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LTC1775

DESCRIPTIO

FEATURES

APPLICATIO S

TYPICAL APPLICATION

High Power

No R

SENSE

Current Mode

Synchronous Step-Down

Switching Regulator

■

Highest Efficiency Current Mode Controller

■

No Sense Resistor Required

■

300mV Maximum Current Sense Voltage

■

Stable High Current Operation

■

Dual N-Channel MOSFET Synchronous Drive

■

Wide V

Range: 4V to 36V

■

Wide V

OUT

Range: 1.19V to V

■

±1% 1.19V Reference

■

Programmable Fixed Frequency with Injection Lock

■

Very Low Drop Out Operation: 99% Duty Cycle

■

Forced Continuous Mode Control Pin

■

Optional Programmable Soft Start

■

Pin Selectable Output Voltage

■

Foldback Current Limit

■

Output Overvoltage Protection

■

Logic Controlled Micropower Shutdown: I

< 30µA

■

Available in 16-Lead Narrow SSOP and SO Packages

The LTC

1775 is a synchronous step-down switching

regulator controller that drives external N-channel power

MOSFETs using few external components. Current mode

control with MOSFET V

sensing eliminates the need for

a sense resistor and improves efficiency. Largely similar

to the LTC1625, the LTC1775 has twice the maximum

sense voltage for high current applications. The frequency

of a nominal 150kHz internal oscillator can be synchro-

nized to an external clock over a 1.5:1 frequency range.

Burst Mode

operation at low load currents reduces

switching losses and low dropout operation extends oper-

ating time in battery-powered systems. A forced continu-

ous mode control pin can assist secondary winding

regulation by disabling Burst Mode operation when the

main output is lightly loaded.

Fault protection is provided by foldback current limiting

and an output overvoltage comparator. An external ca-

pacitor attached to the RUN/SS pin provides soft start

capability for supply sequencing. A wide supply range

allows operation from 4V (4.3V for LTC1775I) to 36V at the

input and 1.19V to V

at the output.

Figure 1. High Efficiency Step-Down Converter

Efficiency vs Load Current

■

Notebook Computers

■

Automotive Electronics

■

Battery Chargers

■

Distributed Power Systems

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

No R

SENSE

and Burst Mode are trademarks of Linear Technology Corporation.

SYNC

LTC1775

RUN/SS

OSENSE

0.33µF

CMDSH-3

2.2nF

10k

SUD50N03-10

MBRS340

SUD50N03-10

VCC

4.7µF

1775 F01

6µH

22µF

30V

×4

BOOST

INTV

PROG

SGND

PGND

OUT

680µF

6.3V

OUT

3.3V

10A

5V TO

28V

0.1µF

LOAD CURRENT (A)

0.01

EFFICIENCY (%)

100

0.1 1 10

1775 F01b

= 10V

f = 150kHz

FCB = INTV

OUT

= 5V

OUT

= 3.3V

Summary of content (24 pages)

