Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - DC/DC voltage regulator Holder MSOP 10 EP

LT3991/LT3991-3.3/LT3991-5

3991fa

Typical applicaTion

DescripTion

55V, 1.2A Step-Down

Regulator with 2.8µA

Quiescent Current

The LT

3991 is an adjustable frequency monolithic buck

switching regulator that accepts a wide input voltage range

up to 55V. Low quiescent current design consumes only

2.8µA of supply current while regulating with no load. Low

ripple Burst Mode operation maintains high efﬁciency at

low output currents while keeping the output ripple below

15mV in a typical application. An internally compensated

current mode topology is used for fast transient response

and good loop stability. A high efﬁciency 0.44Ω switch

is included on the die along with a boost Schottky diode

and the necessary oscillator, control and logic circuitry.

An accurate 1V threshold enable pin can be used to shut

down the LT3991, reducing the input supply current to

700nA.

A capacitor on the SS pin provides a controlled

inrush current (soft-start). A power good ﬂag signals

when V

OUT

reaches 91% of the programmed output volt-

age. The LT3991 is available in small 10-pin MSOP and

3mm

× 3mm DFN packages with exposed pads for low

thermal resistance.

L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks

of Linear Technology Corporation. All other trademarks are the property of their respective

owners.

3.3V Step Down Converter

FeaTures

applicaTions

Ultralow Quiescent Current:

2.8µA I

Regulating 12V

to 3.3V

OUT

Fixed Output Voltages: 3.3V, 5V

2.1µA I

Regulating 12V

Low Ripple Burst Mode

Operation:

Output Ripple < 15mV

P-P

Wide Input Voltage Range: 4.3V to 55V

1.2A Maximum Output Current

Adjustable Switching Frequency: 200kHz to 2MHz

Synchronizable Between 250kHz to 2MHz

Fast Transient Response

Accurate 1V Enable Pin Threshold

Low Shutdown Current: I

= 700nA

Power Good Flag

Soft-Start CapabilityV

Internal Compensation

Saturating Switch Design: 0.44Ω On-Resistance

Output Voltage: 1.19V to 30V

Small Thermally Enhanced 10-Pin MSOP Package

and (3mm × 3mm) DFN Packages

Automotive Battery Regulation

Power for Portable Products

Industrial Supplies

No Load Supply Current

OUT

4.3V TO 55V

OUT

3.3V

1.2A

4.7µF

0.47µF

47µF

118k

f = 400kHz

12µH

GND

SYNC

OFF ON

LT3991-3.3

3991 TA01a

EN./UVLO BOOST

INPUT VOLTAGE (V)

INPUT CURRENT (µA)

2.0

2.5

3991 G06

1.5

1.0

3.0

LT3991-5

LT3991-3.3

OUTPUT IN REGULATION

Summary of content (24 pages)

