LTC1626CS [Linear]
Low Voltage, High Efficiency Step-Down DC/DC Converter; 低电压，高效率降压型DC / DC转换器
| 型号： | LTC1626CS |
| 厂家： | 
Linear |
| 描述： | Low Voltage, High Efficiency Step-Down DC/DC Converter |
| 文件： | 总12页 (文件大小：284K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC1626
Low Voltage, High Efficiency
Step-Down DC/DC Converter
U
FEATURES
DESCRIPTION
The LTC®1626 is a monolithic, low voltage, step-down
current mode DC/DC converter featuring Burst ModeTM
operation at low output current.
■
Wide Input Supply Voltage Range: 2.5V to 6V
■
High Efficiency: Up to 95%
■
Low RDS(ON) Internal Switch: 0.32
Ω (VIN = 4.5V)
■
Current Mode Operation for Excellent Line and Load
Transient Response
Short-Circuit Protected
Low Dropout Operation: 100% Duty Cycle
Built-In Low-Battery Detector
Low Quiescent Current at Light Loads: IQ = 165µA
Ultralow Shutdown Current: IQ = 0.5µA
Peak Inductor Current Independent of Inductor Value
Available in 14-Pin SO Package
The input supply voltage range of 2.5V to 6V makes the
LTC1626 ideal for single cell Li-Ion and 3- or 4-cell NiCd/
NiMH applications. A built-in 0.32Ω switch (VIN = 4.5V)
allows up to 0.6A of output current.
■
■
■
■
■
■
■
The LTC1626 incorporates automatic power saving Burst
Mode operation to reduce gate charge losses when the
load current drops below the level required for continuous
operation. With no load, the converter draws only 165µA.
In shutdown, it draws a mere 0.5µA—making it ideal for
current sensitive applications.
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APPLICATIONS
Theinductorcurrentisuser-programmableviaanexternal
current sense resistor. In dropout, the internal P-channel
MOSFET switch is turned on continuously, maximizing
battery life.
■
Single Cell Li-Ion Step-Down Converters
■
3- or 4-Cell NiMH Step-Down Converters
■
Cellular Telephones
■
5V to 3.3V Conversion
■
3.3V to 2.5V Conversion
■
Inverting Converters
Portable Instruments
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
■
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TYPICAL APPLICATION
V
IN
2.7V TO 6V
Efficiency
100
†
+
C
0.1µF
PWR V
IN
L*
47µF
16V
R **
SENSE
0.1Ω
V
IN
IN
33µH
V
95
90
85
80
75
70
OUT
2.5V
SHDN
LTC1626
SW
D1
0.25A
MBRS130LT
††
+
PGND
C
OUT
3900pF
100µF
470Ω
+
6.3V
I
TH
SENSE
1000pF
100pF
–
10k
SENSE
C
T
V
= 3.5V
IN
L1 = 33µH
= 2.5V
C
T
V
FB
SGND
V
270pF
OUT
10k
R
= 0.1Ω
SENSE
*
COILTRONICS CTX33-4
C
= 270pF
T
** IRC 1206-R100F
1626 F01
†
††
AVX TPSD476KO16
0.01
0.1
OUTPUT CURRENT (A)
1
AVX TPSC107M006R0150
1626 F01a
Figure 1. High Efficiency 2.5V Step-Down Converter
1
LTC1626
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ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Voltages Referred to GND Pin)
TOP VIEW
ORDER PART
Input Supply Voltage (Pins 1, 2, 13)............–0.3V to 7V
Shutdown Input Voltage (Pin 10) ................–0.3V to 7V
Sense–, Sense+ (Pins 7, 8)........... –0.3V to (VIN + 0.3V)
LBO, LBI (Pins 3, 4).................................... –0.3V to 7V
CT, ITH, VFB (Pins 5, 6, 9)............. –0.3V to (VIN + 0.3V)
DC Switch Current (Pin 14) .................................... 1.2A
Peak Switch Current (Pin 14) ................................. 1.6A
Switch Voltage (Pin 14) .......(VIN – 7.5V) to (VIN + 0.3V)
Operating Temperature Range ..................... 0°C to 70°C
Extended Commercial Operating
NUMBER
PWR V
V
1
2
3
4
5
6
7
14
13
12
11
10
9
SW
IN
PWR V
PGND
SGND
SHDN
IN
IN
LBO
LBI
LTC1626CS
C
T
I
TH
V
FB
–
+
SENSE
8
SENSE
S PACKAGE
14-LEAD PLASTIC SO
TJMAX = 125°C, θJA = 110°C/ W
Temperature Range (Note 4) ............. –40°C to 85°C
Junction Temperature (Note 1)............................. 125°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 4.5V, VOUT = 2.5V, VSHDN = 0V, unless otherwise specified.
