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LTC1435

High Efficiency Low Noise

Synchronous Step-Down

Switching Regulator

Figure 1. High Efficiency Step-Down Converter

FEATURES

DESCRIPTION

■

Dual N-Channel MOSFET Synchronous Drive

■

Programmable Fixed Frequency

■

Wide V

Range: 3.5V to 36V Operation

■

Ultrahigh Efficiency

■

Very Low Dropout Operation: 99% Duty Cycle

■

Low Standby Current

■

Secondary Feedback Control

■

Programmable Soft Start

■

Remote Output Voltage Sense

■

Logic Controlled Micropower Shutdown: I

< 25µA

■

Foldback Current Limiting (Optional)

■

Current Mode Operation for Excellent Line and Load

Transient Response

■

Output Voltages from 1.19V to 9V

■

Available in 16-Lead Narrow SO and SSOP Packages

The LTC

1435 is a synchronous step-down switching

regulator controller that drives external N-channel power

MOSFETs using a fixed frequency architecture. Burst

Mode

operation provides high efficiency at low load

currents. A maximum duty cycle limit of 99% provides low

dropout operation which extends operating time in bat-

tery-operated systems.

The operating frequency is set by an external capacitor

allowing maximum flexibility in optimizing efficiency. A

secondary winding feedback control pin, SFB, guarantees

regulation regardless of load on the main output by

forcing continuous operation. Burst Mode operation is

inhibited when the SFB pin is pulled low which reduces

noise and RF interference.

Soft start is provided by an external capacitor which can

be used to properly sequence supplies. The operating

current level is user-programmable via an external current

sense resistor. Wide input supply range allows operation

from 3.5V to 30V (36V maximum).

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

Burst Mode is a trademark of Linear Technology Corporation.

■

Notebook and Palmtop Computers, PDAs

■

Cellular Telephones and Wireless Modems

■

Portable Instruments

■

Battery-Operated Devices

■

DC Power Distribution Systems

APPLICATIONS

TYPICAL APPLICATION

OSC



68pF



330pF

100pF



10k



0.1µF

SENSE

–

SENSE

LTC1435

1000pF

4.7µF



CMDSH-3

B

0.1µF

M2

Si4412DY

D1

MBRS140T3

L1

10µH

M1

Si4412DY

SENSE

0.033Ω



22µF

35V

× 2



4.5V TO 28V

OUT



100µF

10V

× 2

R1

32.4k

R2

22.1k

OUT



2.9V/3.5A

PGND

OSC

OSENSE

BOOST

1435 F01

SGND



RUN/SS

INTV

Summary of content (20 pages)

