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Final Electrical Specifications
LTC1503-1.8/LTC1503-2 High Efficiency Inductorless Step-Down DC/DC Converter
May 1999
FEATURES
s s s s s s s s s s s s
DESCRIPTION
The LTC(R)1503-1.8/LTC1503-2 are switched capacitor step-down DC/DC converters that produce a regulated output from a 2.4V to 6V input. The parts use 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 25% higher than that of a low dropout (LDO) linear regulator. Regulation is achieved by sensing the output voltage and enabling the internal switching network as needed to maintain a fixed output voltage. This method of regulation enables the parts to achieve high efficiency at extremely light loads. Low operating current (25A with no load, 5A in shutdown) and low external parts count (two 1F flying capacitors and two 10F bypass capacitors) make the LTC1503-1.8/LTC1503-2 ideally suited for space constrained battery-powered applications. The parts are fully short-circuit and overtemperature protected. The LTC1503-1.8/LTC1503-2 are available in 8-pin MSOP and SO packages.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Input Voltage Range: 2.4V to 6V Fixed Output Voltages: 1.8V 4%, 2V 4% Output Current: 100mA No Inductors Typical Efficiency 25% Higher than LDOs Low Operating Current: 25A Low Shutdown Current: 5A 600kHz Switching Frequency Shutdown Disconnects Load from VIN Soft-Start Limits Inrush Current at Turn-On Short-Circuit and Overtemperature Protected Available in 8-Pin MSOP and SO Packages
APPLICATIONS
s s s s s s
Cellular Phones Handheld Computers Smart Card Readers Low Power DSP Supplies Portable Electronic Equipment Handheld Medical Instruments
TYPICAL APPLICATION
Efficiency vs Input Voltage
100
Single Li-Ion to 2V DC/DC Converter
LTC1503-2
LTC1503-2 1-CELL Li-Ion OR 3-CELL NiMH VIN C1- C1+ VOUT C2 +
10F 1F
2 3 5
8 C2 - 6 7 1F
10F
EFFICIENCY (%)
4
1
VOUT = 2V IOUT = 100mA
SHDN/SS GND
1503-2 TA01
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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80
60 "IDEAL" LDO 40 VOUT = 2V IOUT = 100mA 20 2 3 4 INPUT VOLTAGE (V)
1503-2 TA02
5
6
1
LTC1503-1.8/LTC1503-2 ABSOLUTE MAXIMUM RATINGS
VIN, VOUT, C1+, C1-, C2 +, C2 - to GND ..... - 0.3V to 6.5V SHDN/SS to GND ......................... - 0.3V to (VIN + 0.3V) VOUT Short-Circuit Duration ............................. Indefinite Commercial Temperature Range ............ - 40C to 85C
PACKAGE/ORDER I FOR ATIO
TOP VIEW VOUT C1- C1+ VIN 1 2 3 4 8 7 6 5 C2 - GND C2+ SHDN/SS
ORDER PART NUMBER LTC1503CMS8-1.8 LTC1503CMS8-2 MS8 PART MARKING LTFX LTHN
VOUT 1 C1- 2 C1+ 3 VIN 4
MS8 PACKAGE 8-LEAD PLASTIC MSOP
TJMAX = 125C, JA = 200C/W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = VIN(MIN) to VIN(MAX), C1 = C2 = 1F, CIN = COUT = 10F unless otherwise noted.
PARAMETER VIN Operating Voltage VOUT VIN Operating Current VIN Shutdown Current Output Ripple Efficiency Switching Frequency SHDN/SS Input Threshold SHDN/SS Input Current VOUT Short-Circuit Current VOUT Turn-On Time VSHDN/SS = 0V (Note 3) VSHDN/SS = VIN VOUT = 0V (Note 4) CSS = 0pF, VIN = 3.3V, COUT = 10F CSS = 10nF LTC1503-1.8, 0mA < IOUT < 100mA LTC1503-2, 0mA < IOUT < 100mA IOUT = 0mA SHDN/SS = 0V LTC1503-X, VIN = 3.6V, IOUT = 100mA LTC1503-2, VIN = 3.6V, IOUT = 100mA Oscillator Free Running
q q q q
CONDITIONS
q q q q q
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC1503C is guaranteed to meet specified performance from 0C to 70C and is designed, characterized and expected to meet these extended temperature limits, but are not tested at - 40C and 85C. The LTC1503I is guaranteed to meet the extended temperature limits.