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
0.1
MAX
UNITS
I
Feedback Pin Current
Feedback Voltage
1
µA
FB
V
0°C to 70°C
–40°C to 85°C
●
●
1.22
1.2
1.25
1.28
1.3
V
V
FB
∆V
Output Voltage Line Regulation
V
= 3.5V to 5.5V, I
= 250mA
–40
0
40
50
mV
mV
OUT
IN
LOAD
Output Voltage Load Regulation
Burst Mode Output Ripple
10mA ≤ I
≤ 250mA
25
50
LOAD
I
= 0
mV
LOAD
P-P
I
Input DC Supply Current (Note 2)
Active Mode
Q
●
●
1.9
165
0.5
3.0
300
5
mA
µA
µA
Sleep Mode
Shutdown
V
V
= V
SHDN IN
V
Low-Battery Trip Point
1.15
1.25
1.35
V
µA
LBTRIP
I
I
Low-Battery Input Bias Current
Low-Battery Output Sink Current
Current Sense Threshold Voltage
±0.5
LBI
LBO
= 0.4V
0.4
1.4
mA
LBO
–
V
V
V
= 2.5V, V = V /2 + 25mV (Forced)
= 2.5V, V = V /2 – 25mV (Forced)
25
155
mV
mV
SENSE
SENSE
SENSE
FB
OUT
+
–
–
V
– V
130
180
0.45
6
SENSE
SENSE
FB
OUT
R
ON Resistance of Switch
Switch Off-Time (Note 3)
SHDN Pin High
0.32
5
Ω
µs
V
ON
t
C = 390pF, I = 400mA
LOAD
4
OFF
T
V
V
Minimum Voltage for Device to Be Shut Down
Maximum Voltage for Device to Be Active
V – 0.4
IN
IHSD
ILSD
INSD
SHDN Pin Low
0.4
V
I
SHDN Pin Input Current
0V ≤ V
≤ 7V
●
±1
µA
SHDN
The
●
denotes specifications that apply over the specified operating
Note 3: In applications where R
off-time increases by approximately 40%.
is placed at ground potential, the
SENSE
temperature range.
Note 1: T is calculated from the ambient temperature T and power
dissipation according to the following formula:
Note 4: C grade device specifications are guaranteed over the 0°C to 70°C
temperature range. In addition, C grade device specifications are assured
over the –40°C to 85°C temperature range by design or correlation, but
are not production tested.
J
A
T = T + (P • 110°C/W)
J
A
D
Note 2: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
2
LTC1626
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TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Input Voltage
(VOUT = 2.5V)
Efficiency vs Output Current
(VOUT = 3.3V)
Efficiency vs Input Voltage
(VOUT = 3.3V)
100
95
90
85
80
75
70
100
98
96
94
92
90
88
86
84
82
80
100
98
96
94
92
90
88
86
84
82
80
L1 = 33µH
= 0.1Ω
R
T
SENSE
= 270pF
C
I
= 100mA
OUT
I
= 100mA
OUT
I
= 250mA
OUT
I
= 250mA
OUT
L1 = 33µH
V
V
= 5V
IN
OUT
= 3.3V
= 0.1Ω
L1 = 33µH
= 0.1Ω
R
SENSE
R
T
SENSE
C
T
= 270pF
C
= 270pF
0.01
0.1
OUTPUT CURRENT (A)
1
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
1626 G02
1626 G01
1626 G03
Operating Frequency
Switch Resistance
Switch Leakage Current
2.0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
100
90
80
70
60
50
40
30
20
10
0
V
= 4.5V
IN
FIGURE 1 CIRCUIT
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
T = 70°C
J
T = 25°C
J
T = 0°C
J
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
0
10 20 30 40 50 60 70 80 90 100
JUNCTION TEMPERATURE (°C)
1626 G06
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
1626 G04
1626 G05
DC Supply Current*
Supply Current in Shutdown
Low Voltage Behavior
5.0
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
T = 25°C
T = 25°C
L1 = 33µH
J
J
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
* DOES NOT INCLUDE
SHUTDOWN = V
IN
R
C
= 0.1Ω
SENSE
T
GATE CHARGE CURRENT
= 270pF
T = 25°C
V
V
= 3.3V
= 2.5V
J
OUT
I
= 250mA
LOAD
OUT
ACTIVE MODE
SLEEP MODE
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
1626 G07
1626 G08
1626 G09
3
LTC1626
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PIN FUNCTIONS
SENSE– (Pin 7): Connects to the (–) Input of the Current
Comparator.
