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LTC1622

Low Input Voltage

Current Mode Step-Down

DC/DC Controller

The LTC

1622 is a constant frequency current mode step-

down DC/DC controller providing excellent AC and DC load

and line regulation. The device incorporates an accurate

undervoltage feature that shuts the LTC1622 down when

the input voltage falls below 2V.

The LTC1622 boasts a ±1.9% output voltage accuracy and

consumes only 350µA of quiescent current. For applica-

tions where efficiency is a prime consideration and the

load current varies from light to heavy, the LTC1622 can

be configured for Burst Mode

operation. Burst Mode

operation enhances low current efficiency and extends

battery run time. Burst Mode operation is inhibited during

synchronization or when the SYNC/MODE pin is pulled low

to reduce noise and possible RF interference.

High constant operating frequency of 550kHz allows the

use of a small inductor. The device can also be synchro-

nized up to 750kHz for special applications. The high

frequency operation and the available 8-lead MSOP pack-

age create a high performance solution in an extremely

small amount of PCB area.

To further maximize the life of the battery source, the

P-channel MOSFET is turned on continuously in dropout

(100% duty cycle). In shutdown, the device draws a mere

15µA.

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1- or 2-Cell Li-Ion Powered Applications

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Cellular Telephones

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Wireless Modems

■

Portable Computers

■

Distributed 3.3V, 2.5V or 1.8V Power Systems

■

Scanners

■

Battery-Powered Equipment

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High Efficiency

■

Constant Frequency 550kHz Operation

■

Range: 2V to 10V

■

Multiampere Output Currents

■

OPTI-LOOP

Compensation Minimizes C

OUT

■

Selectable, Burst Mode

Operation

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Low Dropout Operation: 100% Duty Cycle

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Synchronizable up to 750kHz

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Current Mode Operation for Excellent Line and Load

Transient Response

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Low Quiescent Current: 350µA

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Shutdown Mode Draws Only 15µA Supply Current

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±1.9% Reference Accuracy

■

Available in 8-Lead MSOP

Burst Mode and OPTI-LOOP are a trademarks of Linear Technology Corporation.

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

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

Figure 1. High Efficiency Step-Down Converter

SYNC/MODE

RUN/SS

4.7µH

0.03Ω

IR10BQ015

Si3443DV

159k

OUT

2.5V

1.5A

10k

220pF

C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT

C2: SANYO POSCAP 6TPA47M

D1: INTERNATIONAL RECTIFIER IR10BQ015

2.5V TO 8.5V

75k

1622 F01a

470pF

10µF

10V

LTC1622

47µF

SENSE

–

PDRV

GND

L1: MURATA LQN6C-4R7

R2: DALE WSL-1206 0-03Ω

LOAD CURRENT (mA)

EFFICIENCY (%)

100

1 100 1000 5000

1622 F01b

OUT

= 2.5V

SENSE

= 0.03Ω

= 3.3V

= 6V

= 8.4V

= 4.2V

Efficiency vs Load Current with

Burst Mode Operation Enabled

Summary of content (16 pages)