PAGE 1
LT3991/LT3991-3.3/LT3991-5 55V, 1.2A Step-Down Regulator with 2.8µA Quiescent Current Features n n n n n n n n n n n n n n n n Description Ultralow Quiescent Current: 2.8µA IQ Regulating 12VIN to 3.3VOUT Fixed Output Voltages: 3.3V, 5V 2.1µA IQ Regulating 12VIN Low Ripple Burst Mode® Operation: Output Ripple < 15mVP-P Wide Input Voltage Range: 4.3V to 55V 1.
PAGE 2
LT3991/LT3991-3.3/LT3991-5 Absolute Maximum Ratings (Note 1) VIN, EN Voltage..........................................................55V BOOST Pin Voltage....................................................75V BOOST Pin Above SW Pin..........................................30V FB, VOUT, RT, SYNC, SS Voltage..................................6V PG, BD Voltage..........................................................30V Boost Diode Current.....................................................
PAGE 3
LT3991/LT3991-3.3/LT3991-5 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN = 12V, VBD = 3.3V unless otherwise noted. (Note 2) PARAMETER CONDITIONS Minimum Input Voltage (Note 4) MIN Quiescent Current from VIN VEN Low VEN High, VSYNC Low VEN High, VSYNC Low l LT3991 FB Pin Current VFB = 1.19V l l Internal Feedback Resistor Divider TYP MAX 4 4.3 V 0.7 1.7 1.2 2.7 4.
PAGE 4
LT3991/LT3991-3.3/LT3991-5 Typical Performance Characteristics Efficiency, VOUT = 5V 90 VIN = 12V 90 VIN = 12V 70 EFFICIENCY (%) VIN = 24V 70 VIN = 36V 60 VIN = 48V 40 VOUT = 5V R1 = 1M R2 = 309k 30 0 0.2 30 1 20 1.2 VIN = 36V VIN = 48V 50 40 Efficiency, VOUT = 3.3V 80 0 0.2 0.4 0.6 0.8 LOAD CURRENT (A) 1 VIN = 36V VIN = 48V 30 VIN = 36V 30 VIN = 48V 0 0.01 1.2 0.1 1 10 100 LOAD CURRENT (mA) 1000 3991 G03 No Load Supply Current 3.0 DIODES, INC.
PAGE 5
LT3991/LT3991-3.3/LT3991-5 Typical Performance Characteristics 2.5 VOUT = 3.3V 2.5 0.30 VOUT = 5V 0.20 MINIMUM 1.5 1.0 LOAD REGULATION (%) TYPICAL 2.0 TYPICAL 1.5 MINIMUM 1.0 0.5 0.5 REFERENCED FROM VOUT AT 0.5A LOAD 0.25 2.0 LOAD CURRENT (A) LOAD CURRENT (A) Load Regulation Maximum Load Current Maximum Load Current 3.0 TA = 25°C, unless otherwise noted. 0.15 0.10 0.05 0 –0.05 –0.10 –0.15 –0.20 –0.
PAGE 6
LT3991/LT3991-3.3/LT3991-5 Typical Performance Characteristics Minimum Switch On-Time/ Switch Off-Time 400 800 350 700 600 500 400 300 200 Soft-Start 2.5 tOFF(MIN) 1A LOAD 300 250 SWITCH CURRENT LIMIT (A) 900 SWITCH ON/OFF TIME (ns) SWITCHING FREQUENCY (kHz) LT3991-X Frequency Foldback tOFF(MIN) 0.5A LOAD 200 150 tON(MIN) 100 2.0 1.5 1.0 0.5 50 100 0 TA = 25°C, unless otherwise noted.
PAGE 7
LT3991/LT3991-3.3/LT3991-5 Typical Performance Characteristics Transient Load Response, Load Current Stepped from 0.5A to 1A TA = 25°C, unless otherwise noted. Switching Waveforms; Burst Mode Operation VOUT 100mV/ DIV Switching Waveforms; Full Frequency Continuous Operation VSW 5V/DIV VSW 5V/DIV IL 500mA/ DIV 10µs/DIV VIN = 48V, VOUT = 3.3V COUT = 47µF 3991 G25 IL 500mA/DIV IL 500mA/DIV VOUT 20mV/DIV VOUT 20mV/DIV 5µs/DIV VIN = 48V, VOUT = 3.
PAGE 8
LT3991/LT3991-3.3/LT3991-5 Pin Functions PG (Pin 9): The PG pin is the open-drain output of an internal comparator. PGOOD remains low until the FB pin is within 9% of the final regulation voltage. PGOOD is valid when the LT3991 is enabled and VIN is above 4.3V. at low output loads. Tie to a clock source for synchronization, which will include pulse-skipping at low output loads. When in pulse-skipping mode, quiescent current increases to 1.5mA.
PAGE 9
LT3991/LT3991-3.3/LT3991-5 Operation The LT3991 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, sets an RS flip-flop, turning on the internal power switch. 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 (see Block Diagram).
PAGE 10
LT3991/LT3991-3.3/LT3991-5 Applications Information performance of the LT3991. The feedback resistors should preferably be on the order of MΩ and the Schottky catch diode should have less than 1µA of typical reverse leakage at room temperature. These two considerations are reiterated in the FB Resistor Network and Catch Diode Selection sections. It is important to note that another way to decrease the pulse frequency is to increase the magnitude of each single current pulse.
PAGE 11
LT3991/LT3991-3.3/LT3991-5 Applications Information Operating Frequency Tradeoffs Input Voltage Range Selection of the operating frequency is a tradeoff between efficiency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efficiency, lower maximum input voltage, and higher dropout voltage.
PAGE 12
LT3991/LT3991-3.3/LT3991-5 Applications Information Inductor Selection and Maximum Output Current A good first choice for the inductor value is: L= VOUT + VD fSW ΔIL = where fSW is the switching frequency in MHz, VOUT is the output voltage, VD is the catch diode drop (~0.5V) and L is the inductor value in μH. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher.
PAGE 13