PAGE 1
LTC1435 High Efficiency Low Noise Synchronous Step-Down Switching Regulator U DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LTC®1435 is a synchronous step-down switching regulator controller that drives external N-channel power MOSFETs using a fixed frequency architecture. Burst ModeTM operation provides high efficiency at low load currents. A maximum duty cycle limit of 99% provides low dropout operation which extends operating time in battery-operated systems.
PAGE 2
LTC1435 U W W W SYMBOL PARAMETER Main Control Loop IIN VOSENSE Feedback Current VOSENSE Feedback Voltage ∆VLINEREG Reference Voltage Line Regulation ∆VLOADREG Output Voltage Load Regulation VSFB ISFB VOVL IQ Secondary Feedback Threshold Secondary Feedback Current Output Overvoltage Lockout Input DC Supply Current Normal Mode Shutdown VRUN/SS Run Pin Threshold IRUN/SS Soft Start Current Source ∆VSENSE(MAX) Maximum Current Sense Threshold TG Transition Time Rise Time TG t r TG t f Fall Time BG Transition
PAGE 3
LTC1435 ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 15V, VRUN/SS = 5V unless otherwise noted. The ● denotes specifications which apply over the full operating temperature range.
PAGE 4
LTC1435 U W TYPICAL PERFORMANCE CHARACTERISTICS Input Supply and Shutdown Current vs Input Voltage 100 VOUT = 5V EXTVCC = VOUT 60 VOUT = 3.3V EXTVCC = OPEN 1.0 40 0.5 20 200 VEXTVCC = 0V 180 70°C 0 25°C – 0.3 0 5 10 15 20 INPUT VOLTAGE (V) 140 25°C 120 100 – 55°C 80 60 40 20 SHUTDOWN 0 70°C 160 0.3 EXTVCC – INTVCC (mV) SUPPLY CURRENT (mA) 80 SHUTDOWN CURRENT (µA) 2.0 0.5 ∆INTVCC (%) 2.5 1.
PAGE 5
LTC1435 U W TYPICAL PERFORMANCE CHARACTERISTICS Soft Start: Load Current vs Time Burst Mode Operation VOUT 20mV/DIV RUN/SS 5V/DIV INDUCTOR CURRENT 1A/DIV VITH 200mV/DIV ILOAD = 50mA 1435 G16 1435 G17 U U U PIN FUNCTIONS COSC (Pin 1): External capacitor COSC from this pin to ground sets the operating frequency. RUN/SS (Pin 2): Combination of Soft Start and Run Control Inputs. A capacitor to ground at this pin sets the ramp time to full current output. The time is approximately 0.5s/µF.
PAGE 6
LTC1435 W FUNCTIONAL DIAGRA U U VIN COSC + CIN 1 COSC 4 SFB 13 VIN SGND 5 INTVCC 1.19V REF 1µA DB BOOST 15 – 1.19V TG 16 SHUTDOWN OSC + CB + DROP OUT DET OV S Q R – 1.28V 0.6V SWITCH LOGIC + – SW 14 VOSENSE 6 VFB – – I1 EA + 1.19V Ω R2 gm = 1m + 180k I2 – 4k D1 + VIN + VSEC INTVCC INTVCC CSEC + 12 + – SHUTDOWN R1 5V LDO REG 3µA 6V RUN SOFT START 30k + 4.
PAGE 7
LTC1435 U OPERATION (Refer to Functional Diagram) Main Control Loop Low Current Operation The LTC1435 uses a constant frequency, current mode step-down architecture. During normal operation, the top MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the main current comparator I1 resets the RS latch. The peak inductor current at which I1 resets the RS latch is controlled by the voltage on the ITH pin , which is the output of error amplifier EA.
PAGE 8
LTC1435 U W U U APPLICATIONS INFORMATION 300 The basic LTC1435 application circuit is shown in Figure 1, High Efficiency Step-Down Converter. External component selection is driven by the load requirement and begins with the selection of RSENSE. Once RSENSE is known, COSC and L can be chosen. Next, the power MOSFETs and D1 are selected. Finally, CIN and COUT are selected. The circuit shown in Figure 1 can be configured for operation up to an input voltage of 28V (limited by the external MOSFETs).
PAGE 9
LTC1435 U W U U APPLICATIONS INFORMATION MOSFET is on. Lower inductor values (higher ∆IL) will cause this to occur at higher load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to decrease. The Figure 3 graph gives a range of recommended inductor values vs operating frequency and VOUT. 60 VOUT = 5.0V VOUT = 3.3V VOUT = 2.
PAGE 10
LTC1435 U W U U APPLICATIONS INFORMATION where δ is the temperature dependency of RDS(ON) and k is a constant inversely related to the gate drive current. Both MOSFETs have I2R losses while the topside N-channel equation includes an additional term for transition losses, which are highest at high input voltages.
