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LTC1435A

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

■

Low Minimum On-Time (

≤

300ns) for High

Frequency, Low Duty Cycle Applications

■

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

1435A is a synchronous step-down switching

regulator controller that drives external N-channel power

MOSFETs using a fixed frequency architecture. A wide

duty cycle range of 5% to 99% allows high V

to low V

OUT

DC/DC conversion, as well as low dropout operation that

extends operating time in battery-operated systems. Burst

Mode

operation provides high efficiency at low load

currents.

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 that can be

used to properly sequence supplies. The operating cur-

rent level is user-programmable via an external current

sense resistor. Wide input supply range allows operation

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

■

Notebook and Palmtop Computers, PDAs

■

Cellular Telephones and Wireless Modems

■

Portable Instruments

■

Battery-Operated Devices

■

DC Power Distribution Systems

APPLICATIONS

TYPICAL APPLICATION

Burst Mode is a trademark of Linear Technology Corporation.

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

OSC



43pF



330pF

100pF



10k



0.1µF

SENSE

–

SENSE

LTC1435A

1000pF

4.7µF



CMDSH-3

B

0.1µF

M2

Si4412DY

D1

MBRS140T3

L1

4.7µH

M1

Si4412DY

SENSE

0.033Ω



22µF

35V

× 2



4.5V TO 22V

OUT



100µF

6.3V

× 2

R1

35.7k

R2

102k

OUT



1.6V/3A

PGND

OSC

OSENSE

BOOST

1435A F01

SGND



RUN/SS

INTV

Summary of content (20 pages)