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(Note 1)
Industrial Temperature Range ............... - 40C to 85C Specified Temperature Range (Note 2) ... - 40C to 85C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
TOP VIEW 8 C2 - 7 GND 6 C2+ 5 SHDN/SS
ORDER PART NUMBER LTC1503CS8-1.8 LTC1503CS8-2 LTC1503IS8-1.8 LTC1503IS8-2 S8 PART MARKING 150318 15032 503I18 1503I2
S8 PACKAGE 8-LEAD PLASTIC SO TJMAX = 125C, JA = 150C/W
MIN 2.4 1.728 1.920
TYP 1.8 2.0 25 5 20 81 600
MAX 6 1.872 2.080 50 10
UNITS V V V A A mVP-P % kHz
0.2 - 3.5 -1 8
0.35 -2 20 0.5 8
0.5 -1 1 50
V A A mA ms ms
Note 3: Currents flowing into the device are positive polarity. Currents flowing out of the device are negative polarity. Note 4: When VOUT is less than 150mV, IOUT is limited to much less than the maximum rated output current to prevent damage to the output devices.
LTC1503-1.8/LTC1503-2 TYPICAL PERFOR A CE CHARACTERISTICS
Input Current vs Input Voltage
50 LTC1503-X IOUT = 0mA TA = 25C
OUTPUT VOLTAGE (V) INPUT CURRENT (A)
40
30
20
10
2
Output Voltage vs Input Voltage
2.10 LTC1503-2 IOUT = 50mA TA = 25C 100
OUTPUT VOLTAGE (V)
2.05
EFFICIENCY (%)
2.00
1.95
1.90
2
Efficiency vs Output Current
100 LTC1503-1.8 TA = 25C 100 VIN = 3.3V 80 VIN = 3.6V 60
80
EFFICIENCY (%)
EFFICIENCY (%)
40
20
0 0.01
0.1
UW
3
3
Output Voltage vs Input Voltage
1.90 LTC1503-1.8 IOUT = 50mA TA = 25C 1.85
1.80
1.75
4 INPUT VOLTAGE (V)
5
6
1503 G01
1.70
2
3
4 INPUT VOLTAGE (V)
5
6
1503 G02
Efficiency vs Input Voltage
IOUT = 100mA TA = 25C 80 LTC1503-1.8 60 "IDEAL" LDO 40
4 INPUT VOLTAGE (V)
5
6
1503 G03
20
2
3
4 INPUT VOLTAGE (V)
5
6
1503 G04
Efficiency vs Output Current
LTC1503-2 TA = 25C VIN = 3.3V
VIN = 3.6V
60
40
20
100 1 10 OUTPUT CURRENT (mA)
1000
1503 G05
0 0.01
0.1
100 1 10 OUTPUT CURRENT (mA)
1000
1503 G06
3
LTC1503-1.8/LTC1503-2
PIN FUNCTIONS
VOUT (Pin 1): Regulated Output Voltage. VOUT is disconnected from VIN during shutdown. Bypass VOUT to ground with a 10F low ESR capacitor. C1 - (Pin 2): Flying Capacitor One Negative Terminal. C1 + (Pin 3): Flying Capacitor One Positive Terminal. VIN (Pin 4): Input Voltage. VIN may be between 2.4V and 6V. Bypass VIN to ground with a 10F low ESR capacitor. SHDN/SS (Pin 5): Shutdown/Soft-Start Control. The pin is designed to be driven with an external open-drain output. Holding the SHDN/SS pin below 0.25V will force the part into shutdown mode. An internal pull-up current of 2A will force the SHDN/SS voltage to climb to V IN 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 SHDN/SS pin and ground. Capacitance on the SHDN/SS pin will limit the dV/dt of the pin during turnon which, in turn, will limit the dV/dt of VOUT. By selecting an appropriate soft-start capacitor for a known output capacitor, the user can control the inrush current during turn-on (see Applications Information). If neither of the two functions are desired, the pin may be floated or tied to VIN. C2 + (Pin 6): Flying Capacitor Two Positive Terminal. GND (Pin 7): Ground. Connect to a ground plane for best performance. C2 - (Pin 8): Flying Capacitor Two Negative Terminal.
