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LTC1911-1.5/LTC1911-1.8 Low Noise, High Efficiency, Inductorless Step-Down DC/DC Converter
FEATURES
s s s s s s s s s s s s s
DESCRIPTIO
Low Noise Constant Frequency Operation 2.7V to 5.5V Input Voltage Range No Inductors Typical Efficiency 25% Higher Than LDOs Shutdown Disconnects Load from VIN Output Voltage: 1.8V 4% or 1.5V 4% Output Current: 250mA Low Operating Current: IIN = 180A Typ Low Shutdown Current: IIN = 10A Typ Oscillator Frequency: 1.5MHz Soft-Start Limits Inrush Current at Turn On Short-Circuit and Overtemperature Protected Available in an 8-Pin MSOP Package
The LTC(R)1911 is a switched capacitor step-down DC/DC converter that produces a 1.5V or 1.8V regulated output from a 2.7V to 5.5V input. The part uses switched capacitor fractional conversion to achieve high efficiency over the entire input range. No inductors are required. Internal circuitry controls the step-down conversion ratio to optimize efficiency as the input voltage and load conditions vary.* Typical efficiency is over 25% higher than that of a linear regulator. A unique constant frequency architecture provides a low noise regulated output as well as lower input noise than conventional charge pump regulators. High frequency operation (fOSC = 1.5MHz) simplifies output filtering to further reduce conducted noise. To optimize efficiency, the part enters Burst Mode(R) operation under light load conditions. Low operating current (180A with no load, 10A in shutdown) and low external parts count (two 1F flying capacitors and two 10F bypass capacitors) make the LTC1911 ideally suited for space constrained batterypowered applications. The part is short-circuit and overtemperature protected, and is available in an 8-pin MSOP package.
APPLICATIO S
s s s s s s
Handheld Computers Cellular Phones Smart Card Readers Portable Electronic Equipment Handheld Medical Instruments Low Power DSP Supplies
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. *U.S. Patent #6,438,005
TYPICAL APPLICATIO
90
Single Cell Li-Ion to 1.8V DC/DC Converter
80 100mA LTC1911-1.8 2.7V TO 5.5V INPUT 1-CELL Li-Ion OR 3-CELL NiMH 10F* 1F* 1 2 3 4 *CERAMIC CAPACITOR 30 2 3 4 5 INPUT VOLTAGE (V) 6
1911 G05
VIN C2+ C2- GND
SS/SHDN VOUT C1+ C1-
8 6 7 5 1F*
1911 TA01
EFFICIENCY (%)
70 60 50 IDEAL LDO 40 VOUT = 1.8V
VOUT = 1.8V IOUT = 250mA 10F*
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Efficiency
250mA
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LTC1911-1.5/LTC1911-1.8
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW VIN C2+ C2- GND 1 2 3 4 8 7 6 5 SS/SHDN C1+ VOUT C1-
VIN to GND ...................................................- 0.3V to 6V SS/SHDN to GND ........................ - 0.3V to (VIN + 0.3V) VOUT Short-Circuit Duration ............................ Indefinite Operating Temperature Range (Note 2) .. - 40C to 85C Storage Temperature Range ................. - 40C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER LTC1911EMS8-1.5 LTC1911EMS8-1.8 MS8 PART MARKING LTMY LTNU
MS8 PACKAGE 8-LEAD PLASTIC MSOP
TJMAX = 125C, JA = 160C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER VIN Operating Voltage VOUT VIN Operating Current VIN Shutdown Current Output Ripple VOUT Short-Circuit Current Switching Frequency SS/SHDN Input Threshold SS/SHDN Soft-Start Current Turn-On Time Load Regulation Line Regulation
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VIN = 3.6V, C1 = 1F, C2 = 1F, CIN = 10F, COUT = 10F unless otherwise noted.
