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LT3500

3500fc

FEATURES

APPLICATIONS

DESCRIPTION

Monolithic 2A Step-Down

Regulator Plus Linear

Regulator/Controller

The LT

3500 is a current mode PWM step-down DC/DC

converter with an internal 2.3A switch. The wide input range

of 3V to 40V makes the LT3500 suitable for regulating

power from a wide variety of sources, including automo-

tive batteries, 24V industrial supplies and unregulated

wall adapters.

Resistor-programmable 250kHz to 2.2MHz frequency

range and synchronization capability enable optimization

between efﬁ ciency and external component size. Cycle-

by-cycle current limit, frequency foldback and thermal

shutdown provide protection against a shorted output.

The soft-start feature controls the ramp rate of the output

voltage, eliminating input current surge during start-up,

and also provides output tracking.

The LT3500 contains an internal NPN transistor with feed-

back control which can be conﬁ gured as a linear regulator

or as a linear regulator controller.

The LT3500’s low current shutdown mode (<12µA) enables

easy power management in battery-powered systems.

Wide Input Range: 3V to 36V Operating,

40V Maximum

Short-Circuit Protected Over Full Input Range

2A Output Current Capability

Adjustable/Synchronizable Fixed Frequency

Operation from 250kHz to 2.2MHz

Soft-Start/Tracking Capability

Output Adjustable Down to 0.8V

Adjustable Linear Regulator/Driver with 13mA

Output Capability

Power Good Comparator with Complimentary

Outputs

Low Shutdown Current: 12µA

Thermally Enhanced 3mm × 3mm DFN or 16-Pin

MSOP Package

Automotive Battery Regulation

Industrial Control

Wall Transformer Regulation

Distributed Power Regulation

Dual Step-Down Converters

Output Voltage RippleSwitching Converter Efﬁ ciency

6.8µH

0.47µF

22µF

42.2k

8.06k

2.2µF

0.47µF

6V TO 36V

40.2k

53.6k

330pF

3500 TA01a

OUT1

OUT2

3.3V

B240A

BSTV

SHDN

LT3500

SYNC

LDRV

LFB

8.06k

24.9k

LOAD CURRENT (A)

EFFICIENCY (%)

1.6

3500 TA01b

0.4 0.8 1.2 2.01.40.2 0.6 1.0 1.8

= 12V

OUT2

= 0A

FREQUENCY = 800kHz

500ns/DIV

3500 TA01c

OUT1

= 5V AT 1A

AC COUPLED

2mV/DIV

OUT2

= 3.3V AT 1A

AC COUPLED

2mV/DIV

TYPICAL APPLICATION

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.

Summary of content (28 pages)