PAGE 1
LTC1622 Low Input Voltage Current Mode Step-Down DC/DC Controller U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO High Efficiency Constant Frequency 550kHz Operation VIN Range: 2V to 10V Multiampere Output Currents OPTI-LOOPTM Compensation Minimizes COUT Selectable, Burst Mode Operation Low Dropout Operation: 100% Duty Cycle Synchronizable up to 750kHz Current Mode Operation for Excellent Line and Load Transient Response Low Quiescent Current: 350µA Shutdown Mode Draws Only 15µA Supply Current ±1.
PAGE 2
LTC1622 U W W W ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage (VIN).........................– 0.3V to 10V RUN/SS Voltage .......................................– 0.3V to 2.4V SYNC/MODE Voltage ................................. – 0.3V to VIN SENSE – Voltage .......................................... 2.4V to VIN PDRV Peak Output Current (< 10µs) ......................... 1A Storage Ambient Temperature Range ... – 65°C to 150°C Operating Temperature Range Commercial ............................
PAGE 3
LTC1622 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 4.2V SYMBOL PARAMETER CONDITIONS MIN PDRV tr PDRV tf Gate Drive Rise Time Gate Drive Fall Time CLOAD = 3000pF CLOAD = 3000pF ∆VSENSE(MAX) Maximum Current Sense Voltage 80 ● Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired.
PAGE 4
LTC1622 U W TYPICAL PERFORMANCE CHARACTERISTICS Efficiency vs Load Current for Figure 1 with Burst Mode Operation Defeated Load Step Transient Response Burst Enabled Load Step Transient Response Burst Inhibited 100 VIN = 3.3V VIN = 4.2V 80 70 VIN = 6V 100mV/DIV 100mV/DIV EFFICIENCY (%) 90 VIN = 8.4V 60 50 VOUT = 2.5V RSENSE = 0.03Ω ILOAD = 50mA TO 1.2A VIN = 4.2V 40 ILOAD = 50mA TO 1.2A VIN = 4.
PAGE 5
LTC1622 W FUNCTIONAL DIAGRA U U VIN BURST DEFEAT 1µA VCC Y = “0” ONLY WHEN X IS A CONSTANT “1” OTHERWISE Y = “1” Y X SLOPE COMP SYNC/ 5 MODE OSC 0.36V 0.3V – VFB 3 + – FREQ SHIFT 0.8V VREF VIN 0.8V REFERENCE – + 0.12V EA gm = 0.5m 2.5µA VIN Ω RUN/SS 4 + – VIN RUN/ SOFT-START SLEEP + ICOMP BURST 2 VREF 0.8V 8 EN + – SENSE – 1 ITH S R Q RS1 SWITCHING LOGIC AND BLANKING CIRCUIT VIN PDRV 7 UVLO TRIP = 1.97V + OV 6 SHUTDOWN U OPERATIO VREF + 0.
PAGE 6
LTC1622 (Refer to Functional Diagram) the ITH pin will drop. When the ITH voltage goes below 0.12V, the sleep signal goes high, turning off the external MOSFET. The sleep signal goes low when the ITH voltage rises above 0.22V and the LTC1622 resumes normal operation. The next oscillator cycle will turn the external MOSFET on and the switching cycle repeats. Frequency Synchronization The LTC1622 can be externally driven by a TTL/CMOS compatible clock signal up to 750kHz.
PAGE 7
LTC1622 U W U U APPLICATIONS INFORMATION The basic LTC1622 application circuit is shown in Figure 1. External component selection is driven by the load requirement and begins with the selection of L and RSENSE. Next, the Power MOSFET and the output diode D1 are selected followed by CIN and COUT. VOUT. The inductor’s peak-to-peak ripple current is given by: RSENSE Selection for Output Current where f is the operating frequency.
PAGE 8
LTC1622 U U W U APPLICATIONS INFORMATION preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core materials saturate “hard,” which means that the inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequently, output voltage ripple.
PAGE 9
