High Efficiency Linear and Switching Solutions for Splitting a Digital Supply – Design Note 172 Dave Dwelley and Gary Maulding It can be inconvenient to generate a split supply in a typical digital system. The classic solution is to use a pair of resistors between 5V and GND to create a 2.5V “ground” for analog circuitry (Figure 1). Unfortunately, the resultant “ground” has a painfully high impedance and the resistors draw a large amount of supply current. The output can be buffered with an op amp to lower the impedance, but a specialized op amp is required to handle any significant bypass capacitance at the output. This Design Note presents two alternate methods of creating a split supply that can provide good transient response while conserving supply current. 10.4mA 0.1μF

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100

– 240Ω

2.5V SPLIT SUPPLY ZOUT = 120Ω

240Ω

1μF

D

90 80

+ OPTIONAL OP AMP BUFFER

COUT LIMITED BY OP AMP STABILITY DN172 F01

Figure 1. Resistor Divider Supply Splitter

EFFICIENCY (%)

VCC 5V

loading conditions, far better than any practical resistor divider solution. The LT1118 draws only enough supply current to meet the demands of the load at the split supply, providing nearly 50% power efficiency over a wide range of load currents (Figure 3). Load transient response is excellent, with less than 5μs recovery time from a ± 400mA current load step (Figure 4). At low current levels, the LT1118 is the optimum solution for splitting a digital supply.

F

VCC 5V

0.6mA IN 0.1μF

OUT LT1118-2.5

1μF

2.5V VIRTUAL GROUND ZOUT < 0.1Ω

GND DN172 F02

60 50 A

40 30

B

10

A: LT1118 SOURCE B: LT1118 SINK C: LTC1504/47μH SOURCE D: LTC1504/47μH SINK E: LTC1504/22μH SOURCE F: LTC1504/22μH SINK

0 1

10 100 1000 LOAD CURRENT (SINK/SOURCE)(mA) DN172 F03

Figure 3. Efficiency vs Load Current for Linear and Switching Circuits

VOUT 500mV/DIV AC COUPLED

500mA ILOAD 0 500mA/DIV –500mA 0.1ms/DIV

Figure 2. LT1118-2.5 Supply Splitter 1/98/172_conv

E

70

20

The LT®1118 is a specialized linear regulator designed to source or sink current as necessary to keep its output in regulation. It can handle output capacitors of arbitrarily large size, improving output transient response. Available with a fixed 2.5V output (ideal for splitting 5V supplies), it draws only 600μA quiescent current typically and can source 800mA or sink 400mA, enough to satisfy most analog subsystems. The LT1118 requires only two external components (Figure 2) and features a DC output impedance below 0.1Ω under all

C

DN172 F04

Figure 4. LT1118 Transient Response

At higher power levels, the 50% efficiency of the LT1118 can become a liability in power-sensitive or battery-powered systems since half of all the power drawn from the split supply is wasted heating up the LT1118. The LTC ®1504 addresses this situation by providing as much as 90% efficiency while sourcing or sinking up to 500mA. The LTC1504 is a synchronous switching regulator with on-board power switches. The continuous conduction, synchronous buck architecture inherently sinks current as well as sourcing it, making the circuit an effective supply splitter. Quiescent current is 3mA with typical components. This penalizes efficiency at low current levels when compared to the LT1118, but the intrinsic power conversion abilities of the inductor-based switching architecture allow power efficiencies approaching 90% above 100mA (Figure 3 again). A typical LTC1504 circuit will draw only 56mA from the 5V supply while sourcing 100mA from the 2.5V output—magic! The switching architecture of the LTC1504 requires a few more external components than the LT1118 (Figure 5), and generates a small amount of output noise at the 200kHz switching frequency. Transient recovery is controlled primarily by the value of the external inductor. With a 47μH inductor, switching noise is minimal NC

LEXT

SS NC

500mA ILOAD 0 500mA/DIV –500mA

SPLIT SUPPLY 2.5V ±500mA

DN172 F06

Figure 6. LTC1504 Transient Response with 47μH Inductor

11.8k

LTC1504 GND

VOUT 500mV/DIV AC COUPLED

0.1ms/DIV

SW

VCC 4.7μF CERAMIC

Both the LT1118 and the LTC1504 provide superior supply splitting when compared to simple resistor- or regulator-based circuits. The LT1118 fits best where impedance requirements are critical at low current levels, or where low output noise is paramount. The LTC1504 is the best solution where efficiency, especially at high current levels, is the overriding concern. Both devices can also be used in similar applications where source/sink capability is important, such as SCSI or positive ECL supplies.

SHDN

IMAX SHDN 5V

and the circuit recovers from a ±400mA output load step in 30μs (Figure 6). Switching to a 22μH inductor brings transient recovery time down to 15μs (Figure 7), but output ripple and quiescent current increase. The LTC1504 features a shutdown pin that drops quiescent current below 10μA when the split supply is not required.

+

FB COMP

COUT 12.1k

7.5k

220pF

VOUT 500mV/DIV AC COUPLED

0.01μF COUT: AVX TAJC476M016R LEXT: SUMIDA CDRH73-470 (47μH: LOW RIPPLE, HIGH EFFICIENCY) SUMIDA CDRH73-220 (22μH: FAST TRANSIENT RESPONSE) DN172 F05

500mA ILOAD 0 500mA/DIV –500mA

Figure 5. LTC1504 Supply Splitter 0.1ms/DIV

DN172 F07

Figure 7. LTC1504 Transient Response with 22μH Inductor

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