LT3991/LT3991-3.3/LT3991-5 Applications Information Input Capacitor Bypass the input of the LT3991 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 4.7μF to 10μF ceramic capacitor is adequate to bypass the LT3991 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used (due to longer on-times).
PAGE 14
LT3991/LT3991-3.3/LT3991-5 Applications Information Table 4. Schottky Diodes. The Reverse Current Values Listed Are Estimates Based Off of Typical Curves for Reverse Current vs Reverse Voltage at 25°C. IAVE (A) MBR0520L 20 0.5 MBR0540 40 0.5 620 MBRM120E 20 1 530 MBRM140 40 1 550 B0530W 30 0.5 B0540W 40 0.5 620 1 B120 20 1 500 1.1 B130 30 1 500 1.1 B140 40 1 500 1.1 B150 50 1 700 0.4 B220 20 2 500 20 B230 30 2 500 0.
PAGE 15
LT3991/LT3991-3.3/LT3991-5 Applications Information while using the internal boost diode. For reliable BOOST pin operation with 2.5V outputs use a good external Schottky diode (such as the ON Semi MBR0540), and a 1μF boost capacitor (Figure 3b). For output voltages below 2.5V, the boost diode can be tied to the input (Figure 3c), or to another external supply greater than 2.8V. However, the circuit in Figure 3a is more efficient because the BOOST pin current comes from a lower voltage source.
PAGE 16
LT3991/LT3991-3.3/LT3991-5 Applications Information Enable Pin The LT3991 is in shutdown when the EN pin is low and active when the pin is high. The rising threshold of the EN comparator is 1.01V, with 30mV of hysteresis. The EN pin can be tied to VIN if the shutdown feature is not used. Adding a resistor divider from VIN to EN programs the LT3991 to regulate the output only when VIN is above a desired voltage (see Figure 5).
PAGE 17
LT3991/LT3991-3.3/LT3991-5 Applications Information Soft-Start The SS pin can be used to soft-start the LT3991 by throttling the maximum input current during start-up. An internal 1μA current source charges an external capacitor generating a voltage ramp on the SS pin. The SS pin clamps the internal VC node, which slowly ramps up the current limit. Maximum current limit is reached when the SS pin is about 1.5V or higher.
PAGE 18
LT3991/LT3991-3.3/LT3991-5 Applications Information D4 MBRS140 VIN VIN BOOST EN SW L1 C2 VOUT VOUT LT3991 GND BD FB + BACKUP RPG C4 3991 F07 Figure 8. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output. It Also Protects the Circuit from a Reversed Input. The LT3991 Runs Only When the Input is Present PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout.
PAGE 19
LT3991/LT3991-3.3/LT3991-5 Applications Information loss. The die temperature is calculated by multiplying the LT3991 power dissipation by the thermal resistance from junction to ambient. Also keep in mind that the leakage current of the power Schottky diode goes up exponentially with junction temperature. When the power switch is closed, the power Schottky diode is in parallel with the power converter’s output filter stage.
PAGE 20
LT3991/LT3991-3.3/LT3991-5 Typical Applications 1.8V Step-Down Converter VIN 4.3V TO 37V VIN BD EN OFF ON BOOST 0.47µF PG SS 4.7µF SW LT3991 6.8µH RT 10pF 162k SYNC GND 1M f = 300kHz VOUT 1.8V 1.2A 511k FB 100µF 3991 TA05 12V Step-Down Converter VIN 16V TO 55V VIN EN OFF ON BOOST 0.47µF PG 10µF SS SW LT3991 10µH RT BD 49.9k SYNC GND 10pF 110k f = 800kHz VOUT 12V 1.2A 1M FB 10µF 3991 TA06 3.
PAGE 21
LT3991/LT3991-3.3/LT3991-5 Typical Applications 4V Step-Down Converter with a High Impedance Input Source R + – 10M 48V + VIN EN CBULK 100µF 432k 0.47µF PG SS 4.7µF BOOST 100k LT3991 SW * AVERAGE OUTPUT POWER CANNOT EXCEED THAT WHICH CAN BE PROVIDED BY HIGH IMPEDANCE SOURCE. NAMELY, V2 •η POUT(MAX) = 4R 10µH RT BD 2nF 10pF 118k SYNC f = 400kHz GND VOUT 4V 1.
PAGE 22
LT3991/LT3991-3.3/LT3991-5 Package Description DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev C) R = 0.125 TYP 6 0.40 ± 0.10 10 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) 1.65 ± 0.10 (2 SIDES) 3.00 ±0.10 (4 SIDES) PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER PIN 1 PACKAGE TOP MARK OUTLINE (SEE NOTE 6) 0.25 ± 0.05 0.50 BSC 2.38 ±0.05 (2 SIDES) 5 0.75 ±0.05 0.200 REF 0.00 – 0.05 1 (DD) DFN REV C 0310 0.25 ± 0.05 0.50 BSC 2.38 ±0.
PAGE 23
LT3991/LT3991-3.3/LT3991-5 Revision History REV DATE DESCRIPTION PAGE NUMBER A 01/11 Added 3.3V and 5V fixed voltage options reflected throughout the data sheet. 1-24 3991fa 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 24
LT3991/LT3991-3.3/LT3991-5 Related Parts PART DESCRIPTION COMMENTS LT3970 40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.5µA VIN = 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD <1 µA, 3mm × 2mm DFN-10, MSOP-10 Packages LT3990 62V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.5µA VIN = 4.2V to 62V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD <1 µA, 3mm × 2mm DFN-10, MSOP-10 Packages LT3971 38V, 1.2A, 2.