PAGE 1
LT3500 Monolithic 2A Step-Down Regulator Plus Linear Regulator/Controller DESCRIPTION FEATURES n n n n n n n n n n Wide Input Range: 3V to 36V Operating, 40V Maximum Short-Circuit Protected Over Full Input Range 2A Output Current Capability Adjustable/Synchronizable Fixed Frequency Operation from 250kHz to 2.2MHz Soft-Start/Tracking Capability Output Adjustable Down to 0.
PAGE 2
LT3500 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, PG, PG Operating .................................... 40V/–0.3V SW .............................................................................VIN BST ................................................................ 55V/–0.3V BST Pin Above SW....................................................25V LDRV, SHDN ..............................................................15V FB, LFB, RT/SYNC .......................................................5V SS, VC ......
PAGE 3
LT3500 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TJ = 25°C. VVIN = 15V, VRT/SYNC = 2V, unless otherwise speciﬁed. PARAMETER CONDITIONS l SHDN Threshold SHDN Source Current VSHDN = 0.62V SHDN Current Hysterisis MIN TYP MAX UNITS 710 760 780 mV 1.5 2.5 3.5 μA 1.25 2 3.25 μA 2.4 2.
PAGE 4
LT3500 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TJ = 25°C. VVIN = 15V, VRT/SYNC = 2V, unless otherwise speciﬁed. PARAMETER CONDITIONS LFB Voltage VLDRV = VLFB LFB Line/Load Regulation VVIN = 3V to 40V, VLDRV = VLFB SS to LFB Offset (VSS – VLFB) VSS = 0.8V, VLDRV = VLFB MIN TYP MAX UNITS l 0.784 0.8 0.816 V l 0.776 0.8 0.824 V 90 115 140 mV LFB Bias Current VLFB = 0.
PAGE 5
LT3500 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Quiescent Current vs Temperature Error Ampliﬁer gm vs Temperature 15.0 400 5 12.5 350 4 VSHDN = 0.9V 3 VSHDN = 0.7V 2 TRANSCONDUCTANCE (μmhos) 6 CURRENT (μA) CURRENT (μA) Shutdown Input Currents vs Temperature 10.0 7.5 5.0 2.
PAGE 6
LT3500 TYPICAL PERFORMANCE CHARACTERISTICS Peak Switch Current vs Temperature LDRV Short-Circuit Current vs Temperature 20 100 3.4 19 90 18 80 17 70 CURRENT (A) 3.2 CURRENT (mA) MSE PACKAGE 3.1 3.0 DD PACKAGE 2.9 DUTY CYCLE (%) 3.5 3.3 16 15 14 12 20 2.
PAGE 7
LT3500 TYPICAL PERFORMANCE CHARACTERISTICS LDRV Dropout Voltage vs Temperature 1.45 Switcher Dropout Operation 6 ILDRV = 5mA 5 OUTPUT VOLTAGE (V) 1.40 VOLTAGE (V) 1.35 1.30 1.25 1.20 1.15 1.10 2500 IVOUT1 = 1A TA = 25°C L = 1μH 2000 VVOUT1 = 5V 4 VVOUT1 = 3.3V 3 2 L = 1.5μH 1500 L = 3.3μH 1250 L = 4.7μH 1000 L = 6.8μH 500 0 0 25 50 75 100 125 150 TEMPERATURE (°C) 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) 3500 G22 5.0 5.5 3500 G23 L = 2.2μH 1750 750 1 1.05 1.
PAGE 8
LT3500 PIN FUNCTIONS Driving the RT/SYNC pin with an external clock signal will synchronize the switch to the applied frequency. Synchronization occurs on the rising edge of the clock signal after the clock signal is detected. Each rising clock edge initiates an oscillator ramp reset. A gain control loop servos the oscillator charging current to maintain a constant oscillator amplitude. Hence, the slope compensation remains unchanged.
PAGE 9
R6 C3 C2 C4 R5 100mV SS VC – + 2.75μA 0.76V RT/SYNC SHDN GND VIN – + 2μA 2.5μA R S PRE Q OSCILLATOR AND AGC R S Q + + – LFB 0.7V + – – 0.8V 100mV DRIVER CIRCUITRY – + + SS 115mV + 0.8V Figure 1.
PAGE 10
LT3500 OPERATION The LT3500 is a constant frequency, current mode buck converter with an internal 2.3A switch plus a linear regulator with 13mA output capability. Control of both outputs is achieved with a common SHDN pin, internal regulator, oscillator, undervoltage detect, soft-start, thermal shutdown and power-on reset. During power up, the POR signal sets the soft-start latch, which discharges the SS pin to ensure proper start-up operation.
PAGE 11
