LTC3855 LINER [Linear Technology], LTC3855 Datasheet

Page 1

FEATURES n Wide V Range: 4.5V to 38V ±0.67% Output Voltage Accuracy Over Temperature, Differential Output Voltage Sensing, Allowing Up to ±500mV Line Loss at Remote Ground on Channel 1 n ±1% Output on the Independent 2nd ...

Page 2

LTC3838 ABSOLUTE MAXIMUM RATINGS V Voltage ................................................. –0.3V to 40V IN BOOST1, BOOST2 Voltages ....................... –0.3V to 46V SW1, SW2 Voltages ...................................... –5V to 40V INTV , DRV , DRV , EXTV CC CC1 CC2 CC PGOOD2, RUN1, RUN2, (BOOST1-SW1), ...

Page 3

ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL LTC3838EUHF#PBF LTC3838EUHF#TRPBF LTC3838IUHF#PBF LTC3838IUHF#TRPBF LTC3838EFE#PBF LTC3838EFE#TRPBF LTC3838IFE#PBF LTC3838IFE#TRPBF Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult ...

Page 4

LTC3838 ELECTRICAL CHARACTERISTICS junction temperature range, otherwise specifications are at T SYMBOL PARAMETER Current Sensing V Maximum Valley Current Sense Threshold SENSE(MAX)1,2 + – (V – SENSE1,2 SENSE1,2 V Minimum Valley Current Sense Threshold SENSE(MIN)1,2 + – (V ...

Page 5

ELECTRICAL CHARACTERISTICS junction temperature range, otherwise specifications are at T SYMBOL PARAMETER Internal V Regulator CC V Internally Regulated DRV DRVCC1 CC1 DRV Load Regulation CC1 V EXTV Switchover Voltage EXTVCC CC EXTV Switchover Hysteresis CC EXTV to DRV Voltage ...

Page 6

LTC3838 TYPICAL PERFORMANCE CHARACTERISTICS Transient Response (Forced Continuous Mode) I LOAD 10A/DIV V OUT 50mV/DIV AC-COUPLED I L 10A/DIV 50μs/DIV LOAD TRANSIENT = 0A TO 15A 12V 1.2V OUT FIGURE 17 CIRCUIT, CHANNEL ...

Page 7

TYPICAL PERFORMANCE CHARACTERISTICS Regular Soft Start-Up RUN1 5V/DIV TRACK/SS1 200mV/DIV V OUT 500mV/DIV 3838 G09 C = 10nF 1ms/DIV 12V 1.2V OUT FORCED CONTINUOUS MODE FIGURE 17 CIRCUIT, CHANNEL SGND RNG1 ...

Page 8

LTC3838 TYPICAL PERFORMANCE CHARACTERISTICS Output Regulation vs Input Voltage 0 0.6V OUT LOAD V NORMALIZED 15V OUT IN 0.1 0 –0.1 CHANNEL 1 CHANNEL 2 –0 ...

Page 9

TYPICAL PERFORMANCE CHARACTERISTICS Current Sense Voltage vs ITH Voltage 120 FORCED CONTINUOUS MODE 100 – RNG –40 RNG V = 0.6V RNG –60 0 0.4 0.8 1.2 1.6 2 ...

Page 10

LTC3838 PIN FUNCTIONS (QFN/TSSOP) PHASMD (Pin 4/Pin 8): Phase Selector Input. This pin determines the relative phases of channels and the CLKOUT signal. With zero phase being defined as the rising edge of TG1: Pulling this pin to SGND locks ...

Page 11

PIN FUNCTIONS (QFN/TSSOP) – V (Pin 13/Pin 17): Differential Output Sense OUTSENSE1 Amplifier (–) Input of Channel 1. Connect this pin to the negative terminal of the output load capacitor SENSE1 , SENSE2 (Pins 14, 37/Pins ...

Page 12

LTC3838 PIN FUNCTIONS (QFN/TSSOP) SW1, SW2 (Pins 21, 30/Pins 25, 34): Switch Node Con- nection to Inductors. Voltage swings are from a diode voltage below ground The (–) terminal of the IN bootstrap capacitor connects ...

Page 13

FUNCTIONAL DIAGRAM 1-2μA 5μA PTAT RUN + EN_DRV – 1.2V + – ~0.8V – SENSE V IN ONE-SHOT 250k TIMER 250k FORCED CONTINUOUS MODE MODE/PLLIN PHASE MODE/CLK DETECTOR DETECT CLK1 RT CLOCK PLL/ CLK2 GENERATOR TO CHANNEL ...

Page 14

LTC3838 OPERATION (Refer to Functional Diagram) Main Control Loop The LTC3838 is a controlled on-time, valley current mode step-down DC/DC dual controller with two channels operating out of phase. Each channel drives both main and synchronous N-channel MOSFETs. The two ...

Page 15

OPERATION (Refer to Functional Diagram) from V and can output 5.3V to DRV IN an internal EXTV switch (with on-resistance of around CC 2Ω) can short the EXTV pin to DRV CC If the EXTV pin is below the EXTV ...

Page 16

LTC3838 OPERATION (Refer to Functional Diagram) If the MODE/PLLIN pin is left open or connected to signal ground, the channel will transition into discontinuous mode operation, where a current reversal comparator (I off the bottom MOSFET ( the ...

Page 17

OPERATION (Refer to Functional Diagram) The system can be configured for up to 12-phase opera- tion with a multichip solution. Typical configurations are shown in Table 2 to interleave the phases of the channels. Table 1 PHASMD SGND FLOAT Channel ...

