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Micropower 600kHz Step-Up DC/DC Converter Delivers 5V at 1A from a Li-Ion Cell – Design Note 179 Steve Pietkiewicz performs this function with a minimum of external components. The circuit is detailed in Figure 1. Many designs use a large aluminum electrolytic capacitor (1000μF to 3300μF) at the DC/DC converter output to sustain the output voltage during the transmit time slice, since the amplifier can require more than 1A. The output capacitor, along with the LT1308 compensation network, serves to smooth out the input current demanded from the Li-Ion cell. Efficiency, which reaches 90%, is shown in Figure 2. Transient response of a 0A to 1A load step with typical GSM profiling (1:8 duty cycle, 577μs pulse duration) is 95 V IN = 3.6V
90 EFFICIENCY (%)
Linear Technology introduces a new micropower DC/ DC converter designed to provide high output power from a single cell or higher input voltage. The LT®1308 features an onboard switch capable of handling 2A with a voltage drop of 300mV and operates from an input voltage as low as 1V. The LT1308 features Burst Mode® operation at light load; efficiency is 75% or better for load currents of 1mA. The device switches at 600kHz; this high frequency keeps associated power components small and flat; additionally, troublesome interference problems in the sensitive 455kHz IF band are avoided. The LT1308 is intended for generating power on the order of 2W to 5W. This is sufficient for RF power amplifiers in GSM terminals or for digitalcamera power supplies. The LT1308 is available in the 8-lead SO package. Single Li-Ion Cell to 5V/1A DC/DC Converter for GSM GSM terminals have emerged as a worldwide standard. A common requirement for these products is an efficient, compact, step-up converter that develops 5V from a single Li-Ion cell to power the RF amplifier. The LT1308
V IN = 4.2V
85 80 V IN = 3V 75 70 65
L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
1
10 100 LOAD CURRENT (mA)
1000 DN179 F02
Figure 2. Efficiency of Figure 1’s Circuit Reaches 90% 3V TO 4.2V
SHDN
SW
LBI Li-Ion CELL
+
R1 301k
LT1308 C1 100μF
L1 4.7μH
VIN
LBO
D1 5V 1A
FB
VC RC 47k CC 22nF
GND R2 100k
+
C2 100μF
+
DN179 F01
C1,C2: AVX TPSD107M010R0100 D1: INTERNATIONAL RECTIFIER 10BQ015 L1: COILTRONICS CTX5-1 COILCRAFT DO3316-472
Figure 1. Single Li-Ion Cell to 5V/1A DC/DC Converter 4/98/179_conv
C3 2200μF
depicted in Figure 3. Voltage droop (top trace) is 200mV. Inductor current (bottom trace) increases to 1.7A peak; the input capacitor supplies some of this current, with the remainder drawn from the Li-Ion cell.
extension of the SEPIC (single ended primary inductance converter) topology. The addition of capacitor C6 clamps the SW pin, eliminating a snubber network. Both the 3.3V and 5V outputs are fed back to the LT1308 FB pin, a technique known as split feedback. This compromise results in better overall line and load regulation. The 5V output has more influence than the 3.3V output, as can be seen from the relative values of R2 and R3. Transformer T1 is available from Coiltronics, Inc. (561-241-7876). Efficiency vs input voltage for several load currents on both 3.3V and 5V outputs is pictured in Figure 5. The CCD bias voltages are loaded with 10mA in all cases.
VOUT 200mV/DIV DC OFFSET
INDUCTOR CURRENT 1A/DIV
DN179 F03
1ms/DIV
90
Figure 3. Transient Response of DC/DC Converter: VIN = 3V, 0A to 1A Load Step
85 80 EFFICIENCY (%)
2-Cell Digital-Camera Supply Produces 3.3V, 5V, 18V and –10V
Power supplies for digital cameras must be small and efficient while generating several voltages. The DSP and logic need 3.3V, the ADC and LCD display need 5V and biasing for the CCD element requires 18V and –10V. The power supplies must also be free of low frequency noise, so that post filtering can be done easily. The obvious approach, to use a separate DC/DC converter IC for each output voltage, is not cost effective. A single LT1308, along with an inexpensive transformer, generates 3.3V/200mA, 5V/200mA, 18V/10mA and –10V/10mA from a pair of AA or AAA cells. Figure 4 shows the circuit. A coupled-flyback scheme is used, actually an
VIN 1.6V TO 6V
8
L1A N=1 10μH 1
+
2
SW
C8 1nF
R4 47k
LT1308
65
150mA LOADS
55 50
1
1.5
2
2.5 3 3.5 4 INPUT VOLTAGE (V)
5
Figure 5. Efficiency vs Input Voltage for 100mA, 150mA and 200mA Loads on 3.3V and 5V Outputs
L1C 3 N = 0.3 t
L1B N = 0.7
D1
D2
4
5V 200mA
FB
R1 100k
R2 2.01M
C2 100μF
+
+
3.3V 200mA
D3 7
C3 100μF
t
L1D N = 3.5
CCD BIAS 18V 10mA
+
6 6
C1, C2, C3 = AVX TPSD107M010R0100 C4, C5 = AVX TAJC106M025 C6 = CERAMIC
4.5
DN179 F05
GND
C7 22nF
200mA LOADS
60
3
t
R3 340k
SHDN VC
70
C6 10μF
t
VIN C1 100μF
100mA LOADS
75
D1, D2 = IR10BQ015 D3, D4 = BAT-85 L1 = COILTRONICS CTX02-13973 DN179 F04
t
+ L1E N=2
5 D4
C4 10μF
C5 10μF CCD BIAS –10V 10mA
Figure 4. This Digital-Camera Power Supply Delivers 5V/200mA, 3.3V/200mA, 18V/10mA and –10V/10mA from Two AA Cells Data Sheet Download
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Linear Technology Corporation
For applications help, call (408) 432-1900 dn179f_conv LT/TP 0498 370K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900
●
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