PAGE 1
LTC1775 High Power No RSENSE Current Mode Synchronous Step-Down Switching Regulator TM U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LTC®1775 is a synchronous step-down switching regulator controller that drives external N-channel power MOSFETs using few external components. Current mode control with MOSFET VDS sensing eliminates the need for a sense resistor and improves efficiency.
PAGE 2
LTC1775 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Input Supply Voltage (VIN, TK) ................. 36V to – 0.3V Boosted Supply Voltage (BOOST) ............. 42V to – 0.3V Boosted Driver Voltage (BOOST – SW) ...... 7V to – 0.3V Switch Voltage (SW) ....................................36V to – 5V EXTVCC Voltage ...........................................7V to – 0.3V ITH Voltage ................................................2.7V to – 0.3V FCB, RUN/SS, SYNC Voltages .....
PAGE 3
LTC1775 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 15V unless otherwise noted. SYMBOL PARAMETER CONDITIONS TG tR TG tF TG Transition Time Rise Time Fall Time BG tR BG tF BG Transition Time Rise Time Fall Time MIN TYP MAX UNITS (Note 6) CLOAD = 3300pF CLOAD = 3300pF 50 50 150 150 ns ns (Note 6) CLOAD = 3300pF CLOAD = 3300pF 50 50 150 150 ns ns 5.2 5.4 V – 0.
PAGE 4
LTC1775 U W TYPICAL PERFOR A CE CHARACTERISTICS Efficiency vs Load Current Efficiency vs Input Voltage 100 100 ILOAD = 5A 95 90 95 ILOAD = 5A 70 CONTINUOUS MODE 60 ILOAD = 500mA 85 80 VIN = 10V VOUT = 5V EXTVCC = VOUT 50 0.001 75 90 ILOAD = 500mA 85 80 75 VOUT = 5V FIGURE 1 CIRCUIT 70 0.1 0.01 1.0 LOAD CURRENT (A) EFFICIENCY (%) 80 BURST MODE 90 EFFICIENCY (%) EFFICIENCY (%) Efficiency vs Input Voltage 100 10 0 10 15 20 INPUT VOLTAGE (V) 5 25 VOUT = 3.
PAGE 5
LTC1775 U W TYPICAL PERFOR A CE CHARACTERISTICS Maximum Current Sense Voltage vs Temperature 300 250 200 150 100 50 0 0 0.2 0.4 0.6 DUTY CYCLE 0.8 320 300 250 300 200 SYNC = 0V 150 100 290 50 60 35 85 10 TEMPERATURE (°C) 110 135 0 –40 –15 60 35 85 10 TEMPERATURE (°C) 1775 • G11 1775 • G10 0 110 135 1775 • G12 RUN/SS Pin Current vs Temperature FCB Pin Current vs Temperature Soft Start 0 RUN/SS CURRENT (µA) – 0.5 FCB CURRENT (µA) SYNC = 1.5V 310 280 –40 –15 1.
PAGE 6
LTC1775 U U U PI FU CTIO S EXTVCC (Pin 1): INTVCC Switch Input. When the EXTVCC voltage is above 4.7V, the switch closes and supplies INTVCC power from EXTVCC. Do not exceed 7V at this pin. Leaving VPROG open allows the output voltage to be set by an external resistive divider between the output and VOSENSE. SYNC (Pin 2): Synchronization Input for Internal Oscillator. The oscillator will nominally run at 150kHz when open, 225kHz when tied above 1.2V, and will lock over a 1.5:1 clock frequency range.
PAGE 7
LTC1775 W FU CTIO AL DIAGRA U U TK + 2 SYNC VIN 15 TA × 5.5 INTVCC + – INTVCC CIN – BA × 5.5 0.95V + ITH +– 5 OSC 0.7V RC – 1.19V – + gm = 1m BOOST + – CL – TOP R 0.5V VFB 12 B SLEEP CB TG SWITCH LOGIC/ DROPOUT COUNTER Ω EA 14 INTVCC 11 OVERVOLTAGE +– 0.6V – DB + CVCC BG FCNT 10 3µA M2 PGND + 3 M1 13 SW SHUTDOWN VIN CSS I1 – + RUN/SS S Q ITHB 0.6V I2 + CC1 + REV 9 6V 1.28V 1.19V REF – + VIN 16 OV 1.19V 5.
PAGE 8
LTC1775 U OPERATIO Main Control Loop The LTC1775 is a constant frequency, current mode controller for DC/DC step-down converters. In normal operation, the top MOSFET is turned on when the RS latch is set by the on-chip oscillator and is turned off when the current comparator I1 resets the latch. While the top MOSFET is turned off, the bottom MOSFET is turned on until either the inductor current reverses, as determined by the current reversal comparator I2, or the next cycle begins.