LTC1622 U W U U APPLICATIONS INFORMATION The allowable forward voltage drop in the diode is calculated from the maximum short-circuit current as: VF ≈ PD ISC(MAX ) where PD is the allowable power dissipation and will be determined by efficiency and/or thermal requirements. A fast switching diode must also be used to optimize efficiency. Schottky diodes are a good choice for low forward drop and fast switching times.
PAGE 10
LTC1622 U W U U APPLICATIONS INFORMATION NORMALIZED VOLTAGE (%) 101 is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: VREF 100 Efficiency = 100% – (η1 + η2 + η3 + ...) VITH 99 where η1, η2, etc. are the individual losses as a percentage of input power. 98 97 96 95 2.0 2.2 2.4 2.6 2.8 INPUT VOLTAGE (V) 3.
PAGE 11
LTC1622 U W U U APPLICATIONS INFORMATION Transition Loss = 3(VIN)2IO(MAX)CRSS(f) VOUT LTC1622 Other losses including CIN and COUT ESR dissipative losses, and inductor core losses, generally account for less than 2% total additional loss. R2 ITH VFB + DFB R1 Run/Soft-Start Function The RUN/SS pin is a dual purpose pin that provides the soft-start function and a means to shut down the LTC1622. Soft-start reduces input surge current from VIN by gradually increasing the internal current limit.
PAGE 12
LTC1622 U U W U APPLICATIONS INFORMATION down to 2.7V. Let’s assume that the MOSFET dissipation is to be limited to PP = 250mW and its thermal resistance is 50°C/W. Hence the junction temperature at TA = 25°C will be 37.5°C and δp = 0.005 (37.5 – 25) = 0.0625. The required RDS(ON) is then given by: RDS(ON) ≅ PP ( ) (1+ δp) DC IOUT 2 = 0.11Ω The P-channel MOSFET requirement can be met by an Si6433DQ. The requirement for the Schottky diode is the most stringent when VOUT = 0V, i.e., short circuit.
PAGE 13
LTC1622 U TYPICAL APPLICATIONS LTC1622 1.8V/1.5A Regulator with Burst Mode Operation Disabled C1 47µF 16V 1 R1 10K C3 220pF VFB U1 8 7 2 7 LTC1622 6 GND 3 VIN ITH 3 R2 0.025Ω 1 SENSE – 2 + VIN 2.5V TO 8.5V PDRV 8 R3 93.1k 6 4 SYNC/ 5 MODE 4 RUN/ SS L1 3.3µH 5 R4 75k C4 560pF VOUT 1.8V 1.5A + C2 220µF 6V 1622 TA01 C1: AVX TPSD476M016R0150 C2: AVX TPSD227M006R0100 L1: MURATA LQN6C3R3 R2: DALE WSL-1206 0.025Ω U1: INTERNATIONAL RECTIFIER FETKY TM IRF7422D2 LTC1622 2.
PAGE 14
LTC1622 U TYPICAL APPLICATIONS LTC1622 2.5V/3A Regulator with External Frequency Synchronization 1 SENSE – VIN 7 PDRV LTC1622 6 3 VFB GND 2 R1 10k ITH 4 RUN/ SS C3 220pF + R2 0.01Ω 8 M1 D1 C1 47µF 16V ×2 L1 4.7µH SYNC/ 5 MODE C2 100µF 6.3V ×2 + 650kHz 1.5VP-P C4 560pF VIN 3.3V TO 8.5V R3 158k R4 75k 1622 TA03 L1: COILCRAFT D03316-472 M1: SILICONIX Si3443DV R2: DALE WSL-2512 0.01Ω C1: AVX TPSD476M016R0150 C2: AVX TPSD107M010R0065 D1: MOTOROLA MBR320T3 VOUT 2.
PAGE 15
LTC1622 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. MS8 Package 8-Lead Plastic MSOP (LTC DWG # 05-08-1660) 0.118 ± 0.004* (3.00 ± 0.102) 8 7 6 5 0.118 ± 0.004** (3.00 ± 0.102) 0.193 ± 0.006 (4.90 ± 0.15) 1 2 3 4 0.040 ± 0.006 (1.02 ± 0.15) 0.007 (0.18) 0.034 ± 0.004 (0.86 ± 0.102) 0° – 6° TYP SEATING PLANE 0.012 (0.30) 0.0256 REF (0.65) BSC 0.021 ± 0.006 (0.53 ± 0.015) 0.006 ± 0.004 (0.15 ± 0.
PAGE 16
LTC1622 U TYPICAL APPLICATION Efficiency vs Load Current Small Footprint 3.3V/1A Regulator 2 SENSE – VIN 8 7 PDRV LTC1622 3 6 VFB GND 4 RUN/ SYNC/ 5 MODE SS R1 10k C3 220pF M1 L1 2.2µH ITH R3 232k D1 C2 47µF 6V C4 560pF VIN = 3.5V 90 EFFICIENCY (%) 1 R2 0.025Ω 100 VIN 3.3V TO C1 8.5V 10µF 16V CERAMIC + VOUT 3.3V 1A + VIN = 6V 70 R4 75k 60 VOUT = 3.3V RSENSE = 0.025Ω 50 1622 TA05 1 L1: COILCRAFT D01608C-222 M1: SILICONIX Si3443DY R2: DALE WSL-2010 0.