CONDITIONS
q
MIN 2.7 1.44 1.73
q q q q
TYP 1.5 1.8 180 10 5 12 600
MAX 5.5 1.56 1.87 350 20
UNITS V V V A A mVP-P mVP-P mA
LTC1911-1.5, 0mA IOUT 250mA, VIN = 2.7V to 5.5V LTC1911-1.8, 0mA IOUT 250mA, VIN = 2.7V to 5.5V IOUT = 0mA, VIN = 2.7V to 5.5V SS/SHDN = 0V, VIN = 2.7V to 5.5V IOUT = 10mA IOUT = 250mA VOUT = 0V Oscillator Free Running
1.2
q
1.5 0.6 -2 0.01 0.03 10 0.13 0.3
1.8 1 -1
0.3 -5
VSS/SHDN = 0V (Note 3) VSS/SHDN = VIN CSS = 0pF, VIN = 3.3V CSS = 10nF, VIN = 3.3V 0V IOUT 250mA 0V IOUT 250mA
q
mV/mA %/V
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC1911E is guaranteed to meet specified performance from 0C to 70C. Specifications over the - 40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls.
Note 3: Currents flowing into the device are positive polarity. Currents flowing out of the device are negative polarity.
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MHz V A A ms ms
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LTC1911-1.5/LTC1911-1.8 TYPICAL PERFOR A CE CHARACTERISTICS
Input Operating Current vs Input Voltage
210 200
INPUT CURRENT (A)
190 TA = 25C 180 170 TA = -40C 160 150 2 3 4 INPUT VOLTAGE (V)
1911 G01
11
TA = 25C TA = -40C
OUTPUT VOLTAGE (V)
INPUT CURRENT (A)
TA = 85C
LTC1911-1.5 Output Voltage vs Input Voltage
1.55 IOUT = 250mA TA = -40C TA = 25C 1.53 TA = 85C 100 90
OUTPUT VOLTAGE (V)
EFFICIENCY (%)
1.51
70 250mA 60 50 IDEAL LDO 40
EFFICIENCY (%)
1.49
1.47 30 1.45 2 3 4 INPUT VOLTAGE (V)
LTXXXX * TPCXX
5
LTC1911-1.5 Efficiency vs Output Current
90 80
OUTPUT VOLTAGE (V)
EFFICIENCY (%)
70 60 50 VIN: 40 30 2.8V 3.3V 3.7V 1 10 100 OUTPUT CURRENT (mA) 4.3V 5.1V 5.5V 1000
1911 G07
OUTPUT VOLTAGE (V)
UW
5 6
6
Input Shutdown Current vs Input Voltage
15 VOUT = 0V V(SS/SHDN) = 0V TA = 85C 1.90
LTC1911-1.8 Output Voltage vs Input Voltage
IOUT = 250mA TA = -40C TA = 25C TA = 85C
13
1.85
1.80
9
1.75
7
5
2
3
4 INPUT VOLTAGE (V)
1.70 5 6
1911 G02
2
3
4 INPUT VOLTAGE (V)
5
6
1911 G03
LTC1911-1.5 Efficiency vs Input Voltage (Falling Input Voltage)
90 80 70 60
LTC1911-1.8 Efficiency vs Output Current
100mA 80
VIN: 50 40 30
2.7V 3.2V 3.7V 4.2V 5.1V 5.5V 1000
1911 G06
20
2
3
4 INPUT VOLTAGE (V)
5
6
1911 G05
1
100 10 OUTPUT CURRENT (mA)
LTC1911-1.8 Output Voltage vs Output Current
1.84 1.54 VIN = 3.6V TA = -40C TA = 25C TA = 85C
LTC1911-1.5 Output Voltage vs Output Current
VIN = 3.6V TA = 85C TA = 25C
1.82
1.52
1.80
1.50
TA = -40C
1.78
1.48
1.76
1.46
1.74 0.1
10 1 100 OUTPUT CURRENT (mA)
1000
1911 G08
1.44 0.1
10 1 100 OUTPUT CURRENT (mA)
1000
1911 G09
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LTC1911-1.5/LTC1911-1.8 TYPICAL PERFOR A CE CHARACTERISTICS
Start-Up Time vs Soft-Start Capacitor
100 VIN = 3.6V TA = -40C TA = 25C TA = 85C
30 25
OUTPUT RIPPLE (mVP-P)
START-UP TIME (ms)
10
20 15 COUT = 10F 10 5 COUT = 22F
FREQUENCY (MHz)
1
0.1 0.1
1 10 SOFT-START CAPACITOR (nF)
LTC1911-1.8 Output Voltage Ripple
VOUT 50mV/DIV 2-TO-1 MODE VIN = 5V VOUT 50mV/DIV 3-TO-2 MODE VIN = 3.6V VOUT 50mV/DIV 1-TO-1 MODE VIN = 2.7V IOUT = 250mA 100ns/DIV ALL WAVEFORMS AC COUPLED
1911 G12
PI FU CTIO S
VIN (Pin 1): Input Supply Voltage. VIN may be between 2.7V and 5.5V. Suggested bypass for VIN is a 10F (1F min) ceramic low ESR capacitor. C2 + (Pin 2): Flying Capacitor Two Positive Terminal. C2 - (Pin 3): Flying Capacitor Two Negative Terminal. GND (Pin 4): Ground. Connect to a ground plane for best performance. C1- (Pin 5): Flying Capacitor One Negative Terminal. VOUT (Pin 6): Regulated Output Voltage. VOUT is disconnected from VIN during shutdown. Bypass VOUT with a 10F ceramic low ESR capacitor (4F min, ESR < 0.1 max). C1+ (Pin 7): Flying Capacitor One Positive Terminal. SS/SHDN (Pin 8): Soft-Start/Shutdown Control Pin. This pin is designed to be driven with an external open-drain output. Holding the SS/SHDN pin