LT3500 OPERATION VIN undervoltage detection or thermal shutdown will set the soft-start latch, resulting in a complete soft-start sequence. in the same manner as the switching regulator. The linear output can also be conﬁgured to drive an external NPN to provide a linear regulator with higher current capability. The switch driver operates from either the VIN or BST voltage.
PAGE 12
LT3500 APPLICATIONS INFORMATION where VSW is the voltage drop of the internal switch, and The following example along with the data in Table 1 illustrates the tradeoffs of switch frequency selection. DCMAX = 1 – tOFF(MIN) • Frequency. Example. Figure 3 shows a typical graph of minimum input voltage vs load current for 3.3V and 5V applications. VIN = 25V, VOUT1 = 3.3V, IOUT1 = 2.
PAGE 13
LT3500 APPLICATIONS INFORMATION Note that the LT3500 will regulate if the input voltage is taken above the calculated maximum voltage as long as maximum ratings of the VIN and BST pins are not violated. However operation in this region of input voltage will exhibit pulse skipping behavior. Example: VOUT1 = 3.3V, IOUT1 = 1A, Frequency = 1MHz, Temperature = 25°C, VSW = 0.3V, VD = 0.4V, tON(MIN) = 150ns, tOFF(MIN) = 110ns DCMAX = 1− (110ns)1MHz = 89% VIN(MIN) = 3.3 + 0.4 − 0.4 + 0.3 = 4.06V 0.
PAGE 14
LT3500 APPLICATIONS INFORMATION IOUT1(MAX) =ILIM − ΔIL ΔI =2.3 – L 2 2 If the inductor value is chosen so that the ripple current is small, then the available output current will be near the switch current limit. One approach to choosing the inductor is to start with the simple rule given above, look at the available inductors and choose one to meet cost or space goals. Then use these equations to check that the LT3500 will be able to deliver the required output current.
PAGE 15
LT3500 APPLICATIONS INFORMATION The frequency, VIN to VOUT1 ratio, and maximum load current requirement of the LT3500 along with the input supply source impedance, determine the energy storage requirements of the input capacitor. Determine the worstcase condition for input ripple current and then size the input capacitor such that it reduces input voltage ripple to an acceptable level. Typical values for input capacitors run from 10μF at low frequencies to 2.2μF at higher frequencies.
PAGE 16
LT3500 APPLICATIONS INFORMATION the value of output capacitance required. Also, the current mode control loop doesn’t require the presence of output capacitor series resistance (ESR). For these reasons, you are free to use ceramic capacitors to achieve very low output ripple and small circuit size.
PAGE 17
LT3500 APPLICATIONS INFORMATION The only reason to consider a larger diode is the worstcase condition of a high input voltage and shorted output. With a shorted condition, diode current will increase to a typical value of 3A, determined by the peak switch current limit of the LT3500. This is safe for short periods of time, but it would be prudent to check with the diode manufacturer if continuous operation under these conditions can be tolerated.
PAGE 18
LT3500 APPLICATIONS INFORMATION The boost circuit can also run directly from a DC voltage that is higher than the input voltage by more than 2.5V, as in Figure 5d. The diode is used to prevent damage to the LT3500 in case VX is held low while VIN is present. The circuit eliminates a capacitor, but efﬁciency may be lower and dissipation in the LT3500 may be higher.
PAGE 19
LT3500 APPLICATIONS INFORMATION current proportional to the voltage at the VC pin. Note that the output capacitor integrates this current, and that the capacitor on the VC pin (CC) integrates the error ampliﬁer output current, resulting in two poles in the loop. In most cases a zero is required and comes from either the output capacitor ESR or from a resistor in series with CC.
PAGE 20
