Datasheet

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LT1936

1936fd

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

1.4A, 500kHz Step-Down

Switching Regulator

The LT

1936 is a current mode PWM step-down DC/DC

converter with an internal 1.9A power switch, packaged

in a tiny, thermally enhanced 8-lead MSOP. The wide in-

put range of 3.6V to 36V makes the LT1936 suitable for

regulating power from a wide variety of sources, including

automotive batteries, 24V industrial supplies and unregu-

lated wall adapters. Its high operating frequency allows the

use of small, low cost inductors and ceramic capacitors,

resulting in low, predictable output ripple.

Cycle-by-cycle current limit, frequency foldback and

thermal shutdown provide protection against shorted

outputs, and soft-start eliminates input current surge

during start-up. Transient response can be optimized by

using external compensation components, or board space

can be minimized by using internal compensation. The

low current (<2μA) shutdown mode enables easy power

management in battery-powered systems.

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Wide Input Range: 3.6V to 36V

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Short-Circuit Protected Over Full Input Range

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1.9A Guaranteed Minimum Switch Current

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5V at 1.4A from 10V to 36V Input

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3.3V at 1.4A from 7V to 36V Input

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5V at 1.2A from 6.3V to 36V Input

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3.3V at 1.2A from 4.5V to 36V Input

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Output Adjustable Down to 1.20V

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500kHz Fixed Frequency Operation

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Soft-Start

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Uses Small Ceramic Capacitors

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Internal or External Compensation

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Low Shutdown Current: <2μA

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Thermally Enhanced 8-Lead MSOP Package

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Automotive Battery Regulation

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Industrial Control Supplies

■

Unregulated Wall Adapters

3.3V Step-Down Converter Efﬁ ciency

4.5V TO 36V

ON OFF

0.22μF

10μH

17.4k

22μF

1936 TA01a

4.7μF

OUT

3.3V

1.2A

BOOST

GND

COMP FB

LT1936

SHDN SW

10k

LOAD CURRENT (A)

EFFICIENCY (%)

0.5 1

1936 TA01b

1.5

OUT

= 5V

= 12V

OUT

= 3.3V

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other

trademarks are the property of their respective owners.

Summary of content (20 pages)