below 0.3V will force the LTC1911-X into shutdown mode. An internal pull-up current of 2A will force the SS/SHDN voltage to climb to VIN once the device driving the pin is forced into a Hi-Z state. To limit inrush current on start-up, connect a capacitor between the SS/SHDN pin and GND. Capacitance on the SS/SHDN pin will limit the dV/dt of the pin during turn on which, in turn, will limit the dV/dt of VOUT. By selecting an appropriate soft-start capacitor, the user can control the inrush current for a known output capacitor during turn-on (see Application Information). If neither of the two functions are desired, the pin may be left floating or tied to VIN.
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UW
1911 G10
Output Ripple vs Output Load Current
1.60
Oscillator Frequency vs Input Supply Voltage
TA = -40C COUT = 4.7F 1.55
TA = 25C 1.50
1.45
TA = 85C
0
1.40 0 100 150 200 250 50 OUTPUT LOAD CURRENT (mA) 300 2.5 3.0 3.5 4.0 VIN (V) 4.5 5.0 5.5
1911 G15
100
1911 G11
Output Current Transient Response
250mA IOUT 25mA 4V VIN 500mV/DIV 3V
Line Transient Response
VOUT 20mV/DIV
VOUT 20mV/DIV
VIN = 3.6V
10s/DIV
1911 G13
IOUT = 225mA
20s/DIV
1911 G14
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LTC1911-1.5/LTC1911-1.8
SI PLIFIED BLOCK DIAGRA
1 CIN
VIN
300k
+
50k
-
MODE CONTROL STEP-DOWN CHARGE PUMP
+
150k SHDN
-
RSENSE
ADJ OFFSET
+
AMP1
-
VREF COMP1
OVERTEMP DETECT
1.5MHz OSCILLATOR
VIN
2A
8
600mV
+
1911 BD
+
SS/SHDN
SHDN
W
RA C1+ 7 C1 C1- 5 C2 + 2 C2 3 C2 - VOUT 6 COUT
-
W
+ - - +
COMP2 BURST THRESHOLD 60k
+ -
SHORT-CIRCUIT THRESHOLD
-
AMP2
+ -
600mV 140k SOFT-START VREF RAMP
+
+
1.26V VREF GND 4
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LTC1911-1.5/LTC1911-1.8
APPLICATIO S I FOR ATIO
General Operation
The LTC1911 uses a switch capacitor-based DC/DC conversion to provide the efficiency advantages associated with inductor-based circuits as well as the cost and simplicity advantages of a linear regulator. The LTC1911's unique constant frequency architecture provides a low noise regulated output as well as lower input noise than conventional switch-capacitor charge pump regulators. The LTC1911 uses an internal switch network and fractional conversion ratios to achieve high efficiency over widely varying VIN and output load conditions. Internal control circuitry selects the appropriate step-down conversion ratio based on VIN and load conditions to optimize efficiency. The part has three possible step-down modes: 2-to-1, 3-to-2 or 1-to-1 step-down mode. Only two external flying caps are needed to operate in all three modes. 2-to-1 mode is chosen when VIN is greater than two times the desired VOUT. 3-to-2 mode is chosen when VIN is greater than 1.5 times VOUT but less than 2 times VOUT. 1to-1 mode is chosen when VIN falls below 1.5 times VOUT. An internal load current sense circuit controls the switch point of the step-down ratio as needed to maintain output regulation over all load conditions. Regulation is achieved by sensing the output voltage and regulating the amount of charge transferred per cycle. This method of regulation provides much lower input and output ripple than that of conventional switched capacitor charge pumps. The constant frequency charge transfer also makes additional output or input filtering much less demanding than conventional switched capacitor charge pumps. The LTC1911 also has a Burst Mode function that delivers a minimum amount of charge for one cycle then goes into a low current state until the output drops enough to require another burst of charge. Burst Mode operation allows the LTC1911 to achieve high efficiency even at light loads. The part has shutdown capability as well as user-controlled inrush current limiting. In addition, the part has shortcircuit and overtemperature protection.