LT3500 APPLICATIONS INFORMATION If the synchronization signal changes between high and low impedance states during power up (VOL, Hi-Z), connect the synchronization circuitry to the LT3500 as shown in the Typical Applications section. This will allow the LT3500 to start up with a switching frequency determined by the equivalent resistance from the RT/SYNC pin to ground. Shutdown and Undervoltage Lockout Figure 8 shows how to add an undervoltage lockout (UVLO) to the LT3500.
PAGE 21
LT3500 APPLICATIONS INFORMATION At power-up, a reset signal sets the soft-start latch and discharges the SS pin to approximately 0V to ensure proper start-up. When the SS pin is fully discharged the latch is reset and the internal 2.75μA current source starts to charge the SS pin. When the SS pin voltage is below 100mV, the VC pin is pulled low which disables switching. As the SS pin voltage rises above 100mV, the VC pin is released and the outputs are regulated to the SS voltage.
PAGE 22
LT3500 APPLICATIONS INFORMATION NMOS times the output current of the regulator. This also increases the overall efﬁciency of the system. However, the minimum VIN increases to 2V plus the VGS at full load of the transistor. Additionally, due to a lack of beta current limiting, a shorted output can cause the switcher output of the LT3500 to collapse. VOUT2 AC COUPLED 20mV/DIV LOAD STEP 2.5mA TO 7.5mA 5mA/DIV To compensate the linear regulator, simply add a ceramic capacitor from the LDRV pin to ground.
PAGE 23
LT3500 APPLICATIONS INFORMATION VIN LT3500 SW VIN LT3500 SW VIN LT3500 SW GND GND GND 3500 F11 (11a) (11b) (11c) Figure 11. Subtracting the Current when the Switch is On (11a) from the Current when the Switch is Off (11b) Reveals the Path of the High Frequency Switching Current (11c). Keep this Loop Small. The Voltage on the SW and BST Traces will Also be Switched; Keep These Traces as Short as Possible.
PAGE 24
LT3500 TYPICAL APPLICATIONS High Efﬁciency Linear Regulator Efﬁciency vs Load Current 90 D2 BAT54 VIN C1 2.2μF BST L1 C5 0.47μF 3.3μH LT3500 80 EFFICIENCY (%) 4.5V TO 36V SW D1 B240A SHDN SS C2 0.47μF R1 25.5k FB RT/SYNC LDRV C3 220pF R6 40.2k C7 22μF R2 8.06k VC R5 49.9k GND PG PG LFB R3 24.9k M1 ZXMN2A03E6 R7 10k 70 60 VOUT2 3.3V 50 C8 22μF R4 8.06k 0 0.4 0.6 0.8 1.0 1.2 LOAD CURRENT (A) 0.2 1.4 3500 TA02a 3500 TA02b 5V/1.5A, 3.3V/0.
PAGE 25
LT3500 PACKAGE DESCRIPTION DD Package 12-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1725 Rev A) 0.70 ±0.05 3.50 ±0.05 2.10 ±0.05 2.38 ±0.05 1.65 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.45 BSC 2.25 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 3.00 ±0.10 (4 SIDES) R = 0.115 TYP 7 0.40 ± 0.10 12 2.38 ±0.10 1.65 ± 0.10 PIN 1 NOTCH R = 0.20 OR 0.25 × 45° CHAMFER PIN 1 TOP MARK (SEE NOTE 6) 6 0.200 REF 1 0.23 ± 0.05 0.45 BSC 0.75 ±0.05 2.
PAGE 26
LT3500 PACKAGE DESCRIPTION MSE Package 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev A) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 p 0.102 (.112 p .004) 5.23 (.206) MIN 2.845 p 0.102 (.112 p .004) 0.889 p 0.127 (.035 p .005) 8 1 1.651 p 0.102 (.065 p .004) 1.651 p 0.102 3.20 – 3.45 (.065 p .004) (.126 – .136) 0.305 p 0.038 (.0120 p .0015) TYP 16 0.50 (.0197) BSC 4.039 p 0.102 (.159 p .004) (NOTE 3) RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 0.35 REF 0.
PAGE 27
LT3500 REVISION HISTORY (Revision history begins at Rev C) REV DATE DESCRIPTION PAGE NUMBER C 01/11 Corrected Pin Conﬁguration for MSE package 2 3500fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PAGE 28
LT3500 TYPICAL APPLICATION 1.8V/2A Step-Down Regulator 4.5V TO 36V C1 2.2μF VIN VOUT2 3.3V 10mA LDRV R3 R4 24.9k 8.06k LT3500 C6 1μF D2 LFB C2 R5 0.47μF 40.2k C3 220pF SHDN SS BST C5 0.47μF RT/SYNC SW FB VC PG PG D1 L1 2.2μH R6 49.9k R1 10k R2 8.06k VOUT1 1.8V 2A C7 22μF 3500 TA05 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1766 60V, 1.2A (IOUT), 200kHz High Efﬁciency Step-Down DC/DC Converter VIN: 5.5V to 60V, VOUT(MIN) = 1.20V, IQ = 2.