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LT1534 Ultralow Noise Switching Regulator Controls EMI Design Note 178 Jeff Witt Today’s circuit designer is often challenged to assemble a high performance system by combining sensitive analog electronics with potentially noisy DC/ DC converters. Requirements for a small, efficient, cost effective solution are in conflict with acceptable noise performance—noisy switching regulators call for filtering, shielding and layout revisions that add bulk and expense. Most electromagnetic interference (EMI) problems associated with DC/DC converters are due to high speed switching of large currents and voltages. To maintain high efficiency, these switch transitions are designed to occur as quickly as possible. The result is input and output ripple that contains very high harmonics of the switching frequency. These fast edges also couple through stray magnetic and electric fields to nearby signal lines, making efforts to filter the supply lines ineffective. The LT®1534 ultralow noise switching regulator provides an effective and flexible solution to this problem. Using two external resistors, the user can program the slew rates of the current through the internal 2A power switch and the voltage on it. Noise performance can be evaluated and improved with the circuit operating in the final system. The system designer need sacrifice only as much efficiency as is necessary to meet the
VIN 3.3V
required noise performance. With the controlled slew rates, system performance is less sensitive to layout, and shielding requirements can be greatly reduced; expensive layout and mechanical revisions can be avoided. The LT1534’s internal oscillator can be programmed over a broad frequency range (20kHz to 250kHz) with good initial accuracy. It can also be synchronized to an external signal placing the switching frequency and its harmonics away from sensitive system frequencies. Low Noise Boost Regulator In Figure 1, the LT1534 boosts 3.3V to supply 650mA at 5V with its oscillator synchronized to an external 50kHz clock. The circuit relies on the low ESR of capacitor C2 to keep the output ripple low at the fundamental frequency; slew rate control reduces the high frequency ripple. Figure 2 shows waveforms of the circuit as it delivers 500mA. The top trace shows the voltage on the collector of the internal bipolar power switch (the COL pins), and the middle trace shows the switch current. The lowest trace is the output ripple. The slew rates are programmed to their fastest here, resulting in good efficiency (83%), but also generating excessive high 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. D1 1N5817
L1 50μH
+
C1 33μF 6.3V
14 1 4
50kHz
11 8 10 6.8k
COL COL
SYNC SHDN
PGND
LT1534
2 15
L2 28nH
FB
VC RT
CT
6
5
16.9k
GND RVSL 9 3.3nF
4k TO 25k
5V 650mA
7.50k
7
RCSL
13
1nF
C2 47μF 6.3V ×2
16
NFB
15nF 220pF
VIN
NC
+
10Ω
DN178 F01
2.49k
12 4k TO 25k
C1: MATSUSHITA ECGC0JB330 C2: MATSUSHITA ECGC0JB470 L1: COILTRONICS CTX50-4 L2: COILCRAFT B08T
Figure 1. The LT1534 Boosts 3.3V to 5V. The Resistors on the RVSL and RCSL Pins Program the Slew Rates of the Voltage On the Power Switch (COL Pins) and the Current Through It 4/98/178_conv
VCOL 5V/DIV
VCOL 5V/DIV
ICOL 1A/DIV
ICOL 1A/DIV
VOUT AC COUPLED 50mV/DIV
VOUT AC COUPLED 50mV/DIV
DN178 F02
5μs/DIV
5μs/DIV
DN178 F03
Figure 2. High Slew Rates (RCSL = RVSL = 4k) Result in Good Efficiency But Excess High Frequency Ripple
Figure 3. Low Slew Rates (RCSL = RVSL = 24k) Result in an Output Without Troublesome High Frequency Transients
frequency ripple. Figure 3 shows the same waveforms with the slew rates reduced. The large high frequency transients have been eliminated.
16-lead narrow plastic SO. The current mode architecture provides fast transient response and cycle-bycycle current limit. Undervoltage lockout and thermal shutdown provide further protection. The large input range (2.7V to 23V) and high switch voltage (25V), combined with a 12μA shutdown mode, result in a very flexible part suitable for battery-powered operation. The LT1534 can directly regulate either positive or negative output voltages.
Low Noise Bipolar Supply Many high performance analog systems require quiet bipolar supplies. This circuit (Figure 4) will generate ± 5V from a wide input range of 3V to 12V, with a total output power of 1.5W. By using a 1:1:1 transformer, the primary and secondary windings can be coupled using capacitors C2 and C3, allowing the LT1534 to control the switch transitions at the output rectifiers as well as at the switch collector. Secondary damping networks are not required.
The LT1533, closely related to the LT1534, provides two slew rate-controlled 1A power switches. Optimized for push-pull topologies, the LT1533 provides even greater opportunity for reducing DC/DC converter noise. For further applications, consult the LT1533 and LT1534 data sheets and Linear Technology’s Application Note 70.
Additional LT1534 Features The LT1534 is a complete, low noise switching regulator with an internal 2A power switch, packaged in a
C2 10μF 16V
+
C1 10μF 16V
1
14
8
10 6.8k
VIN SYNC
COL
SHDN
COL
CT
6
5
16.9k
15
PGND
9 1500pF
13 4k TO 25k
L2 28nH
16
7
RCSL
GND RVSL
VOUT1 5V
t6
4t 2
3 t 12
5t
9
+
C4 47μF 6.3V
2
FB
VC RT
0.01μF 220pF
LT1534
NFB
T1
+
4 11
1N5817
+
VIN 3V TO 12V
10Ω
C3 10μF 1nF 16V
t 11 1N5817
+
8 t 10 7.50k VOUT1 2.49k
7
12 4k TO 25k
C5 47μF 6.3V
DN178 F04
VOUT2 – 5V
TOTAL OUTPUT CURRENT IS 300mA C1, C2, C3: MATSUSHITA ECGCICB6R8 C4, C5: MATSUSHITA ECGC0JB470 L2: COILCRAFT B08T T1: COILTRONICS VP2-0216
Figure 4. A Low Noise, Wide Input Range ± 5V Supply Data Sheet Download
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