BLOCK DIAGRA
VIN CIN
680k
330k
990k
SHORT CIRCUIT
MODE SKIP
600kHz OSCILLATOR VIN 2A
REG ENABLE
COMP1
SOFT-START
350mV
VREF RAMP
SHDN/SS
+
SHDN
350mV
+
-
LTC1503-2
1503-2 BD
4
+
COMP2
-
-
-
+
+
+
+
-
+
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MODE CONTROL STEP-DOWN CHARGE PUMP
C1+
C1- C2 +
+ -
VOUT
C2 - VOUT COUT
+
150mV 800k
10mV
1.2M
1.2V VREF
GND
LTC1503-1.8/LTC1503-2
APPLICATIONS INFORMATION
General Operation The two most common methods for providing regulated step-down DC/DC conversion are linear DC/DC conversion (used by LDOs) and inductor-based DC/DC conversion. Linear regulation provides low cost and low complexity, but the conversion efficiency is poor since all of the load current must come directly from VIN. Inductor-based stepdown conversion provides the highest efficiency, but the solution cost and circuit complexity are much higher. The LTC1503-X provides the efficiency advantages associated with inductor-based circuits as well as the cost and simplicity advantages of an inductorless converter. The LTC1503-X is a switched capacitor step-down DC/DC converter. The part 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, VOUT and load conditions to optimize efficiency. The part has three possible step-down modes: 2-to-1, 3-to-2 or 1-to-1 (gated switch) 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. 1-to-1 mode is chosen when VIN falls below 1.5 times VOUT. An internal mode skip function will switch the step-down ratio as needed to maintain output regulation under heavy load conditions. Regulation is achieved by sensing the divided down output voltage and enabling the charge pump as needed to boost the output back into regulation. This method of regulation allows the LTC1503-X to achieve high efficiency at very light loads. The part has shutdown capability as well as user controlled inrush current limiting. In addition, the part can withstand an indefinite short-circuit condition on VOUT and is also overtemperature protected. Step-Down Charge Pump Operation Figure 1a shows the charge pump switch configuration that is used for 2-to-1 step down. When the charge pump is enabled on this mode, a two phase nonoverlapping clock generates the switch control signals. On phase one of the clock, flying capacitor C1 is connected through
VIN C1 (EXTERNAL) C1- S3 2
1503 F01a
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S4 2
C1+
S1 1 VOUT
S2 1
Figure 1a. Step-Down Charge Pump in 2-to-1 Mode
switches S1 and S2 across VOUT. If the voltage on C1 is greater than the voltage on COUT, charge is transferred from C1 onto COUT. On phase two, the top plate of C1 is connected to VIN and the bottom plate is connected to VOUT. If the voltage across C1 is less than VIN /2 during phase two, charge will be transferred from C1 onto COUT thereby boosting the voltage on COUT and raising the voltage across C1. Thus, in 2-to-1 mode, charge transfer from C1 onto COUT occurs on both phases of the clock, and the voltage on COUT is driven towards 1/2VIN until the output is back in regulation. Since charge current is sourced from ground on phase one of the clock, current multiplication is realized with respect to VIN, i.e., IVOUT equals approximately 2 * IVIN. This results in significant efficiency improvement relative to a linear regulator. 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, C1 and C2 are connected in series across VOUT. If the sum of the voltages across C1 and C2 is greater than VOUT, charge is transferred from the flying caps onto COUT thereby reducing the average voltage on the flying caps and raising the voltage on the output capacitor. On phase two, the two flying capacitors are connected on parallel between VIN and VOUT. Since the average voltage across the two capacitors during phase one is VOUT /2, charge will be transferred from VIN to VOUT through the two flying caps if VIN minus VOUT /2 is greater than VOUT. In this manner, charge is again transferred from the flying caps to the output on both phases of the clock, and the voltage on COUT is driven towards (2/3)VIN until the part is back in regulation. As in 2-to-1 mode, charge current is sourced from ground on phase one of the clock which results in increased power efficiency. IVOUT in 3-to-2 mode equals approximately (3/2)IVIN.