PWR VIN (Pins 1, 13): Supply for the Power MOSFET and
Its Driver. Decouple this pin properly to ground.
SENSE+ (Pin 8): The (+) Input to the Current Comparator.
VIN (Pin 2): Main Supply for All the Control Circuitry in
A built-in offset between Pins 7 and 8 in conjunction with
the LTC1626.
RSENSE sets the current trip threshold.
LBO (Pin 3): Open-Drain Output of the Low-Battery Com-
parator. This pin will sink current when Pin 4 (LBI) goes
below 1.25V. During shutdown, this pin is high imped-
ance.
VFB (Pin 9): This pin serves as the feedback pin from an
external resistive divider used to set the output voltage.
SHDN (Pin 10): Shutdown Pin. Pulling this pin to VIN
keeps the internal switch off and puts the LTC1626 in
micropower shutdown. If not used, connect to SGND.
LBI (Pin 4): The (–) Input of the Low-Battery Comparator.
The (+) input is connected to a reference voltage of 1.25V.
If not used, connect to VIN.
SGND (Pin 11): Small-Signal Ground. Must be routed
separately from other grounds to the (–) terminal of COUT
.
CT (Pin 5): External capacitor CT from Pin 5 to ground sets
the switch off-time. The operating frequency is dependent
on the input voltage and CT.
PWR GND (Pin 12): Switch Driver Ground. Connects to
the (–) terminal of CIN.
ITH (Pin 6): Feedback Amplifier Decoupling Point. The
current comparator threshold is proportional to Pin 6
voltage.
SW (Pin 14): Drain of the P-Channel MOSFET Switch.
Cathode of the Schottky diode must be connected closely
to this pin.
W
BLOCK DIAGRAM
–
+
SENSE
7
PWR V
SENSE
8
IN
1
13
PWR GND
12
SW
14
9
V
FB
–
+
V
–
SLEEP
R
S
C
25mV TO 150mV
Q
+
+
–
+
V
OS
S
I
–
+
TH
V
TH2
13k
G
6
V
TH1
–
+
V
LBO
3
IN
T
2
+
–
REFERENCE
10 SHDN
A3
OFF-TIME
CONTROL
–
SENSE
V
LBI
4
FB
1626 BD
5
C
T
11
SGND
4
LTC1626
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OPERATIO
The nominal off-time of the LTC1626 is set by an external
timingcapacitorconnectedbetweentheCT pinandground.
The operating frequency is then determined by the off-
decreasesslightly.Thiscausestheoutputofthegainstage
(Pin 6) to increase the current comparator threshold, thus
tracking the load current.
time and the difference between VIN and VOUT
.
When the load is relatively light, the LTC1626 automati-
cally switches to Burst Mode operation. The current loop
is interrupted when the output voltage reaches the desired
regulated value. The hysteretic voltage comparator V trips
when VOUT is above the desired output voltage, turning off
the switch and causing the timing capacitor to discharge.
This capacitor discharges past VTH1 until its voltage drops
below VTH2. Comparator S then trips and a sleep signal is
generated. Thecircuitnowentersintosleepmodewiththe
power MOSFET turned off. In sleep mode, the LTC1626 is
in standby and the load current is supplied by the output
capacitor. All unused circuitry is shut off, reducing quies-
cent current from 1.9mA to 165µA. When the output
capacitor discharges by the amount of the hysteresis of
thecomparatorV,theP-channelswitchturnsonagainand
the process repeats itself. During Burst Mode operation,
The output voltage is set by an external divider returned to
the VFB pin. A voltage comparator V and a gain block G
compare the divided output voltage with a reference
voltage of 1.25V.