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Step-Down Charge Transfer Operation Figure 1a shows the switch configuration that is used for 2-to-1 step down mode. In this mode, a 2-phase clock generates the switch control signals. On phase one of the clock, the top plate of C1 is connected to VIN through RA and S4, the bottom plate is connected to VOUT through S3. The amount of charge transferred to C1 (and VOUT) is set by the value of RA. On phase two, flying capacitor C1 is connected to VOUT through S1 and to GND through S2. The charge that was transferred onto C1 from the previous cycle is now transferred to the output. Thus, in 2-to-1 mode, charge is transferred to VOUT on both phases of the clock. Since charge current is sourced from GND on the second phase of the clock, current multiplication is realized with respect to VIN, i.e., IOUT equals approximately 2 * IIN. This results in significant efficiency improvement relative to a linear regulator. The value of RA is set by the control loop of the regulator.
RA VIN C1 C1- S3 1
1911 F01a
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S4 1
C1+
S1 2 VOUT
S2 2
Figure 1a. Step-Down Charge Transfer in 2-to-1 Mode
The 3-to-2 conversion mode also uses a nonoverlapping clock for switch control but requires two flying capacitors and a total of seven switches (see Figure 1b). On phase one of the clock, the two capacitors are connected in parallel to VIN through RA by switches S5 and S7, and to VOUT through S4 and S6. The amount of charge transferred to C1|| C2 (and VOUT) is set by the regulator control loop which determines the value of RA. On phase two, C1 and C2 are connected in series from VOUT to GND through switches S1, S2 and S3. On phase two, half of the charge
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LTC1911-1.5/LTC1911-1.8
APPLICATIO S I FOR ATIO
transferred to the parallel combination of C1 and C2 is transferred to the VOUT. In this manner, charge is again transferred from the flying capacitors to the output on both phases of the clock. As in 2-to-1 mode, charge current is sourced from GND on phase two of the clock resulting in increased power efficiency. IOUT in 3-to-2 mode equals approximately (3/2)IIN. In 1-to-1 mode (see Figure 1c), switch S1 is always closed connecting the top plate of C1 to VOUT. Switch S2 remains closed for almost the entire clock period, opening only briefly at the end of clock phase one. In this manner, VOUT is connected to VIN through RA. The value of RA is set by the regulator control loop which determines the amount of current transferred to VOUT during the on period of S2. The LTC1911 acts much like a linear regulator in this mode. Since all of the VOUT current is sourced from VIN, the efficiency in 1-to-1 mode is approximately equal to that of a linear regulator.