PAGE 1
LTC1435A High Efficiency Low Noise Synchronous Step-Down Switching Regulator U DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Dual N-Channel MOSFET Synchronous Drive Programmable Fixed Frequency Wide VIN Range: 3.
PAGE 2
LTC1435A 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 tON(MIN) Minimum On-Time TG Transition Time Rise Time Fall Time B
PAGE 3
LTC1435A ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 15V, VRUN/SS = 5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Oscillator fOSC Oscillator Frequency COSC = 100pF (Note 4) 112 125 138 kHz The ● denotes specifications which apply over the full operating temperature range.
PAGE 4
LTC1435A 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 10 15 20 INPUT VOLTAGE (V) 25°C 120 100 – 55°C 80 60 20 0 5 140 40 SHUTDOWN 0 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
LTC1435A 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 1435A G16 1435A 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
LTC1435A W FUNCTIONAL DIAGRA U U VIN + CIN COSC 1 COSC 4 SFB 13 VIN SGND 5 INTVCC 1.19V REF 1µA DB BOOST 15 – 1.19V CB + SHUTDOWN OSC + TG 16 DROP OUT DET OV S Q R – 1.28V 0.6V SWITCH LOGIC + – VOSENSE 6 VFB – – I1 EA R2 + Ω 1.19V gm = 1m 180k VSEC D1 4k + SW 14 I2 – + VIN + INTVCC INTVCC CSEC + 12 + R1 5V LDO REG – SHUTDOWN 3µA RUN SOFT START 6V + 4.
PAGE 7
LTC1435A U OPERATION (Refer to Functional Diagram) on and periodically forces a brief off period to allow CB to recharge. either of which causes drive to be returned to the TG pin on the next cycle. The main control loop is shut down by pulling the RUN/SS pin low. Releasing RUN/SS allows an internal 3µA current source to charge soft start capacitor CSS. When CSS reaches 1.3V, the main control loop is enabled with the ITH voltage clamped at approximately 30% of its maximum value.
PAGE 8
LTC1435A U W U U APPLICATIONS INFORMATION The value of COSC is calculated from the desired operating frequency:  1.37(104 )   – 11 COSC (pF) =   Frequency (kHz)    A graph for selecting COSC vs frequency is given in Figure 2. As the operating frequency is increased the gate charge losses will be higher, reducing efficiency (see Efficiency Considerations). The maximum recommended switching frequency is 400kHz. 300 The inductor value also has an effect on low current operation.
PAGE 9
LTC1435A U W U U APPLICATIONS INFORMATION a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core loss and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation.
PAGE 10
LTC1435A U U W U APPLICATIONS INFORMATION CIN and COUT Selection In continuous mode, the source current of the top N-channel MOSFET is a square wave of duty cycle VOUT/ VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: CIN required IRMS ≈ IMAX [ ( VOUT VIN − VOUT )] 1/ 2 VIN This formula has a maximum at V IN = 2V OUT , where IRMS = IOUT/2.
PAGE 11
LTC1435A U W U U APPLICATIONS INFORMATION control power to be derived from the output during normal operation (4.8V < VOUT < 9V) and from the internal regulator when the output is out of regulation (start-up, short circuit). Do not apply greater than 10V to the EXTVCC pin and ensure that EXTVCC < VIN. Significant efficiency gains can be realized by powering INTVCC from the output, since the VIN current resulting from the driver and control currents will be scaled by a factor of Duty Cycle/Efficiency.
PAGE 12
LTC1435A U W U U APPLICATIONS INFORMATION The external resistive divider is connected to the output as shown in Figure 5 allowing remote voltage sensing. 1.19V ≤ VOUT ≤ 9V R2 VOSENSE 100pF LTC1435A SGND R1 1435A F05 Figure 5. Setting the LTC1435A Output Voltage 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 LTC1435A. Soft start reduces surge currents from VIN by gradually increasing the internal current limit.
PAGE 13
LTC1435A U W U U APPLICATIONS INFORMATION on and off again. It is determined by internal timing delays and the gate charge required to turn on the top MOSFET. Low duty cycle applications may approach this minimum on-time limit. If the duty cycle falls below what can be accommodated by the minimum on-time, the LTC1435A will begin to skip cycles. The output voltage will continue to be regulated, but the ripple current and ripple voltage will increase. Therefore this limit should be avoided.
PAGE 14
LTC1435A U W U U APPLICATIONS INFORMATION By powering EXTVCC from an output-derived source, the additional VIN current resulting from the driver and control currents will be scaled by a factor of Duty Cycle/Efficiency. For example, in a 20V to 5V application, 10mA of INTVCC current results in approximately 3mA of VIN current. This reduces the midcurrent loss from 10% or more (if the driver was powered directly from VIN) to only a few percent. 3.
PAGE 15
LTC1435A U U W U APPLICATIONS INFORMATION conduct during double battery operation, but must still clamp the input voltage below breakdown of the converter. Although the LTC1435A has a maximum input voltage of 36V, most applications will be limited to 30V by the MOSFET BVDSS. Design Example As a design example, assume VIN = 12V(nominal), VIN = 22V(max), VOUT = 1.6V, IMAX = 3A and f = 250kHz, RSENSE and COSC can immediately be calculated: COSC = 1.37(104)/250 – 11 = 43pF Referring to Figure 3, a 4.
PAGE 16
LTC1435A U U W U APPLICATIONS INFORMATION 4. Does the (+) plate of CIN connect to the drain of the topside MOSFET(s) as closely as possible? This capacitor provides the AC current to the MOSFET(s). 6. Keep the switching node SW away from sensitive smallsignal nodes. Ideally the switch node should be placed at the furthest point from the LTC1435A. 5. Is the INTVCC decoupling capacitor connected closely between INTVCC and the power ground pin? This capacitor carries the MOSFET driver peak currents. 7.
PAGE 17
LTC1435A U TYPICAL APPLICATIONS Dual Output 5V and Synchronous 12V Application VIN 5.4V TO 28V COSC 68pF 1 CSS 0.1µF RC 10k 2 CC1 470pF 3 CC2 51pF 4 COSC TG RUN/SS BOOST ITH SW SFB VIN LTC1435A 5 M1 Si4412DY 14 T1 10µH 1:1.42 13 7 VOSENSE BG SENSE – PGND 0.1µF 12 + RSENSE 0.033Ω + 100pF 6 IRLL014 4.7k 15 CMDSH-3 INTVCC SGND 16 0.01µF CIN 22µF 35V ×2 + M2 Si4412DY MBRS140T3 SENSE + EXTVCC COUT 100µF 10V ×2 + 10 R2 20k 1% 1000pF 8 VOUT 5V/3.5A R1 35.7k 1% 4.
PAGE 18
LTC1435A U TYPICAL APPLICATIONS Constant-Current/Constant-Voltage High Efficiency Battery Charger E1 VIN + C1* 22µF 35V E3 GND E3 SHDN + C2* 22µF 35V R7 1.5M C4 0.1µF C11 56pF C5 0.1µF LTC1435A C13 0.033µF R5 1k 1 C12 0.1µF 2 3 C14 1000pF 4 5 C9 100pF C15 0.1µF LT1620 1 2 3 4 AVG SENSE IOUT PROG GND VCC NIN 6 PIN COSC TG RUN/SS BOOST SW ITH VIN SFB SGND INTVCC VOSENSE BG 7 SENSE – PGND 8 SENSE + EXTVCC 16 Q1 Si4412DY 15 D1 14 13 + C6 0.
PAGE 19
LTC1435A U TYPICAL APPLICATIONS Dual Output 5V and 12V Application VIN 5.4V TO 28V COSC 68pF 1 CSS 0.1µF RC 10k CIN 22µF 35V ×2 + 2 CC1 510pF 3 CC2 51pF 4 COSC RUN/SS ITH VIN CMDSH-3 7 SENSE – VOUT 5V/3.5A RSENSE 0.033Ω 4.7µF 11 M2 IRF7403 R1 35.7k 1% MBRS140T3 PGND SENSE + COUT 100µF 10V ×2 + 10 R2 20k 1% 1000pF 8 CSEC 3.3µF 25V 0.1µF + BG VOSENSE + 12 100pF 6 24V T1 10µH 1:2.
PAGE 20
LTC1435A U TYPICAL APPLICATION Low Dropout 2.9V/3A Converter VIN 3.5V TO 20V COSC 68pF 1 CSS 0.1µF RC 10k 2 CC1 330pF CC2 51pF INTVCC 3 4 COSC TG RUN/SS BOOST ITH SW SFB VIN LTC1435A 5 SGND 16 14 13 CMDSH-3 INTVCC 7 VOSENSE BG SENSE – PGND CIN 22µF 35V ×2 + 15 L1 10µH 0.1µF 12 RSENSE 0.033Ω VOUT 2.9V/3A + 100pF 6 M1 1/2 Si9802DY 4.7µF 11 M2 1/2 Si9802DY MBRS140T3 100pF SENSE + EXTVCC + 10 R2 24.9k 1% 1000pF 8 R1 35.