Tooptimizeefficiency,theLTC1626automaticallyswitches
between continuous and Burst Mode operation. The volt-
age comparator is the primary control element when the
device is in Burst Mode operation, while the gain block
controls the output voltage in continuous mode.
When the load is heavy, the LTC1626 is in continuous
operation. During the switch “ON” time, current compara-
tor C monitors the voltage between the SENSE+ and
SENSE– pins connected across an external shunt in series
with the inductor. When the voltage across the shunt
reaches the comparator’s threshold value, its output sig-
nal changes state, resetting the flip-flop and turning the
internal P-channel MOSFET off. The timing capacitor
connected to the CT pin is now allowed to discharge at a
rate determined by the off-time controller.
the peak inductor’s current is set at 25mV/RSENSE
.
To avoid the operation of the current loop interfering with
Burst Mode operation, a built-in offset VOS is incorporated
in the gain stage. This prevents the current from increas-
inguntiltheoutputvoltagehasdroppedbelowaminimum
threshold.
When the voltage on the timing capacitor has discharged
pastVTH1,comparatorTtrips,setstheflip-flopandcauses
the switch to turn on. Also, the timing capacitor is
recharged. The inductor current will again ramp up until
the current comparator C trips. The cycle then repeats.
When the load current increases, the output voltage
In dropout, the P-channel MOSFET is turned on continu-
ously (100% duty cycle) providing low dropout operation
with VOUT VIN.
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APPLICATIONS INFORMATION
determines the peak inductor current. Depending upon
the load current condition, the threshold of the compara-
tor lies between 25mV/RSENSE and 150mV/RSENSE. The
maximum output current of the LTC1626 is:
The basic LTC1626 application circuit is shown in Figure
1. External component selection is driven by the load
requirementandbeginswiththeselectionofRSENSE.Once
RSENSE is known, CT and L can be chosen. Next, the
Schottky diode D1 is selected followed by CIN and COUT
.
IOUT(MAX) = 150mV/RSENSE – IRIPPLE/2 (A)
RSENSE Selection for Output Current
Where IRIPPLE is the peak-to-peak inductor ripple current.
At a relatively light load, the LTC1626 is in Burst Mode
operation. In this mode, the peak current is set at 25mV/
RSENSE. To fully benefit from Burst Mode operation, the
RSENSE is chosen based on the required output current.
Withthecurrentcomparatormonitoringthevoltagedevel-
oped across RSENSE, the threshold of the comparator
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LTC1626
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APPLICATIONS INFORMATION
inductor current should be continuous during burst peri-
Operating Frequency Considerations
ods. Hence, the peak-to-peak inductor ripple current must
For most applications, the LTC1626 should be operated in
the 100kHz to 300kHz range. This range can be extended,
however, up to 600kHz, to accommodate smaller size/
valued inductors, such as low profile types, with a slight
decrease in efficiency due to gate charge losses. Some
experimentation may be required to determine the opti-
mum operating frequency for a particular set of external
components and operating conditions.
not exceed 25mV/RSENSE
.
To account for light load conditions, the IOUT(MAX) is then
given by:
IOUT(MAX) = 150mV/RSENSE – 25mV/2RSENSE (A)
= 137.5mV/RSENSE (A)
Solving for RSENSE and allowing a margin of variations in
the LTC1626 and external component values yields:
CT and L Selection
RSENSE = 100mV/IOUT(MAX) (Ω)
The value of CT is calculated from the desired continuous
mode operating frequency:
The LTC1626 switch is capable of supplying a maximum
of 1.2A of output current. Therefore, the minimum value
of RSENSE that can be used is 0.083Ω. A graph for
selecting RSENSE versus maximum output current is given
in Figure 2.
V − V
(
)
IN
OUT
C =
F
( )
T
V + V 3300 V −V
f
IN
IN
D
BE
O
whereVD isthedropacrosstheSchottkydiodeD1andVBE
is a base-emitter voltage drop (0.6V).