PAGE 11
LTC1435 U W U U APPLICATIONS INFORMATION INTVCC Regulator An internal P-channel low dropout regulator produces the 5V supply which powers the drivers and internal circuitry within the LTC1435. The INTVCC pin can supply up to 15mA and must be bypassed to ground with a minimum of 2.2µF tantalum or low ESR electrolytic. Good bypassing is necessary to supply the high transient currents required by the MOSFET gate drivers.
PAGE 12
LTC1435 U U W U APPLICATIONS INFORMATION + 1.19V ≤ VOUT ≤ 9V + VIN 1µF CIN BAT85 0.22µF R2 VOSENSE BAT85 TG SGND BAT85 N-CH EXTVCC R1 LTC1435 • F05 VN2222LL L1 RSENSE VOUT + LTC1435 Figure 5. Setting the LTC1435 Output Voltage COUT SW BG 100pF LTC1435 VIN 3.3V OR 5V N-CH RUN/SS RUN/SS D1 PGND CSS LTC1435 • F04b Figure 4b. Capacitive Charge Pump for EXTVCC CSS LTC1435 • F06 Figure 6.
PAGE 13
LTC1435 U W U U APPLICATIONS INFORMATION Foldback Current Limiting Efficiency Considerations As described in Power MOSFET and D1 Selection, the worst-case dissipation for either MOSFET occurs with a short-circuited output, when the synchronous MOSFET conducts the current limit value almost continuously. In most applications this will not cause excessive heating, even for extended fault intervals.
PAGE 14
LTC1435 U W U U APPLICATIONS INFORMATION MOSFET and the synchronous MOSFET. If the two MOSFETs have approximately the same RDS(ON), then the resistance of one MOSFET can simply be summed with the resistances of L and RSENSE to obtain I2R losses. For example, if each RDS(ON) = 0.05Ω, RL = 0.15Ω, and RSENSE = 0.05Ω, then the total resistance is 0.25Ω. This results in losses ranging from 3% to 10% as the output current increases from 0.5A to 2A.
PAGE 15
LTC1435 U W U U APPLICATIONS INFORMATION Design Example As a design example, assume VIN = 12V(nominal), VIN = 22V(max), VOUT = 3.3V, IMAX = 3A and f = 250kHz, RSENSE and COSC can immediately be calculated: RSENSE = 100mV/3A = 0.033Ω COSC = 1.37(104)/250 – 11 = 43pF Referring to Figure 3, a 10µH inductor falls within the recommended range.
PAGE 16
LTC1435 U U W U APPLICATIONS INFORMATION + M1 1 CSS 2 TG RUN/SS BOOST 16 CIN 15 VIN CC1 RC COSC + COSC 3 CC2 4 5 ITH SW LTC1435 SFB VIN INTVCC SGND 14 13 VOSENSE BG 7 SENSE – PGND 8 SENSE + EXTVCC D1 12 – 100pF 6 CB 0.1µF DB 11 + M2 4.7µF 10 1000pF 9 L1 – R1 + R2 COUT VOUT RSENSE BOLD LINES INDICATE HIGH CURRENT PATHS + LTC1435 • F08 Figure 8. LTC1435 Layout Diagram U TYPICAL APPLICATIONS Dual Output 5V and Synchronous 12V Application VIN 5.
PAGE 17
LTC1435 U TYPICAL APPLICATIONS 3.3V/4.5A Converter with Foldback Current Limiting VIN 4.5V TO 28V COSC 68pF 1 CSS 0.1µF RC 10k 2 CC1 330pF CC2 51pF 3 4 INTVCC COSC TG RUN/SS LTC1435 5 14 SW SFB ITH PIN 3 13 VIN IN4148 7 SENSE – PGND SENSE + EXTVCC M2 Si4410DY 9 + MBRS140T3 10 100pF R2 20k 1% 1000pF 8 VOUT 3.3V/4.5A R1 35.7k 1% 4.7µF 11 BG VOSENSE RSENSE 0.025Ω + 100pF 6 L1 10µH 0.
PAGE 18
LTC1435 U TYPICAL APPLICATIONS Constant-Current/Constant-Voltage High Efficiency Battery Charger E1 VIN + C1* 22µF 35V E3 GND E3 SHDN + C2* 22µF 35V 1 2 3 C14 1000pF 4 5 C9 100pF U2 LT1620 1 2 3 4 AVG SENSE PROG GND VCC NIN 6 7 8 C15 0.1µF IOUT PIN C4 0.1µF C11 56pF C12 0.1µF C13 0.033µF R5 1k R7 1.5M COSC TG RUN/SS BOOST C5 0.
PAGE 19
LTC1435 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. G Package 16-Lead Plastic SSOP (0.209) (LTC DWG # 05-08-1640) 0.239 – 0.249* (6.07 – 7.33) 16 15 14 13 12 11 10 9 0.205 – 0.212** (5.20 – 5.38) 0.068 – 0.078 (1.73 – 1.99) 0.301 – 0.311 (7.65 – 7.90) 0° – 8° 0.0256 (0.65) BSC 0.022 – 0.037 (0.55 – 0.95) 0.005 – 0.009 (0.13 – 0.22) *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.
PAGE 20
LTC1435 U TYPICAL APPLICATION Low Dropout 2.9V/3A Converter VIN 3.5V TO 25V COSC 68pF 1 CSS 0.1µF RC 10k 2 CC1 330pF CC2 51pF INTVCC 3 4 COSC TG RUN/SS BOOST ITH SW SFB LTC1435 5 SGND VIN 16 14 13 CMDSH-3 INTVCC 7 VOSENSE SENSE – BG PGND CIN 22µF 35V ×2 + 15 L1 10µH 0.1µF 12 RSENSE 0.033Ω VOUT 2.9V/3A + 100pF 6 M1 1/2 Si9925DY 11 4.7µF M2 1/2 Si9925DY 100pF MBRS140T3 SENSE + EXTVCC + 10 R2 24.9k 1% 1000pF 8 R1 35.