Datasheet

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LT1611

Inverting 1.4MHz Switching

Regulator in SOT-23

■

Very Low Noise: 1mV

P–P

Output Ripple

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–5V at 150mA from a 5V Input

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Better Regulation Than a Charge Pump

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Effective Output Impedance: 0.14Ω

■

Uses Tiny Capacitors and Inductors

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Internally Compensated

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Fixed Frequency 1.4MHz Operation

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Low Shutdown Current: <1µA

■

Low V

CESAT

Switch: 300mV at 300mA

■

Tiny 5-Lead SOT-23 Package

The LT

1611 is the industry’s first inverting 5-lead SOT-23

current mode DC/DC converter. Intended for use in small,

low power applications, it operates from an input voltage

as low as 1.1V and switches at 1.4MHz, allowing the use

of tiny, low cost capacitors and inductors 2mm or less in

height. Its small size and high switching frequency enable

the complete DC/DC converter function to consume less

than 0.25 square inches of PC board area. Capable of

generating –5V at 150mA from a 5V supply or –5V at

100mA from a 3V supply, the LT1611 replaces nonregulated

“charge pump” solutions in many applications.

The LT1611 operates in a dual inductor inverting topology

which filters the input side as well as the output side of the

DC/DC converter. Fixed frequency switching ensures a

clean output free from low frequency noise typically present

with charge pump solutions. No load quiescent current of

the LT1611 is 3mA, while in shutdown quiescent current

drops to 0.5µA. The 36V switch allows V

to V

OUT

differential of up to 33V.

The LT1611 is available in the 5-lead SOT-23 package.

■

MR Head Bias

■

Digital Camera CCD Bias

■

LCD Bias

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GaAs FET Bias

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Positive-to-Negative Conversion

OUT

–5V

150mA

1611 TA01

L1A

22µH

L1B

22µH

GND

LT1611

C1: AVX TAJB226M010

C2: TAIYO YUDEN LMK212BJ105MG

C3: TAIYO YUDEN JMK325BJ226MM (1210 SIZE)

D1: MBR0520

L1: SUMIDA CLS62-220 OR 2× MURATA LQH3C220 (UNCOUPLED)

22µF

1µF

10k

29.4k

1200pF

NFB

SHDN

Figure 1. 5V to –5V, 150mA Low Noise Inverting DC/DC Converter

, LTC and LT are registered trademarks of Linear Technology Corporation.

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

OUT

20mV/DIV

AC COUPLED

LOAD CURRENT

150mA

50mA

100µs/DIV

1611 F10

Transient Response

Summary of content (12 pages)