0.5
0.4
0.3
0.2
0.1
0
The complete expression for operating frequency is given
by:
1
V − V
IN OUT
V + V
IN D
f ≈
Hz
( )
O
t
OFF
where:
t
= 3300 C V − V
sec
OFF
T
IN
BE
0
0.2
0.4
0.6
0.8
1.0
MAXIMUM OUTPUT CURRENT (A)
Figure 3 is a graph of operating frequency versus power
supply voltage for the 2.5V regulator circuit shown in
Figure 1 (CT = 270pF). Note that the frequency is relatively
constant with supply voltage but drops as the supply
voltage approaches the regulated output voltage.
1626 F02
Figure 2. Selecting RSENSE
Duringashortcircuitoftheregulatoroutputtoground,the
peak current is determined by:
To maintain continuous inductor current at light load, the
inductor must be chosen to provide no more than 25mV/
ISC = 150mV/RSENSE (A)
RSENSE of peak-to-peak ripple current. This results in the
In this condition, the LTC1626 automatically extends the
off-time period of the P-channel MOSFET switch to allow
the inductor current to decay far enough to prevent any
current buildup. The resulting ripple current causes the
following expression for L:
5
L ≥ 5.2 10
R
C
V
REG
H
SENSE
T
average current to be approximately IOUT(MAX)
.
Using an inductance smaller than the above value will
result in inductor current being discontinuous. As a con-
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LTC1626
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APPLICATIONS INFORMATION
sequence, the LTC1626 will delay entering Burst Mode
operation and efficiency will be degraded at low currents.
the P-channel switch duty cycle. At high input voltages,
the diode conducts most of the time. As VIN approaches
VOUT, the diode conducts only a small fraction of the time.
The most stressful condition for the diode is when the
regulator output is shorted to ground.
200
FIGURE 1 CIRCUIT
180
160
140
120
100
80
Under short-circuit conditions, the diode must safely
handle ISC(PK) at close to 100% duty cycle. Most LTC1626
circuits will be well served by either an MBRM5819 or an
MBRS130LT3. An MBR0520LT1 is a good choice for
60
I
OUT(MAX) ≤ 500mA.
40
20
Input Capacitor (CIN) Selection
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
In continuous mode, the input current of the converter is
a square wave of duty cycle VOUT/VIN. To prevent large
voltage transients, a low effective series resistance (ESR)
input capacitor must be used. In addition, the capacitor
must handle a high RMS current. The CIN RMS current is
given by:
INPUT VOLTAGE (V)
1626 F03
Figure 3. Operating Frequency vs Supply Voltage
for Circuit Shown in Figure 1
Inductor Core Selection
With the value of L selected, the type of inductor must be
chosen. Basically, there are two kinds of losses in an
inductor—core and copper losses.
1/2
]
I
V
V − V
(
)
OUT OUT IN
OUT
[
I
≈
A
RMS
V
IN
Core losses are dependent on the peak-to-peak ripple
current and core material. However, they are independent
of the physical size of the core. By increasing inductance,
the peak-to-peak inductor ripple current will decrease,
therefore reducing core loss. Utilizing low core loss mate-
rial,suchasmolypermalloyorKoolMµ® willallowtheuser
to concentrate on reducing copper loss and preventing
saturation.
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst case is commonly used
to design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours life-
time.Thismakeitadvisabletofurtherderatethecapacitor,
or choose a capacitor rated at a higher temperature than
required. Do not underspecify this component. An addi-
tional0.1µFceramiccapacitorisalsorequiredonPWRVIN
for high frequency decoupling.
Althoughhigherinductancereducescoreloss,itincreases
copper loss as it requires more windings. When space is
not a premium, larger wire can be used to reduce the wire
resistance. This also prevents excessive heat dissipation
in the inductor.
Output Capacitor (COUT) Selection
The selection of COUT is driven by the ESR for proper
operation of the LTC1626. The required ESR of COUT is:
Catch Diode Selection
ESRCOUT < 50mV/IRIPPLE
Losses in the catch diode depend on forward drop and
switching times. Therefore, Schottky diodes are a good
choice for low drop and fast switching times.
where IRIPPLE is the ripple current of the inductor. For the
case where the IRIPPLE is 25mV/RSENSE, the required ESR
of COUT is:
The catch diode carries the load current during the off-
time. The average diode current is therefore dependent on
Kool Mµ is a registered trademark of Magnetics, Inc.