RA VIN C1 C1- S4 1 S5 1 C1+ S1 2 VOUT
S7 1
S2 2
C2 + C2 C2 - S6 1
1911 F01b
S3 2 GND
Figure 1b. Step-Down Charge Transfer in 3-to-2 Mode
RA VIN
S2 C1
C1+
S1 VOUT
C1-
1911 F01c
Figure 1c. Step-Down Charge Transfer in 1-to-1 Mode
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Mode Selection The optimal step-down conversion mode is chosen based on VIN and output load conditions. Two internal comparators are used to select the default step-down mode based on the input voltage. Each comparator has an adjustable offset built in that increases (decreases) in proportion to the increasing (decreasing) output load current. In this manner, the mode switch point is optimized to provide peak efficiency over all supply and load conditions. Each comparator also has built-in hysteresis of about 300mV to ensure that the LTC1911 does not oscillate between modes when a transition point is reached. Soft-Start/Shutdown Operation The SS/SHDN pin is used to implement both low current shutdown and soft-start. The soft-start feature limits inrush currents when the regulator is initially powered up or taken out of shutdown. Forcing a voltage lower than 0.6V (typ) on the SS/SHDN pin will put the LTC1911 into shutdown mode. Shutdown mode disables all control circuitry and forces VOUT into a high impedance state. A 2A pull-up current on the SS/SHDN pin will force the part into active mode if the pin is left floating or is driven with an open-drain output that is in a high impedance state. If the pin is not driven with an open-drain device, it must be forced to a logic high voltage of 2.2V (min) to ensure proper VOUT regulation. The SS/SHDN pin should not be driven to a voltage higher than VIN. To implement softstart, the SS/SHDN pin must be driven with an open-drain device and a capacitor must be connected from the SS/ SHDN pin to GND. Once the open-drain device is turned off, the 2A pull-up current will begin charging the external soft-start capacitor and force the voltage on the pin to ramp towards VIN. As soon as the shutdown threshold is reached (0.6V typ), the internal reference voltage that controls the VOUT regulation point will follow the ramp voltage on the SS/SHDN pin (minus a 0.6V offset to account for the shutdown threshold) until the reference reaches its final band gap voltage. This occurs when the voltage on the SS/SHDN pin reaches approximately 1.9V. Since the ramp rate on the SS/SHDN pin controls the ramp rate on VOUT, the average inrush current can be controlled through the selection of CSS and COUT. For example, a
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LTC1911-1.5/LTC1911-1.8
APPLICATIO S I FOR ATIO
4.7nF capacitor on SS/SHDN results in a 3ms ramp time from 0.6V to 1.9V on the pin. If COUT is 10F, the 3ms VREF ramp time results in an average COUT charge current of only 6mA (see Figure 2).
VOUT LTC1911 8 ON OFF VCTRL SS/SHDN 6 COUT RLOAD
CSS
(2a)
VCTRL 2V/DIV
VOUT 1V/DIV
CSS = 0nF COUT = 10F RLOAD = 10
2ms/DIV
1911 F02b
(2b)
VCTRL 2V/DIV
VOUT 1V/DIV
CSS = 4.7nF COUT = 10F ROUT = 10
2ms/DIV
1911 F02c
(2c) Figure 2. Shutdown/Soft-Start Operation
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Low Current Burst Mode Operation To improve efficiency at low output currents, a Burst Mode function was included in the design of the LTC1911. An output current sense circuit is used to detect when the required output current drops below 30mA typ. When this occurs, the oscillator shuts down and the part goes into a low current operating state. The LTC1911 will remain in the low current operating state until VOUT has dropped enough to require another burst of current. Unlike traditional charge pumps who's burst current is dependant on many factors (i.e., supply, switch strength, capacitor selection, etc.), the LTC1911 burst current is set by the burst threshold. This means that the output ripple voltage during Burst Mode operaton will be fixed and is typically 5mV for COUT = 10F. Short-Circuit/Thermal Protection The LTC1911 has built-in short-circuit current limiting as well as overtemperature protection. During short-circuit conditions it will automatically limit its output current to approximately 600mA. The LTC1911 will shut down if the junction temperature exceeds approximately 160C. Under normal operating conditions, the LTC1911 should not go into thermal shutdown but it is included to protect the IC in cases of excessively high ambient temperatures, or in cases of excessive power dissipation inside the IC (i.e., overcurrent or short circuit). The charge transfer will reactivate once the junction temperature drops back to approximately 150C. The LTC1911 can cycle in and out of thermal shutdown indefinitely without latch-up or damage until the fault condition is removed. VOUT Ripple and Capacitor Selection The type and value of capacitors used with the LTC1911 determine several important parameters such as regulator control loop stability, output ripple and charge pump strength. The value of COUT directly controls the amount of output ripple for a given load current. Increasing the size of COUT will reduce the output ripple.
