Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - DC/DC voltage regulator Converter, SEPIC MSOP 8 EP

LT1767/LT1767-1.8/

LT1767-2.5/LT1767-3.3/LT1767-5

sn1767 1767fas

Monolithic 1.5A, 1.25MHz

Step-Down Switching Regulators

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1.5A Switch in a Small MSOP Package

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Constant 1.25MHz Switching Frequency

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High Power Exposed Pad (MS8E) Package

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Wide Operating Voltage Range: 3V to 25V

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High Efficiency 0.22Ω Switch

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1.2V Feedback Reference Voltage

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Fixed Output Voltages of 1.8V, 2.5V, 3.3V, 5V

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2% Overall Output Tolerance

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Uses Low Profile Surface Mount Components

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Low Shutdown Current: 6µA

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Synchronizable to 2MHz

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Current Mode Loop Control

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Constant Maximum Switch Current Rating at All Duty

Cycles*

The LT

1767 is a 1.25MHz monolithic buck switching

regulator. A high efficiency 1.5A, 0.22Ω switch is included

on the die together with all the control circuitry required to

complete a high frequency, current mode switching regu-

lator. Current mode control provides fast transient re-

sponse and excellent loop stability.

New design techniques achieve high efficiency at high

switching frequencies over a wide operating range. A low

dropout internal regulator maintains consistent perfor-

mance over a wide range of inputs from 24V systems to Li-

Ion batteries. An operating supply current of 1mA im-

proves efficiency, especially at lower output currents.

Shutdown reduces quiescent current to 6µA. Maximum

switch current remains constant at all duty cycles. Syn-

chronization allows an external logic level signal to in-

crease the internal oscillator from 1.4MHz to 2MHz.

The LT1767 is available in an 8-pin MSOP fused leadframe

package and a low thermal resistance exposed pad pack-

age. Full cycle-by-cycle short-circuit protection and ther-

mal shutdown are provided. High frequency operation

allows the reduction of input and output filtering compo-

nents and permits the use of chip inductors.

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DSL Modems

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Portable Computers

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Wall Adapters

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Battery-Powered Systems

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Distributed Power

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

Efficiency vs Load Current

12V to 3.3V Step-Down Converter

BOOST

LT1767-3.3

OUTPUT

3.3V

1.2A*

12V

1767 TA01

0.1µF

1.5nF

4.7k

UPS120

10µF

CERAMIC

2.2µF

CERAMIC

CMDSH-3

5µH

FBSHDN

OPEN

HIGH

= ON

GND

SYNC

*MAXIMUM OUTPUT CURRENT IS SUBJECT TO THERMAL DERATING.

LOAD CURRENT (A)

0.2 0.4 0.6 0.8 1 1.2 1.4

EFFICIENCY (%)

1767 TA01a

= 10V

OUT

= 5V

OUT

= 3.3V

*Patent Pending

APPLICATIO S

FEATURES

TYPICAL APPLICATIO

DESCRIPTIO

Summary of content (16 pages)