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LTC1626
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APPLICATIONS INFORMATION
ESRCOUT < 2RSENSE
The LBO is an N-channel open drain that goes low when
the battery voltage drops below the threshold voltage. In
shutdown, the comparator is disabled and LBO is in the
high impedance state. Figure 4 is a schematic diagram
detailing the low-battery comparator connection and op-
eration.
To avoid overheating, the output capacitor must be sized
to handle the ripple current generated by the inductor. The
worst-case RMS ripple current in the output capacitor is
given by:
IRMS < 150mV/2RSENSE (ARMS)
V
IN
LTC1626
Generally, once the ESR requirements for COUT have been
met, the RMS current rating far exceeds the IRIPPLE
requirement.
R4
1%
LBO
+
–
1.25V
LBI
C
R3
1%
FILTER
1626 F04
0.01µF
In some surface mount applications, multiple capacitors
may have to be paralleled to meet the capacitance, ESR or
RMS current handling requirement of the application.
Aluminum electrolyte and dry tantalum capacitors are
bothavailableinsurfacemountconfigurations.Inthecase
oftantalum,itiscriticalthatthecapacitorsaresurgetested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalums, available
in case heights ranging from 2mm to 4mm. Other capaci-
tor types include Sanyo OS-CON, Nichicon PL series and
Sprague 595D series. Consult the manufacturer for other
specific recommendations.
Figure 4. Low-Battery Comparator
Setting the Output Voltage
The LTC1626 develops a 1.25V reference voltage between
the feedback pin VFB and the signal ground as shown in
Figure 5. By selecting resistor R1, a constant current is
caused to flow through R1 and R2 which sets the desired
output voltage. The regulated output voltage is deter-
mined by:
When the capacitance of COUT is made too small, the
outputrippleatlowfrequencieswillbelargeenoughtotrip
the voltage comparator. This causes Burst Mode opera-
tion to be activated when the LTC1626 would normally be
in continuous mode operation. The effect will be most
pronounced with low RSENSE values and can be improved
at higher frequencies.
R2
R1
V
= 1.25 1+
OUT
R1 should be ≤ 10k to ensure that sufficient current flows
through the divider to maintain accuracy and to provide a
minimum load for the regulator output at elevated
temperatures. (See Switch Leakage Current curve in Typi-
cal Performance Characteristics section.)
Low-Battery Detection
To prevent stray pickup, a 100pF capacitor is suggested
across R1, located close to the LTC1626.
The low-battery detector senses the input voltage through
anexternalresistivedivider. Thisdividedvoltageconnects
to the (–) input of a voltage comparator (LBI) and is
compared to an internal 1.25V reference voltage. Neglect-
ingLBIinputbiascurrent,thefollowingexpressionisused
for setting the trip voltage threshold:
V
OUT
R2
1%
V
LTC1626
SGND
FB
R1
10k
1%
100pF
1626 F05
R4
V
= 1.25 1+
_
LB TRIP
R3
Figure 5. Setting the Output Voltage
8
LTC1626
U
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APPLICATIONS INFORMATION
Thermal Considerations
regulator when it is operating in a 25°C ambient tempera-
ture is:
In a majority of applications, the LTC1626 does not
dissipate much heat due to its high efficiency. However, in
applications where the switching regulator is running at
high duty cycles or the part is in dropout with the switch
turned on continuously (DC), some thermal analysis is
required. The goal of the thermal analysis is to determine
whetherthepowerdissipatedbytheregulatorexceedsthe
maximum junction temperature. The temperature rise is
given by:
TJ = (0.113 • 110) + 25 = 38°C
Remembering that the above junction temperature is
obtained from an RDS(ON) at 25°C, we might recalculate
the junction temperature based on a higher RDS(ON) since
it increases with temperature. However, we can safely
assume that the actual junction temperature will not
exceed the absolute maximum junction temperature
of 125°C.
TRISE = PD • θJA
Board Layout Considerations
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.