PAGE 9
LTC1775 U W U U APPLICATIO S I FOR ATIO The LTC1775 requires two external N-channel power MOSFETs, one for the top (main) switch and one for the bottom (synchronous) switch. Important parameters for the power MOSFETs are the breakdown voltage V(BR)DSS, threshold voltage VGS(TH), on-resistance RDS(ON), reverse transfer capacitance CRSS and maximum current ID(MAX). The gate drive voltage is set by the 5.2V INTVCC supply. Consequently, logic level threshold MOSFETs must be used in LTC1775 applications.
PAGE 10
LTC1775 U W U U APPLICATIO S I FOR ATIO paid to the resulting thermal issues. Under DC conditions, the maximum power that can be dissipated by a MOSFET switch limits the current through it: IDS(MAX)= P RDS(ON) = TJ(MAX) – TA θ JA RDS(ON) ρTJ(MAX) For example, the SUD50N03-10 with TJ(MAX) = 175°C, TA =70°, θJA = 30° C/W, RDS(ON) = 0.019Ω, ρTJ(MAX) = 1.8 can operate with a maximum DC current of 10A.
PAGE 11
LTC1775 U W U U APPLICATIO S I FOR ATIO A reasonable starting point is to choose a ripple current that is about 40% of IO(MAX). Note that the largest ripple current occurs at the highest VIN.
PAGE 12
LTC1775 U W U U APPLICATIO S I FOR ATIO Normally, the top and bottom MOSFETs will be of the same type. A bottom MOSFET with lower RDS(ON) than the top may be chosen if the resulting increase in short-circuit current is tolerable. However, the bottom MOSFET should never be chosen to have a higher nominal RDS(ON) than the top MOSFET. Inductor Core Selection Once the value for L is known, the type of inductor must be selected.
PAGE 13
LTC1775 U W U U APPLICATIO S I FOR ATIO Schottky diode must be placed next to the synchronous switch to minimize this effect. One also might consider using a power switch with an integrated Schottky diode, or omitting the diode altogether in high current applications. CIN and COUT Selection In continuous mode, the drain current of the top MOSFET is approximately a square wave of duty cycle VOUT / VIN.
PAGE 14
LTC1775 U W U U APPLICATIO S I FOR ATIO VPUMP ≈ 2(VOUT – VD) < 7V EXTVCC Connection + The LTC1775 contains an internal P-channel MOSFET switch connected between the EXTVCC and INTVCC pins. Whenever the EXTVCC pin is above 4.7V the internal 5.2V regulator shuts off, the switch closes and INTVCC power is supplied via EXTVCC until EXTVCC drops below 4.5V. This allows the MOSFET gate drive and control power to be derived from the output or other external source during normal operation.
PAGE 15
LTC1775 U W U U APPLICATIO S I FOR ATIO voltage is between 4.7V and 5.2V, INTVCC will be connected to the output and the gate drive is reduced. The resulting increase in RDS(ON) will also lower the current limit. Even applications with VOUT > 5.2V will traverse this region during start-up and must take into account the reduced current limit. An external bootstrap capacitor (CB in the functional diagram) connected to the BOOST pin supplies the gate drive voltage for the topside MOSFET.
PAGE 16
LTC1775 U W U U APPLICATIO S I FOR ATIO Run/Soft Start Function The RUN/SS pin is a dual purpose pin that provides a soft start function and a means to shut down the LTC1775. Soft start reduces surge currents from VIN by gradually increasing the controller’s current limit ITH(MAX). This pin can also be used for power supply sequencing. Pulling the RUN/SS pin below 1.4V puts the LTC1775 into a low quiescent current shutdown (IQ < 30µA). This pin can be driven directly from logic as shown in Figure 9.