PAGE 1
LT1611 Inverting 1.4MHz Switching Regulator in SOT-23 U DESCRIPTIO FEATURES The LT®1611 is the industry’s first inverting 5-lead SOT-23 current mode DC/DC converter. Intended for use in small, low power applications, it operates from an input voltage as low as 1.1V and switches at 1.4MHz, allowing the use of tiny, low cost capacitors and inductors 2mm or less in height. Its small size and high switching frequency enable the complete DC/DC converter function to consume less than 0.
PAGE 2
LT1611 W U PACKAGE/ORDER INFORMATION U W W W (Note 1) VIN Voltage .............................................................. 10V SW Voltage ................................................– 0.4V to 36V NFB Voltage ............................................................. – 3V Current into NFB Pin ............................................. ±1mA SHDN Voltage .......................................................... 10V Maximum Junction Temperature ..........................
PAGE 3
LT1611 U W TYPICAL PERFOR A CE CHARACTERISTICS Efficiency, VOUT = – 5V 80 –1.245 6 –1.240 5 NFB PIN BIAS CURRENT (µA) 85 VIN = 5V 75 –1.235 70 VIN = 3V VNFB (V) EFFICIENCY (%) NFB Pin Bias Current vs Temperature VNFB vs Temperature 65 –1.230 –1.225 60 –1.220 55 –1.215 50 0 25 50 75 100 LOAD CURRENT (mA) 125 150 –1.
PAGE 4
LT1611 U U U PIN FUNCTIONS SW (Pin 1): Switch Pin. Minimize trace area at this pin to keep EMI down. GND (Pin 2): Ground. Tie directly to local ground plane. NFB (Pin 3): Negative Feedback Pin. Minimize trace area. Reference voltage is –1.23V. Connect resistive divider tap here. The suggested value for R2 is 10k. Set R1 and R2 according to: VOUT − 1.23 R1 = 1.23  + 4.5 • 10− 6  R2  SHDN (Pin 4): Shutdown Pin. Tie to 1V or more to enable device. Ground to shut the device down.
PAGE 5
LT1611 U OPERATIO boost converter, generating a negative output voltage, which is directly regulated. The circuit schematic is detailed in Figure 3. Only one inductor is required, and the two diodes can be in a single SOT-23 package. Output noise is the same as in a boost converter, because current is delivered to the output only during the time when the LT1611’s internal switch is off. When Q1 turns off during the second phase of switching, the SW node voltage abruptly increases to (VIN + |VOUT|).
PAGE 6
LT1611 U OPERATIO –(VIN + VOUT) VCESAT L1 SW C2 L2 SWX VIN –VOUT D1 Q1 + C1 C3 RLOAD + 1611 F05 Figure 5. Switch-On Phase of Inverting Converter. L1 and L2 Current Have Positive dI/dt VIN + VOUT+ VD L1 SW VD C2 L2 SWX VIN –VOUT Q1 + D1 C1 C3 RLOAD + 1611 F06 Figure 6. Switch-Off Phase of Inverting Converter. L1 and L2 Current Have Negative dI/dt Transient Response The inverting architecture of the LT1611 can generate a very low ripple output voltage.
PAGE 7
LT1611 U OPERATIO C2 1µF L1A 22µH VIN 5V L1B 22µH D1 VIN –VOUT SW SHDN + C1 R1 LT1611 CPL NFB C3 RL2 50Ω + GND RL1 100Ω R2 10k C1: AVX TAJB226M010 C2: TAIYO YUDEN LMK212BJ105MG C3: SEE TEXT D1: MBR0520 L1A, L1B: SUMIDA CLS62-220 1611 F07 Figure 7. Switching RL2 Provides 50mA to 150mA Load Step for LT1611 5V to – 5V Converter Figure 8 shows the output voltage with a 50mA to 150mA load step, using an AVX TAJ “B” case 22µF tantalum capacitor at the output.
PAGE 8
LT1611 U OPERATIO In Figure 11 (also shown on the first page), output capacitor C3 is replaced by a ceramic unit. These large value ceramic capacitors have ESR of about 2mΩ and result in very low output ripple. At the 20mV/division scale, output voltage ripple cannot be seen. Figure 12 pictures the output and switch nodes at 200ns per division. The output voltage ripple is approximately 1mVP–P. Again, good layout is mandatory to achieve this level of performance.
PAGE 9
LT1611 U OPERATIO Start-Up/Soft-Start measured at VIN, is limited to a peak value of 450mA as the time required to reach final value increases to 700µs. In Figure 16, CSS is increased to 0.1µF, resulting in a lower peak input current of 240mA with a VOUT ramp time of 2.1ms. CSS can be increased further for an even slower ramp, if desired. Diode D2 serves to quickly discharge CSS when VSS is driven low to shut down the device.
PAGE 10
LT1611 U OPERATIO COMPONENT SELECTION Output Current The LT1611 will deliver 150mA at – 5V from a 5V ±10% input supply. If a higher voltage supply is available, more output current can be obtained. Figure 17’s schematic shows how to get more current. Although the LT1611’s maximum voltage allowed at VIN is 10V, the SW pin can handle higher voltage (up to 36V). In Figure 17, the VIN pin of the LT1611 is driven from a 5V supply, while input inductor L1A is driven from a separate 12V supply.
PAGE 11
LT1611 U OPERATIO Capacitors ceramic can be used with little trade-off in circuit performance. Some capacitor types appropriate for use with the LT1611 are listed in Table 2. As described previously, ceramic capacitors can be used with the LT1611 provided loop stability is considered. For lower cost applications, small tantalum units can be used. A value of 22µF is acceptable, although larger capacitance values can be used. ESR is the most important parameter in selecting an output capacitor.
PAGE 12
LT1611 U TYPICAL APPLICATIO S 4-Cell to –10V Inverting Converter C2 1µF L1A 15µH 4-Cell to –10V Inverting Converter Efficiency 85 L1B 15µH VIN 80 D1 SHUTDOWN VIN SW LT1611 SHDN VOUT –10V/60mA 68.1k NFB GND EFFICIENCY (%) C1 22µF + + VIN = 6.5V 75 10k C3 6.8µF VIN = 5V VIN = 3.