PAGE 1
LT1936 1.4A, 500kHz Step-Down Switching Regulator FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION Wide Input Range: 3.6V to 36V Short-Circuit Protected Over Full Input Range 1.9A Guaranteed Minimum Switch Current 5V at 1.4A from 10V to 36V Input 3.3V at 1.4A from 7V to 36V Input 5V at 1.2A from 6.3V to 36V Input 3.3V at 1.2A from 4.5V to 36V Input Output Adjustable Down to 1.
PAGE 2
LT1936 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VIN Voltage ................................................. –0.4V to 36V BOOST Voltage .........................................................43V BOOST Above SW Voltage ........................................20V SHDN Voltage ............................................ –0.4V to 36V FB, VC, COMP Voltage .................................................6V Operating Temperature Range (Note 2) LT1936E ............................................
PAGE 3
LT1936 ELECTRICAL CHARACTERISTICS The ● denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 12V, VBOOST = 17V, unless otherwise noted. (Note 2) VC Clamp 1.8 V VC Switch Threshold 0.7 V Internal Compensation R 50 kΩ 150 pF Internal Compensation C VCOMP = 1V COMP Pin Leakage VCOMP = 1.8V, E and I Grades H Grade Switching Frequency VFB = 1.
PAGE 4
LT1936 TYPICAL PERFORMANCE CHARACTERISTICS Maximum Load Current 1.8 VOUT = 5V VOUT = 3.3V 1.6 LOAD CURRENT (A) LOAD CURRENT (A) 1.6 L = 15μH 1.4 L = 10μH 1.2 L = 10μH 1.4 L = 6.8μH 1.2 1.0 1.0 0 5 10 15 20 INPUT VOLTAGE (V) 25 0 30 5 10 15 20 INPUT VOLTAGE (V) 1936 G04 25 Feedback Voltage 200 100 0.5 1.0 SWITCH CURRENT (A) Switching Frequency UVLO (V) 3.4 3.2 1.190 3.0 –50 –25 1.185 25 50 75 100 125 150 TEMPERATURE (°C) 1.5 600 3.6 1.
PAGE 5
LT1936 TYPICAL PERFORMANCE CHARACTERISTICS Minimum Input Voltage 5.0 VOUT = 5V TA = 25°C L = 15μH 6.5 Switch Current Limit 3.0 VOUT = 3.3V TA = 25°C L = 10μH SWITCH CURRENT LIMIT (A) Minimum Input Voltage 7.0 INPUT VOLTAGE (V) 6.0 5.5 5.0 4.0 3.5 2.5 2.0 1.5 1.0 0.5 4.5 4.0 3.0 10 100 LOAD CURRENT (mA) 1000 1 10 100 LOAD CURRENT (mA) 1936 G13 VSW 10V/DIV VSW 10V/DIV IL 500mA/DIV IL 500mA/DIV VOUT 20mV/DIV VOUT 20mV/DIV 1μs/DIV 1936 G16 VIN = 12V VOUT = 3.
PAGE 6
LT1936 PIN FUNCTIONS BOOST (Pin 1): The BOOST pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. FB (Pin 6): The LT1936 regulates its feedback pin to 1.200V. Connect the feedback resistor divider tap to this pin. Set the output voltage according to VOUT = 1.200V (1 + R1/R2). A good value for R2 is 10k. VIN (Pin 2): The VIN pin supplies current to the LT1936’s internal regulator and to the internal power switch.
PAGE 7
LT1936 OPERATION (Refer to Block Diagram) The LT1936 is a constant frequency, current mode stepdown regulator. A 500kHz oscillator enables an RS ﬂip-ﬂop, turning on the internal 1.9A power switch Q1. An ampliﬁer and comparator monitor the current ﬂowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC. An error ampliﬁer measures the output voltage through an external resistor divider tied to the FB pin and servos the VC pin.
PAGE 8
LT1936 APPLICATIONS INFORMATION FB Resistor Network Inductor Selection and Maximum Output Current The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to: A good ﬁrst choice for the inductor value is V R1=R2 OUT – 1 1.200 R2 should be 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram.
PAGE 9
LT1936 APPLICATIONS INFORMATION inductance may result in discontinuous mode operation, which is okay but further reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Linear Technology Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. Choosing L greater than 1.6 (VOUT + VD) μH prevents subharmonic oscillations at all duty cycles.
PAGE 10
LT1936 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES COMMENTS Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, Tantalum EEF Series Murata (408) 749-9714 (404) 436-1300 AVX Taiyo Yuden (864) 963-6300 www.kemet.com Ceramic, Tantalum www.sanyovideo.com Ceramic, Polymer, Tantalum www.murata.com Ceramic www.avxcorp.com Ceramic, Tantalum www.taiyo-yuden.
PAGE 11