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1626. These items are also illustrated graphically in
the layout diagram of Figure 6. Check the following in your
layout:
The junction temperature is given by:
TJ = TRISE + TAMBIENT
1. Are the signal and power grounds separated? The
LTC1626 signal ground (Pin 11) must return to the
(–) plate of COUT. The power ground (Pin 12) returns
to the anode of the Schottky diode and the (–) plate
of CIN.
As an example, consider the case when the LTC1626 is in
dropout at an input voltage of 3V with a load current of
0.5A. FromtheTypicalPerformanceCharacteristicsgraph
of Switch Resistance, the ON resistance of the P-channel
switch is 0.45Ω. Therefore, power dissipated by the
part is:
2. Does the (+) plate of CIN connect to the power VIN (Pins
1, 13) as close as possible? This capacitor provides the
AC current to the internal P-channel MOSFET and its
driver.
PD = I2 • RDS(ON) = 113mW
The SO package junction-to-ambient thermal resistance
θJA is110°C/W.Therefore,thejunctiontemperatureofthe
V
IN
BOLD LINES INDICATE
HIGH CURRENT PATHS
1
2
PWR V
IN
14
13
SW
PWR V
IN
V
IN
D1
+
LTC1626
0.1µF
C
IN
L
3
4
5
6
7
12
11
10
9
PGND
SGND
LBO
LBI
C
1k
3900pF
T
R1
R2
SHUTDOWN
C
I
SHDN
T
C
OUT
V
FB
+
TH
R
SENSE
8
–
+
SENSE
SENSE
V
OUT
1000pF
1626 F06
Figure 6. LTC1626 Layout Diagram (See Board Layout Checklist)
9
LTC1626
U
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APPLICATIONS INFORMATION
3. Is the input decoupling capacitor (0.1µF) connected
closely between power VIN (Pins 1, 13) and power
ground (Pin 12)? This capacitor carries the high fre-
quency peak currents.
divider R1-R2 must be connected between the (+) plate
of COUT and the signal ground.
6. AretheSENSE–andSENSE+ leadsroutedtogetherwith
minimum PC trace spacing? The 1000pF capacitor
between Pin 7 and Pin 8 should be as close as possible
to the LTC1626.
4. Is the Schottky diode closely connected between the
power ground (Pin 12) and switch output (Pin 14)?
5. Does the LTC1626 SENSE– (Pin 7) connect to a point
close to RSENSE and the (+) plate of COUT? The resistor
7. Is SHDN (Pin 10) actively pulled to ground during
normal operation? The shutdown pin is high imped-
ance and must not be allowed to float.
U
TYPICAL APPLICATIONS
Single Cell Li-Ion to 2.5V Converter
(V = 2.7V TO 4.5V)
IN
SINGLE
Li-ION
CELL
†
+
+
C
IN
L1*
0.1µF
47µF
16V
R **
SENSE
0.1Ω
PWR V
V
IN
IN
22µH
V
OUT
LBI
SW
2.5V
0.25A
D1
MBR0520LT1
LBO
SHDN
SHUTDOWN
PGND
††
LTC1626
+
C
OUT
+
–
I
TH
SENSE
SENSE
V
100µF
10V
1000pF
100pF
1k
3900pF
C
T
10k
10k
270pF
C
T
FB
SGND
1626 TA01
*
SUMIDA CDRH62-220
** IRC 1206-R100F
†
AVX TPSD476K016
AVX TPSD107K010
††
3- to 4-Cell NiCd/NiMH to 2.5V Converter
(V = 2.7V TO 6V)
IN
†
+
+
C
IN
3- OR 4-CELL
NiCd OR NiMH
L1*
R4
0.1µF
47µF
16V
R **
SENSE
0.1Ω
PWR V
V
IN
IN
22µH
V
OUT
LBI
SW
2.5V
0.25A
D1
MBR0520LT1
LBO
R3
SHDN
SHUTDOWN
PGND
††
LTC1626
+
C
OUT
+
–
I
TH
SENSE
SENSE
V
100µF
10V
1000pF
100pF
1k
C
T
†††
R1
270pF
3900pF
10k
C
T
FB
†††
R2
10k
SGND
1626 TA02
†††
FOR 3.3V:
*
SUMIDA CDRH62-220
** IRC 1206-R100F
R1 = 15k, 1%
†
AVX TPSD476K016
AVX TPSD107K010
R2 = 9.09k, 1%
††
10
LTC1626
U
TYPICAL APPLICATIONS
Low Profile (3mm Maximum Height) 2.8V Converter
V
IN
†
3V TO 6V
C
IN
†††
+
4.7µF
22µF
16V
TANT
L1*
15µH
R
**
SENSE
0.1Ω
CERAMIC
PWR V
V
IN
IN
V
OUT
LBI
SW
2.8V
0.25A
D1
MBR0520LT1
LBO
SHDN
SHUTDOWN
PGND
††
LTC1626
+
C
OUT
+
–
I
TH
SENSE
SENSE
V
100µF
6.3V
1000pF
100pF
1k
3900pF
R1
15k
1%
C
T
56pF
C
T
FB
R2
SGND
12.1k
1%
1626 TA03
*
COILCRAFT DO3308-153
** IRC 1206-R100F
†
AVX TPSC226M016R0375
††
†††
AVX TPSC107M006R0150
MURATA GRM230Y5V475Z16
U
PACKAGE DESCRIPTION
Dimensions in inches (millimeters) unless otherwise noted.