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LTC1911-1.5/LTC1911-1.8
APPLICATIO S I FOR ATIO
To reduce output noise and ripple, it is suggested that a low ESR ( 0.1) ceramic capacitor (10F or greater) be used for COUT. Tantalum and Aluminum capacitors are not recommended because of their high ESR (equivalent series resistance). Both the style and value of COUT can significantly affect the stability of the LTC1911. As shown in the Block Diagram, the part uses a control loop to adjust the strength of the charge pump to match the current required at the output. The error signal of this loop is stored directly on the output charge storage capacitor. The charge storage capacitor also serves to form the dominant pole for the control loop. To prevent ringing or instability it is important for the output capacitor to maintain at least 4F of capacitance over all conditions (See Ceramic Capacitor Selection Guidelines). Likewise excessive ESR on the output capacitor will tend to degrade the loop stability of the LTC1911. The closedloop output resistance of the part is designed to be 0.13. For a 250mA load current change, the output voltage will change by about 33mV. If the output capacitor has 0.13 or more of ESR, the closed-loop frequency response will cease to roll-off in a simple 1-pole fashion and poor load transient response or instability could result. Ceramic capacitors typically have exceptional ESR performance, and combined with a tight board layout, should yield excellent stability and load transient performance. VIN Capacitor Selection The constant frequency architecture used by the LTC1911 makes input noise filtering much less demanding than with conventional regulated charge pumps. Depending on the mode of operation the input current of the LTC1911 can vary from IOUT to 0mA on a cycle-by-cycle basis. Lower ESR will reduce the voltage steps caused by changing input current, while the absolute capacitor value will determine the level of ripple. For optimal input noise and ripple reduction, it is recommended that a low ESR ceramic capacitor be used for CIN. A tantalum capacitor may be used for CIN but the higher ESR will lead to more input noise. The LTC1911 will operate with capacitors
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less than 1F but the increasing input noise will feed through to the output causing degraded performance. For best performance a 1F or greater capacitor is suggested for CIN. Aluminum capacitors are not recommended because of their high ESR. Flying Capacitor Selection Warning: A polarized capacitor such as tantalum or aluminum should never be used for the flying capacitors since their voltage can reverse upon start-up of the LTC1911. Ceramic capacitors should always be used for the flying capacitor. The flying capacitor controls the strength of the charge pump. In order to achieve the rated output current it is necessary for the flying capacitor to have at least 0.4F of capacitance over operating temperature with a 2V bias (See Ceramic Capacitor Selection Guidelines). If only 100mA or less of output current is required the flying capacitor minimum can be reduced to 0.15F. Ceramic Capacitor Selection Guidelines Capacitors of different materials lose their capacitance with higher temperature and voltage at different rates. For example, a ceramic capacitor made of X7R material will retain most of its capacitance from - 40C to 85C whereas a Z5U or Y5V style capacitor will lose considerable capacitance over that range (60% to 80% loss typ). Z5U and Y5V capacitors may also have a very strong voltage coefficient causing them to lose an additional 60% or more of their capacitance when the rated voltage is applied. Therefore, when comparing different capacitors it is often more appropriate to compare the amount of achievable capacitance for a given case size rather than discussing the specified capacitance value. For example, over rated voltage and temperature conditions, a 4.7F, 10V, Y5V ceramic capacitor in a 0805 case may not provide any more capacitance than a 1F, 10V, X7R available in the same 0805 case. In fact, over bias and temperature range, the 1F, 10V, X7R will provide more capacitance than the 4.7F, 10V, Y5V. The capacitor manufacturer's data sheet should be consulted to determine what value of capacitor
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LTC1911-1.5/LTC1911-1.8
APPLICATIO S I FOR ATIO
is needed to ensure that minimum capacitance values are met over operating temperature and bias voltage. Table 1 is a list of ceramic capacitor manufacturers and how to contact them.
Table 1. Ceramic Capacitor Manufacturers
AVX Kemet Murata Taiyo Yuden Vishay 1-(803)-448-1943 1-(864) 963-6300 1-(800) 831-9172 1-(800) 348-2496 1-(800) 487-9437 www.avxcorp.com www.kemet.com www.murata.com www.t-yuden.com www.vishay.com
Layout Considerations Due to the high switching frequency and transient currents produced by the LTC1911, careful board layout is necessary for optimal performance. A true ground plane and short connections to all capacitors will optimize performance, reduce noise and ensure proper regulation over all conditions. Figure 3 shows the recommended layout configuration.