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LTC1503-1.8/LTC1503-2
APPLICATIONS INFORMATION
S5 2 VIN C1 (EXTERNAL) S4 C1- 2 C1+ S1 1 VOUT
S7 2
S2 1
C2 + C2 (EXTERNAL) S6 C2 - 2
S3 1 GND
1503 F01b
Figure 1b. Step-Down Charge Pump in 3-to-2 Mode
In 1-to-1 mode, switch S1 and S2 are connected in series between VIN and VOUT as needed to boost VOUT back into regulation (see Figure 1c). The REG ENABLE signal from the main comparator (COMP1) controls switches S1 and S2 directly. 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.
S2 VIN C1 (EXTERNAL) C1-
1503 F01c
C1+
S1 VOUT
Figure 1c. Step-Down Charge Pump in 1-to-1 Mode
Mode Selection and Mode Skipping The optimal step-down conversion mode is chosen based on VIN to VOUT differential voltage and output load conditions. Two internal comparators are used to select the default step-down mode based on the VIN and VOUT voltage. A separate comparator (COMP2) is used to sense a droop on VOUT due to a heavy output load and force the charge pump to skip to a higher output current mode to
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maintain regulation. This will only occur as VIN/VOUT nears a 3-to-2 or 1-to-1 transition point. For example, under light load conditions, the LTC1503-X can operate in 2-to-1 mode when VIN equals 4.1V with greater than 90% efficiency. However, when the load is increased, the part can no longer supply enough output current in 2-to-1 mode to maintain regulation. This causes VOUT to droop below the regulation point until COMP2 trips and forces the part to skip from 2-to-1 mode to 3-to-2 mode. The COMP2 threshold is about 17mV (VOUT referred) below the main comparator regulation point. Hysteresis in COMP2 will force the part to transition in and out of mode skipping. This will result in a slight VOUT decrease of approximately 20mV under mode skipping conditions. Shutdown/Soft-Start Operation The SHDN/SS 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 0.35V (typ) will put the part into shutdown mode. Shutdown mode disables all control circuitry and forces the charge pump VOUT into a high impedance state. A 2A pullup current on the SHDN/SS 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 SHDN/SS pin should not be driven to a voltage higher than VIN. To implement soft-start, the SHDN/SS pin must be driven with an open-drain device and a capacitor must be connected from the SHDN/SS pin to GND. Once the opendrain device is turned off, a 2A pull-up current will begin charging the external SS capacitor and force the voltage on the pin to ramp towards VIN. As soon as the SHDN threshold is reached (0.35V typ), the internal reference voltage which controls the VOUT regulation point will follow the ramp voltage on the SHDN/SS pin (minus a 0.35V offset to account for the SHDN threshold) until the reference reaches its final band gap voltage. This occurs when the voltage on the SHDN/SS pin reaches
LTC1503-1.8/LTC1503-2
APPLICATIONS INFORMATION
approximately 1.9V. Since the ramp rate on the SHDN/SS pin controls the ramp rate on VOUT, the average inrush current can be controlled through selection of CSS and COUT. For example, a 10nF capacitor on SHDN/SS results in a 8ms ramp time from 0.35V to 1.9V on the pin. If COUT is 10F, the 8ms VREF ramp time results in an average COUT charge current of only 2.5mA. Capacitor Selection For best performance, it is recommended that low ESR capacitors be used for CIN and COUT to reduce noise and ripple. If the ESR of the output capacitor is too high (> 0.5), both efficiency and output load regulation may be degraded. The CIN and COUT capacitors should be either ceramic or tantalum and should be 10F or greater. If the input source impedance is very low (< 0.5), CIN may not be needed. Ceramic capacitors are recommended for the flying caps C1 and C2 with values of 0.47F to 2.2F. Smaller values may be used in low output current applications (e.g., IOUT < 10mA). For best performance choose the same capacitance value for both C1 and C2. Output Ripple Normal LTC1503-X operation produces voltage ripple on the VOUT pin. Output voltage ripple is required for the parts to regulate. Low frequency ripple exists due to the hysteresis in the sense comparator and propagation delays in the charge pump enable/disable circuits. High frequency ripple is also present mainly from the ESR (equivalent series resistance) in the output capacitor. Typical output ripple (VIN = 3.6V) under maximum load is 20mV peak-to-peak with a low ESR 10F output capacitor. The magnitude of ripple voltage depends on several factors. High input voltages increase the output ripple since more charge is delivered to COUT per charging cycle. Large output current load and/or a small output capacitor (< 10F) results in higher ripple due to higher output voltage dV/dt. High ESR capacitors (ESR > 0.5) on the output pin cause high frequency voltage spikes on VOUT with every clock cycle. There are several ways to reduce the output voltage ripple (see Figure 2). A larger COUT capacitor (22F or greater) will reduce both the low and high frequency ripple due to the lower COUT charging and discharging dV/dt and the lower ESR typically found with higher value (larger case size) capacitors. A low ESR ceramic output capacitor will minimize the high frequency ripple, but will not reduce the low frequency ripple unless a high capacitance value is chosen. A reasonable compromise is to use a 10F to 22F tantalum capacitor in parallel with a 1F to 3.3F ceramic capacitor on VOUT to reduce both the low and high frequency ripple. An RC filter may also be used to reduce high frequency voltage spikes.