PAGE 1
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 Monolithic 1.5A, 1.25MHz Step-Down Switching Regulators U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 1.5A Switch in a Small MSOP Package Constant 1.25MHz Switching Frequency High Power Exposed Pad (MS8E) Package Wide Operating Voltage Range: 3V to 25V High Efficiency 0.22Ω Switch 1.2V Feedback Reference Voltage Fixed Output Voltages of 1.8V, 2.5V, 3.
PAGE 2
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 W W U W ABSOLUTE MAXIMUM RATINGS (Note 1) Input Voltage .......................................................... 25V BOOST Pin Above SW ............................................ 20V Max BOOST Pin Voltage .......................................... 35V SHDN Pin ............................................................... 25V FB Pin Voltage .......................................................... 6V FB Pin Current ...............................
PAGE 3
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 15V, VC = 0.8V, Boost = VIN + 5V, SHDN, SYNC and switch open unless otherwise noted. PARAMETER CONDITION FB Input Resistance LT1767-1.8 LT1767-2.5 LT1767-3.3 LT1767-5 MIN TYP MAX ● ● ● ● 10.5 14.7 19 29 15 21 27.5 42 21 30 39 60 UNITS kΩ kΩ kΩ kΩ Error Amp Voltage Gain 0.4V < VC < 0.
PAGE 4
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U W TYPICAL PERFOR A CE CHARACTERISTICS SHDN Threshold vs Temperature SHDN Supply Current vs VIN 1.40 SHDN IP Current vs Temperature 7 –12 SHDN = 0V 6 –10 1.36 1.34 SHUTTING DOWN 5 SHDN INPUT (µA) VIN CURRENT (µA) SHDN THRESHOLD (V) 1.38 4 3 –8 –6 –4 2 1.32 –2 1 1.
PAGE 5
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U U U PIN FUNCTIONS FB: The feedback pin is used to set output voltage using an external voltage divider that generates 1.2V at the pin with the desired output voltage. The fixed voltage 1.8V, 2.5V, 3.3V and 5V versions have the divider network included internally and the FB pin is connected directly to the output. If required, the current limit can be reduced during start up or short-circuit when the FB pin is below 0.
PAGE 6
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 W BLOCK DIAGRAM The LT1767 is a constant frequency, current mode buck converter. This means that there is an internal clock and two feedback loops that control the duty cycle of the power switch. In addition to the normal error amplifier, there is a current sense amplifier that monitors switch current on a cycle-by-cycle basis. A switch cycle starts with an oscillator pulse which sets the RS flip-flop to turn the switch on.
PAGE 7
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U U W U APPLICATIONS INFORMATION FB RESISTOR NETWORK If an output voltage of 1.8V, 2.5V, 3.3V or 5V is required, the respective fixed option part, -1.8, -2.5, -3.3 or -5, should be used. The FB pin is tied directly to the output; the necessary resistive divider is already included on the part. For other voltage outputs, the adjustable part should be used and an external resistor divider added. The suggested resistor (R2) from FB to ground is 10k.
PAGE 8
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U U W U APPLICATIONS INFORMATION Table 2. Surface Mount Solid Tantalum Capacitor ESR and Ripple Current E Case Size ESR (Max, Ω ) Ripple Current (A) AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 AVX TAJ 0.7 to 0.9 0.4 0.1 to 0.3 0.7 to 1.1 and reduces the current at which discontinuous operation occurs.
PAGE 9
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U U W U APPLICATIONS INFORMATION saturation of the inductor. In these applications, the soft-start circuit shown in Figure 10 should be used. 2. Calculate peak inductor current at full load current to ensure that the inductor will not saturate. Peak current can be significantly higher than output current, especially with smaller inductors and lighter loads, so don’t omit this step.
PAGE 10
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U W U U APPLICATIONS INFORMATION suitable, but the ESR should be <1Ω to ensure it can be fully recharged during the off time of the switch. The capacitor value is derived from worst-case conditions of 700ns on-time, 50mA boost current, and 0.7V discharge ripple. This value is then guard banded by 2x for secondary factors such as capacitor tolerance, ESR and temperature effects.
PAGE 11
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U U W U APPLICATIONS INFORMATION prevent subharmonic switching is reduced. This type of subharmonic switching only occurs at input voltages less than twice output voltage. Higher inductor values will tend to eliminate this problem. See Frequency Compensation section for a discussion of an entirely different cause of subharmonic switching before assuming that the cause is insufficient slope compensation.
PAGE 12
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U U W U APPLICATIONS INFORMATION D2 CMDSH-3 C2 0.1µF L1 5µH BOOST VIN 12V C3 2.2µF CERAMIC VIN OPEN OR HIGH = ON OUTPUT 2.5V 1.2A VSW LT1767-2.5 SHDN SYNC FB VC GND CC 1.5nF C1 10µF CERAMIC D1 UPS120 RC 4.
PAGE 13
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U W U U APPLICATIONS INFORMATION LT1767 CURRENT MODE POWER STAGE gm = 2.5mho PINDUCTOR = (ILOAD) (LDCR) – 500k GND ESR ESL C1 C1 + CF 1767 F07 Figure 7. Model for Loop Response 80 180 VOUT = 5V COUT = 100µF, 0.1Ω CC = 330pF RC/CF = N/C ILOAD = 500mA 60 GAIN (dB) 150 120 PHASE 20 0 90 PHASE (DEG) 40 Figure 8 shows the overall loop response with a 330pF VC capacitor and a typical 100µF tantalum output capacitor.
PAGE 14
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U W U U APPLICATIONS INFORMATION The zero produced by the ESR of the tantalum output capacitor is very useful in maintaining stability. Ceramic output capacitors do not have a zero due to very low ESR, but are dominated by their ESL. They form a notch in the 1MHz to 10MHz range. Without this zero, the VC pole must be made dominant. A typical value of 2.2nF will achieve this.
PAGE 15
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U U W U APPLICATIONS INFORMATION during switch on time, causing current to flow, and energy to build in L1B and C4. At switch off, the energy stored in both L1B and C4 supply the –5V rail. This reduces the current in L1A and changes L1B current waveform from square to triangular. For details on this circuit, including maximum output currents, see Design Note 100. CMDSH-3 0.1µF UPS120* REMOVABLE INPUT VSW VIN 83k 3.3V, 1A LT1767-3.
PAGE 16
LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5 U PACKAGE DESCRIPTION MS8E Package 8-Lead Plastic MSOP (Reference LTC DWG # 05-08-1662) 0.889 ± 0.127 (.035 ± .005) 2.794 ± 0.102 (.110 ± .004) 5.23 (.206) MIN 2.083 ± 0.102 3.2 – 3.45 (.082 ± .004) (.126 – .136) 0.42 ± 0.04 (.0165 ± .0015) TYP 0.65 (.0256) BSC BOTTOM VIEW OF EXPOSED PAD OPTION 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 0.254 (.010) 8 7 6 5 1 2.06 ± 0.102 (.080 ± .004) 1.83 ± 0.102 (.072 ± .004) 3.00 ± 0.102 (.118 ± .004) NOTE 4 4.