 5.5V
LOBATB
1μF
CER
VIN
RUN
1M
LOBATB
22μH
VOUT
1.1V
SW
LTC3620
FMIN/MODE
FMIN/MODE VFB
GND
22pF
432k
523k
1μF
CER
3620 TA03a
Efficiency vs VIN
100
90
90
80
80
70
EFFICIENCY (%)
EFFICIENCY (%)
Efficiency vs Load Current
100
60
50
40
TA = 25°C
VIN = 3.6V
VOUT = 1.1V
fMIN = 20kHz
fMIN = 100kHz
fMIN = 200kHz
30
20
10
0
0.1
1
LOAD CURRENT (mA)
70
60
50
40
30
20
TA = 25°C
10 VOUT = 1.1V
fMIN = 200kHz
0
3.5
2.5
10
IOUT = 500μA
IOUT = 1mA
IOUT = 10mA
4.5
VIN (V)
3620 TA03b
Spectral Content,
FMIN/MODE = 20kHz Clock
–40
–100
–120
1kHz
RBW = 3Hz
VOUT = 1.1V
VIN = 3.6V
IOUT = 500μA
TA = 25°C
30kHz
2.99kHz/DIV
3620 TA03d
99.9kHz
–59.9dBm
–60
POWER RATIO (dBm)
POWER RATIO (dBm)
POWER RATIO (dBm)
20.0kHz
–64.9dBm
–80
–140
Spectral Content,
FMIN/MODE = 200kHz Clock
–40
–60
–80
–100
–120
–140
6.5
3620 TA03c
Spectral Content,
FMIN/MODE = 100kHz Clock
–40
5.5
1kHz
VOUT = 1.1V
VIN = 3.6V
IOUT = 1mA
TA = 25°C
150kHz
14.9kHz/DIV
3620 TA03e
–60
199.7kHz
–80
–100
–120
–140
1kHz
VOUT = 1.1V
VIN = 3.6V
IOUT = 1mA
TA = 25°C
220kHz
21.9kHz/DIV
3620 TA03f
3620f
14
LTC3620
PACKAGE DESCRIPTION
DC Package
8-Lead Plastic DFN (2mm × 2mm)
(Reference LTC DWG # 05-08-1719 Rev A)
0.70 p0.05
2.55 p0.05
1.15 p0.05 0.64 p0.05
(2 SIDES)
PACKAGE
OUTLINE
0.25 p 0.05
0.45 BSC
1.37 p0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
R = 0.05
TYP
2.00 p0.10
(4 SIDES)
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
R = 0.115
TYP
5
8
0.40 p 0.10
0.64 p 0.10
(2 SIDES)
PIN 1 NOTCH
R = 0.20 OR
0.25 s 45o
CHAMFER
(DC8) DFN 0106 REVØ
4
0.200 REF
1
0.23 p 0.05
0.45 BSC
0.75 p0.05
1.37 p0.10
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
3620f
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
LTC3620
TYPICAL APPLICATIONS
High Efficiency Low Power Step-Down Converter,
LTC3620-1 Internally Programmed, 1.1VOUT
1μF
CER
VIN
1M
RUN
LOBATB
LTC3620-1
SW
22μH
VOUT
1.1V
FMIN/MODE VFB
EFFICIENCY (%)
LOBATB
100
90
90
80
80
70
70
60
50
40
TA = 25°C
VOUT = 1.1V
FMIN/MODE = 0V
VIN = 3V
VIN = 3.6V
VIN = 5.5V
30
2.2μF
CER
GND
3620 TA04a
20
10
0
0.1
1
LOAD CURRENT (mA)
10
EFFICIENCY (%)
VIN
2.9V TO 5.5V
Efficiency vs VIN
Efficiency vs Load Current
100
60
50
40
TA = 25°C
VOUT = 1.1V
FMIN/MODE = 0V
IOUT = 500μA
IOUT = 1mA
IOUT = 10mA
30
20
10
0
2.5
3.5
4.5
VIN (V)
5.5
3620 G21
6.5
3620 TA04c
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PART NUMBER
DESCRIPTION
COMMENTS
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LTC3411A
1.25A IOUT, 4MHz, Synchronous Step-Down DC/DC Converter
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Dual 400mA/800mA IOUT , 2.25MHz, Synchronous Step-Down
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1A IOUT, 4MHz, Synchronous Step-Down DC/DC Converter
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LTC3631-5
VIN: 4.5V to 45V (60VMAX), VOUT(MIN) = 0.8V,
IQ = 12μA, ISD < 1μA, 3mm × 3mm DFN Package, MSOP-8E
50V, 20mA (IOUT), Ultralow Quiescent Current Synchronous
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VIN: 4.5V to 50V (60VMAX), VOUT(MIN) = 0.8V,
IQ = 12μA, ISD < 1μA, 3mm × 3mm DFN Package, MSOP-8E
LTC3642/LTC3642-3.3/ 45V, 50mA (IOUT), Ultralow Quiescent Current Synchronous
Step-Down DC/DC Converter
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VIN: 4.5V to 45V (60VMAX), VOUT(MIN) = 0.8V,
IQ = 12μA, ISD < 1μA, 3mm × 3mm DFN Package, MSOP-8E
LTC3632
3620f
16 Linear Technology Corporation
LT 0809 • PRINTED IN USA
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