PAGE 17
LTC1775 U W U U APPLICATIO S I FOR ATIO cycle skipping can occur with correspondingly larger current and voltage ripple. Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power (×100%). Percent efficiency can be expressed as: %Efficiency = 100% – (L1 + L2 + L3 + ...) losses ranging from 2% to 8% as the output current increases from 0.5A to 2A for a 5V output. I2R losses cause the efficiency to drop at high output currents. 3.
PAGE 18
LTC1775 U W U U APPLICATIO S I FOR ATIO Automotive Considerations: Plugging into the Cigarette Lighter As battery-powered devices go mobile, there is a natural interest in plugging into the cigarette lighter in order to conserve or even recharge battery packs during operation. But before you connect, be advised: you are plugging into the supply from hell. The main power line in an automobile is the source of a number of nasty potential transients, including load dump, reverse and double battery.
PAGE 19
LTC1775 U W U U APPLICATIO S I FOR ATIO A short circuit to ground will result in a folded back current of: ISC = CIN is chosen for an RMS current rating of at least 5A at temperature. COUT is chosen with an ESR of 0.013Ω for low output ripple. The output ripple in continuous mode will be highest at the maximum input voltage and is approximately: 80mV  1 (15V)(0.5µs) +  = 6.2A (0.013Ω)(1.1)  2  6µH ∆VO = (∆IL(MAX))(ESR) = (4.3A)(0.013Ω) = 56mV with a typical value of RDS(ON) and ρ(50°C) = 1.1.
PAGE 20
LTC1775 U W U U APPLICATIO S I FOR ATIO PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1775. These items are also illustrated graphically in the layout diagram of Figure 12. Check the following in your layout: 1) Connect the TK lead directly to the drain of the topside MOSFET. Then connect the drain to the (+) plate of CIN. This capacitor provides the AC current to the top MOSFET.
PAGE 21
LTC1775 U TYPICAL APPLICATIO S 3.3V/5A Fixed Output VIN 5V TO 28V RF 1Ω LTC1775 16 VIN EXTVCC 2 15 SYNC TK 3 14 SW RUN/SS CC1 2.2nF 4 CSS RC INTVCC 0.1µF 10k 5 CC2 220pF 6 7 8 TG FCB BOOST ITH INTVCC SGND VOSENSE VPROG BG PGND CIN 22µF 30V ×2 + CF 0.1µF 1 M1 Si4412DY L1 10µH 13 VOUT 3.3V 5A 12 DB CMDSH-3 11 CB 0.1µF 10 + 9 D1 MBRS140T3 + M2 Si4412DY CVCC 4.7µF COUT 100µF 10V 0.
PAGE 22
LTC1775 U TYPICAL APPLICATIO S –5V/2.5A Positive to Negative Converter RF 1Ω VIN 5V TO 10V CF 0.1µF 1 2 EXTVCC VIN SYNC TK 16 M1 Si4412DY 15 3 CC1 2.2nF CSS RC 10k 0.1µF CC2 220pF 14 SW RUN/SS LTC1775 4 13 TG FCB 5 12 BOOST ITH 6 7 8 INTVCC SGND VOSENSE VPROG BG DB CMDSH-3 D1 MBR140T3 10 + COUT 680µF 6.3V M2 Si4412DY + PGND CIN 220µF 16V L1 12µH CB 0.22µF 11 + CVCC 4.7µF VOUT –5V 2.
PAGE 23
LTC1775 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. GN Package 16-Lead Plastic SSOP (Narrow 0.150) (LTC DWG # 05-08-1641) 0.189 – 0.196* (4.801 – 4.978) 0.009 (0.229) REF 16 15 14 13 12 11 10 9 0.229 – 0.244 (5.817 – 6.198) 0.150 – 0.157** (3.810 – 3.988) 1 0.015 ± 0.004 × 45° (0.38 ± 0.10) 0.007 – 0.0098 (0.178 – 0.249) 2 3 4 5 6 7 0.053 – 0.068 (1.351 – 1.727) 8 0.004 – 0.0098 (0.102 – 0.249) 0° – 8° TYP 0.016 – 0.050 (0.406 – 1.270) 0.0250 (0.
PAGE 24
LTC1775 U TYPICAL APPLICATIO S 2.5V/5A Adjustable Output RF 1Ω LTC1775 16 VIN EXTVCC 15 2 SYNC TK 14 3 SW RUN/SS CC2 220pF 4 5 6 7 OPEN 8 TG FCB ITH SGND BOOST INTVCC VOSENSE VPROG BG PGND M1 1/2 FDS8936A L1 6.1µH 13 12 D1 MBRS140 R2 11k 1% M2 1/2 FDS8936A R1 10k 1% DB CB CMDSH-3 0.22µF 11 10 9 CIN 15µF 35V ×3 + CF 0.1µF 1 CC1 2.2nF CSS RC INTVCC 0.1µF 10k VIN 5V TO 25V + CVCC 4.7µF VOUT 2.