LT1936 APPLICATIONS INFORMATION complicated and the best values depend on the application and in particular the type of output capacitor. A practical approach is to start with one of the circuits in this data sheet that is similar to your application and tune the compensation network to optimize the performance. Stability should then be checked across all operating conditions, including load current, input voltage and temperature.
PAGE 12
LT1936 APPLICATIONS INFORMATION BOOST Pin Considerations Capacitor C3 and diode D2 are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22μF capacitor and fast switching diode (such as the 1N4148 or 1N914) will work well. Figure 3 shows two ways to arrange the boost circuit. The BOOST pin must be at least 2.3V above the SW pin for best efﬁciency. For outputs of 3V and above, the standard circuit (Figure 3a) is best. For outputs between 2.8V and 3V, use a 0.
PAGE 13
LT1936 APPLICATIONS INFORMATION Minimum Input Voltage VOUT = 5V Minimum Input Voltage VOUT = 3.3V 6.0 VOUT = 3.3V TA = 25°C L = 10μH 5.5 5.0 4.5 4.0 3.0 0 TO START 6 TO RUN 5 TO RUN 3.5 VOUT = 5V TA = 25°C L = 15μH 7 TO START INPUT VOLTAGE (V) INPUT VOLTAGE (V) 8 10 100 LOAD CURRENT (mA) 4 1000 1 10 100 LOAD CURRENT (mA) 1000 1936 F04b 1936 F04a Figure 4.
PAGE 14
LT1936 APPLICATIONS INFORMATION the SHDN pin, the SW pin current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, then parasitic diodes inside the LT1936 can pull large currents from the output through the SW pin and the VIN pin. Figure 6 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input.
PAGE 15
LT1936 APPLICATIONS INFORMATION Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT1936 circuits. However, these capacitors can cause problems if the LT1936 is plugged into a live supply (see Linear Technology Application Note 88 for a complete discussion).
PAGE 16
LT1936 APPLICATIONS INFORMATION One method of damping the tank circuit is to add another capacitor with a series resistor to the circuit. In Figure 8b an aluminum electrolytic capacitor has been added. This capacitor’s high equivalent series resistance damps the circuit and eliminates the voltage overshoot. The extra capacitor improves low frequency ripple ﬁltering and can slightly improve the efﬁciency of the circuit, though it is likely to be the largest component in the circuit.
PAGE 17
LT1936 TYPICAL APPLICATIONS 5V Step-Down Converter D2 VIN 6.3V TO 36V ON OFF C1 4.7μF VIN BOOST SHDN SW LT1936 COMP VC C3 0.22μF L1 15μH D1 R1 31.6k VOUT 5V 1.2A FB GND R2 10k C2 22μF 1936 TA04 1.8V Step-Down Converter Efﬁciency, 1.8V Output 90 D2 BOOST SHDN SW C1 4.7μF L1 4.7μH D1 LT1936 COMP C3 0.22μF R1 10k VOUT 1.8V 1.3A FB VC GND C2 47μF ×2 R2 20k 2.0 VOUT = 1.8V TA = 25°C VIN = 5V 80 1.5 VIN = 12V 70 1.0 60 0.
PAGE 18
LT1936 TYPICAL APPLICATIONS 2.5V Step-Down Converter D2 VIN 3.6V TO 36V ON OFF VIN BOOST SHDN SW C1 4.7μF LT1936 COMP C3 1μF L1 6.2μH D1 R1 11k VOUT 2.5V 1.2A TA > 0°C FB VC GND R2 10k C2 47μF D1: DFLS140L D2: MBRO540 L1: TOKO D63CB 1936 TA07a Efﬁciency, 2.5V Output 100 Minimum Input Voltage 5.5 VOUT = 2.5V TA = 25°C 5.0 INPUT VOLTAGE (V) 90 EFFICIENCY (%) VOUT = 2.5V VIN = 5V 80 VIN = 12V TO START TA = –45°C 4.5 TO START TA = 25°C 4.0 TO RUN TA = –45°C 70 3.
PAGE 19
LT1936 PACKAGE DESCRIPTION MS8E Package 8-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1662 Rev E) BOTTOM VIEW OF EXPOSED PAD OPTION 2.06 ± 0.102 (.081 ± .004) 1 0.889 ± 0.127 (.035 ± .005) 2.794 ± 0.102 (.110 ± .004) 0.29 REF 1.83 ± 0.102 (.072 ± .004) 0.05 REF 5.23 (.206) MIN 2.083 ± 0.102 3.20 – 3.45 (.082 ± .004) (.126 – .136) DETAIL “B” 8 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 0.65 (.0256) BSC 0.42 ± 0.038 (.0165 ± .
PAGE 20
LT1936 TYPICAL APPLICATION 2.5V Step-Down Converter Minimum Input Voltage 5.5 D2 ON OFF VIN BOOST SHDN SW C1 4.7μF C3 0.22μF D1 LT1936 COMP VC L1 8.2μH R1 11k FB GND VOUT = 2.5V 5.0 VOUT 2.5V 1.3A R2 10k C2 47μF INPUT VOLTAGE (V) VIN 3.6V TO 20V CONNECTING THE BOOST CIRCUIT TO THE INPUT LOWERS THE MINIMUM INPUT VOLTAGE TO RUN AND TO START TO LESS THAN 3.7V AT ALL LOADS 4.5 4.0 3.5 D1: DFLS140L D2: 1N4148 L1: TOKO D63CB 1936 TA09a 3.