VIN SS/SHDN
C2
GND
: CONNECT TO GND PLANE ON BACK OF BOARD
Figure 3. Recommended Component Placement and Grounding
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Additional output filtering can be achieved by placing a second output capacitor, connected to the ground plane, about 2cm or more from the LTC1911 output capacitor (C4). The inductance of the trace running to the second output capacitor will significantly attenuate the high speed switching transients of the LTC1911. Even small capacitors as low as 0.1F will provide excellent results. Thermal Management The power dissipation in the LTC1911 can cause the junction temperature to rise at rates of up to 160C/W. If the specified operating conditions are followed, the junction temperature should never exceed the 160C thermal shutdown temperature. The junction temperature can come very close and possibly exceed the specified 125C operating junction temperature. To reduce the maximum junction temperature, a good thermal connection to the PC board is recommended. Connecting the GND pin (Pin 4) to a ground plane, and maintaining a solid ground plane under the device on two layers of the PC board, can reduce the thermal resistance of the package and PC board considerably.
C3 U1 C1 C4 OUT
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LTC1911-1.5/LTC1911-1.8
PACKAGE DESCRIPTIO
5.23 (.206) MIN
0.42 0.04 (.0165 .0015) TYP
RECOMMENDED SOLDER PAD LAYOUT DETAIL "A" 0 - 6 TYP 4.88 0.1 (.192 .004) 3.00 0.102 (.118 .004) NOTE 4
0.254 (.010) GAUGE PLANE
0.18 (.077) SEATING PLANE 0.22 - 0.38 (.009 - .015) 0.13 0.05 (.005 .002)
MSOP (MS8) 0102
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
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.
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MS8 Package 8-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1660)
0.889 0.127 (.035 .005) 3.2 - 3.45 (.126 - .136) 0.65 (.0256) BSC 3.00 0.102 (.118 .004) (NOTE 3) 8 7 65 0.52 (.206) REF 0.53 0.015 (.021 .006) DETAIL "A" 1 1.10 (.043) MAX 23 4 0.86 (.034) REF 0.65 (.0256) BCS
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LTC1911-1.5/LTC1911-1.8
TYPICAL APPLICATIO
2.7V TO 5.5V INPUT 1-CELL Li-Ion OR 3-CELL NiMH 10F* 1F*
*CERAMIC CAPACITOR
RELATED PARTS
PART NUMBER LTC1514 LTC1515 LT1776 LTC3250-1.5 LTC3251 LTC3404 LTC3405A LTC3406B LTC3411 LTC3412 LTC3440 DESCRIPTION 50mA, 650kHz, Step-Up/Down Charge Pump with Low Battery Comparator 50mA, 650kHz, Step-Up/Down Charge Pump with Power On Reset 500mA (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 250mA, 1.5MHz, High Efficiency, Step-Down Charge Pump 500mA, 1MHz to 1.6MHz, Spread Spectrum, Step-Down Charge Pump 600mA (IOUT), 1.4MHz, Synchronous Step-Down DC/DC Converter 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter COMMENTS VIN = 2.7V to 10V, VOUT = 3V to 5V, IQ = 60A, ISD = 10A, S8 Package VIN = 2.7V to 10V, VOUT = 3.3V or 5V, IQ = 60A, ISD = <1A, S8 Package 90% Efficiency, VIN = 7.4V to 40V, VOUT = 1.24V, IQ = 3.2mA, ISD = 30A, N8,S8 Packages 85% Efficiency, VIN = 3.1V to 5.5V, VOUT = 1.5V, IQ = 35A, ISD = <1A, ThinSOT Package 85% Efficiency, VIN = 3.1V to 5.5V, VOUT = 0.9V to 1.6V, IQ = 9A, ISD = <1A, MS Package 95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 10A, ISD = <1A, MS8 Package 95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 20A, ISD = <1A, ThinSOT Package 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.6V, IQ = 20A, ISD = <1A, ThinSOT Package 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60A, ISD = <1A, MS Package 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60A, ISD = <1A, TSSOP16E Package 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 2.5V, IQ = 25A, ISD = <1A, MS Package
ThinSOT is a trademark of Linear Technology Corporation.
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
U
DC/DC Converter with Shutdown and Soft-Start
LTC1911-1.5 1 2 3 4 VIN C2+ C2- GND SS/SHDN C1+ VOUT 8 7 6 1F* 10F*
1911 TA03
10nF VOUT = 1.5V IOUT = 250mA 2N7002 ON OFF
5 C1-
1911f LT/TP 1102 2K * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2001

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