S Package
14-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.337 – 0.344*
(8.560 – 8.738)
13
12
11 10
8
14
9
0.228 – 0.244
(5.791 – 6.197)
0.150 – 0.157**
(3.810 – 3.988)
1
2
3
4
5
6
7
0.010 – 0.020
(0.254 – 0.508)
× 45°
0.053 – 0.069
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
0.008 – 0.010
(0.203 – 0.254)
0° – 8° TYP
0.050
(1.270)
TYP
0.014 – 0.019
(0.355 – 0.483)
0.016 – 0.050
0.406 – 1.270
S14 0695
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
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 represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
11
LTC1626
TYPICAL APPLICATIONS
U
Single Li-Ion to 3.3V Buck-Boost Converter
(V = 2.5V TO 4.2V)
IN
L1B
L1A
SINGLE
Li-ION
CELL
†
+
3
2
+
C
†
IN
100µF
L1A
0.1µF
100µF
16V
TOP VIEW
16V
PWR V
V
IN
IN
33µH
+
V
1
4
OUT
LBI
SW
3.3V
1
2
L1B
L1A
D1
MBRS130LT1
LBO
SHDN
SHUTDOWN
1k
PGND
15k
1%
††
MANUFACTURER
PART NO.
4
3
+
C
OUT
LTC1626
100µF
COILTRONICS
DALE
CTX33-4
LPT4545-330LA
L1B
I
TH
10V
33µH
9.09k
1%
C
T
100pF
V
75pF
FB
3900pF
V
IN
(V)
I (mA)
OUT
1626 TA05
SGND
C
T
2.5
200
350
500*
500*
500*
+
–
SENSE
3.0
3.5
4.0
4.2
SENSE
R
SENSE
*
1000pF
0.1Ω
*
IRC 1206-R100F
†
††
AVX TPSE107M016R0100
AVX TPSD107M010R0065
*DESIGN LIMIT
5V to 3.3V Converter
V
IN
5V
†
+
C
IN
L1*
47µH
0.1µF
100µF
10V
R
**
SENSE
0.1Ω
PWR V
V
IN
IN
V
3.3V
0.5A
OUT
LBI
SW
D1
MBRS130LT1
LBO
SHDN
SHUTDOWN
1k
PGND
††
LTC1626
+
C
OUT
+
–
I
TH
SENSE
SENSE
V
220µF
10V
1000pF
100pF
C
T
15k
1%
270pF
3900pF
C
T
FB
SGND
9.09k
1%
1626 TA04
*
COILCRAFT DO3316-473
** IRC 1206-R100F
†
AVX TPSD107K010
AVX TPSE227K010
††
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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High Efficiency Step-Down and Inverting DC/DC Converters
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LTC1265
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LTC1438/LTC1439
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Multiple Output Capability
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Q
1626f LT/TP 0398 4K • PRINTED IN USA
12 Linear Technology Corporation
●
1630McCarthyBlvd., Milpitas, CA95035-7417 (408)432-1900
●
●
FAX: (408) 434-0507 TELEX: 499-3977 www.linear-tech.com
LINEAR TECHNOLOGY CORPORATION 1997
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