LTC1503-X VOUT
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VOUT 10F TANTALUM 0.5 1F CERAMIC
LTC1503-X VOUT
+
10F TANTALUM
+
VOUT 10F TANTALUM
1503-2 F02
Figure 2. Output Ripple Reduction Techniques
Protection Features The LTC1503-X contains both thermal shutdown and short-circuit protection features. The charge pump will shut down when the junction temperature reaches approximately 150C and will resume operation once the junction temperature has dropped back to 125C. The part will limit output current to 20mA (typ) when a short-circuit condition (VOUT < 150mV) exists to prevent damage to the internal switches. The part can survive an indefinite short from VOUT to GND.
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LTC1503-1.8/LTC1503-2
TYPICAL APPLICATION
DC/DC Converter with Shutdown and Soft-Start
LTC1503-1.8 4 1-CELL Li-Ion OR 3-CELL NiMH 10F 1F 2 3 5 OFF ON 2N7002 VIN C1- C1+ VOUT C2- 1 8 1F 10F VOUT = 1.8V IOUT = 100mA
PACKAGE DESCRIPTION
MS8 Package 8-Lead Plastic MSOP
(LTC DWG # 05-08-1660)
0.118 0.004* (3.00 0.102) 8 76 5
0.192 0.004 (4.88 0.10)
1 0.040 0.006 (1.02 0.15) 0.007 (0.18) 0.021 0.006 (0.53 0.015) 0 - 6 TYP SEATING PLANE 0.012 (0.30) 0.0256 REF (0.65) TYP
* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
RELATED PARTS
PART NUMBER LTC1514/LTC1515/LTC1516 LTC1555/LTC1556 LTC1627 DESCRIPTION Micropower, Regulated 5V Charge Pump DC/DC Converters SIM Power Supply and Level Translator Monolithic Sync Step-Down Switching Regulator COMMENTS 2V to 10V Input Range; Up to 50mA Output Current: Short-Circuit and Overtemperature Protected Step-Up/Step-Down Charge Pump Generates 5V 2.65V to 8.5V Input Range; VOUT from 0.8V, IOUT to 500mA; Low Dropout Operation; 100% Duty Cycle
150312i LT/TP 0599 4K * PRINTED IN USA
8
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
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SHDN/SS GND
10nF
1503-2 TA03
Dimensions in inches (millimeters) unless otherwise noted. S8 Package 8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 - 0.197* (4.801 - 5.004) 8 7 6 5
0.118 0.004** (3.00 0.102)
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157** (3.810 - 3.988)
23
4 0.034 0.004 (0.86 0.102)
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254)
1 0.053 - 0.069 (1.346 - 1.752) 0- 8 TYP
2
3
4 0.004 - 0.010 (0.101 - 0.254)
0.006 0.004 (0.15 0.102)
MSOP (MS8) 1197
0.016 - 0.050 0.406 - 1.270
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) TYP
SO8 0996
*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
(c) LINEAR TECHNOLOGY CORPORATION 1999

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