Page 18

LTC3838 APPLICATIONS INFORMATION Once the required output voltage and operating frequency have been determined, external component selection is driven by load requirement, and begins with the selec- tion of inductors and current sense method (either sense resistors R or inductor ...

Page 19

APPLICATIONS INFORMATION LTC3838 + V V OUTSENSE1 R R FB2 FB1 Figure 2. Differential Output Sensing Used to Correct Line Loss Variations in a High Power Distributed System with a Shared Ground Plane enough so that it will not affect ...

Page 20

LTC3838 APPLICATIONS INFORMATION The inductor value has a direct effect on ripple current. The inductor ripple current ∆I decreases with higher L inductance or frequency and increases with higher V ⎛ ⎞ ⎛ ⎞ ΔI = OUT OUT ...

Page 21

APPLICATIONS INFORMATION R Inductor Current Sensing SENSE The LTC3838 can be configured to sense the inductor currents through either low value series current sensing resistors ( inductor DC resistance (DCR). The SENSE choice between the two current sensing ...

Page 22

LTC3838 APPLICATIONS INFORMATION For example, Figure 4a illustrates the voltage waveform across a 2mΩ sense resistor with a 2010 footprint for a 1.2V/15A converter operating at 100% load. The waveform is the superposition of a purely resistive component and a ...

Page 23

APPLICATIONS INFORMATION < 10A, set R to 10Ω and C to 1000pF . This will provide good starting point. The filter components need to be placed close to the IC. The positive and negative sense traces need ...

Page 24

LTC3838 APPLICATIONS INFORMATION The maximum power loss related to duty cycle, and will occur in continuous mode at the maximum input voltage – V • IN(MAX) OUT = P R1 LOSS ...

Page 25

APPLICATIONS INFORMATION The term (1 + δ) is generally given for a MOSFET in the form of a normalized R vs temperature curve in the DS(ON) power MOSFET data sheet. For low voltage MOSFETs, 0.5% per degree (°C) can be ...

Page 26

LTC3838 APPLICATIONS INFORMATION more attractive since it can provide a larger capacitance for more damping. An aluminum-electrolytic capacitor with a ripple current rating that is high enough to handle all of the ripple current by itself will be very large. ...

Page 27

APPLICATIONS INFORMATION C Selection OUT The selection of output capacitance C determined by the effective series resistance, ESR, to minimize voltage ripple. The output voltage ripple ∆V in continuous mode is determined by: ⎛ 1 ΔV ≤ Δ ...

Page 28

LTC3838 APPLICATIONS INFORMATION and BOOST capacitor under all operating conditions. Vari- able frequency in response to load steps offers superior transient performance but requires higher instantaneous gate drive. Gate charge demands are greatest in high frequency low duty factor applications ...

Page 29

APPLICATIONS INFORMATION For applications where the main input power never exceeds 5.3V, tie the DRV and DRV CC1 CC2 through a small resistor, (such as 1Ω to 2Ω) as shown in Figure 8 to minimize the voltage drop caused by ...

Page 30

LTC3838 APPLICATIONS INFORMATION When a channel is configured to soft-start by itself, a ca- pacitor should be connected to its TRACK/SS pin. TRACK pulled low until the RUN pin voltage exceeds 1.2V and UVLO is released, at which ...

Page 31

APPLICATIONS INFORMATION V must be set higher than V OUT1 OUT2 ratiometric tracking, the master channel’s feedback divider can be also used to provide TRACK/SS voltage for the slave channel, since the additional divider, if used, should be of the ...

Page 32

LTC3838 APPLICATIONS INFORMATION I LOAD CLOCK INPUT PHASE AND FREQUENCY LOCKED SW V OUT Figure 10. Phase and Frequency Locking Behavior During Transient Conditions mode at light load and switch into continuous conduction at the R programmed frequency as load ...

Page 33

APPLICATIONS INFORMATION TG- TOP GS MOSFET) DEAD-TIME DELAYS BOTTOM MOSFET NEGATIVE INDUCTOR CURRENT V IN FCM IN SW DURING BG-TG DEAD TIME, DURING TG-BG DEAD TIME, NEGATIVE INDUCTOR CURRENT THE RATE ...

Page 34

LTC3838 APPLICATIONS INFORMATION If the maximum duty cycle is reached, due to a drooping input voltage for example, the output will drop out of regulation. The minimum input voltage to avoid dropout is OUT V IN(MIN) D MAX ...

Page 35

APPLICATIONS INFORMATION The regulator loop response can also be checked by looking at the load transient response. An output current pulse of 20% to 100% of full-load current having a rise time of 1μs to 10μs will produce V transient-response ...

Page 36

LTC3838 APPLICATIONS INFORMATION to the inductor current setpoint. A load transient will result in a quick change of this load current setpoint, i.e., a negative spike of the first derivative of the ITH voltage. The LTC3838 uses a detect transient ...

Page 37

APPLICATIONS INFORMATION as long as the OV condition is not present. When inductor current drops to zero and starts to reverse, BG will turn back on in forced continuous mode (e.g., the MODE/ PLLIN pin tied to INTV , or ...

Page 38

LTC3838 APPLICATIONS INFORMATION 3. DRV current. This is the sum of the MOSFET driver CC and INTV control currents. The MOSFET driver cur- CC rents result from switching the gate capacitance of the power MOSFETs. Each time a MOSFET gate ...

Page 39

APPLICATIONS INFORMATION V IN 4.5V TO 26V + C IN2 C IN1 10μF 220μF 3 3.57k MT1 L1 0.56μH V OUT1 1.2V 15A + C C OUT1 OUT2 100μF 330μ MB1 100 FORCED CONTINUOUS MODE DISCONTINUOUS MODE 90 ...

Page 40

LTC3838 APPLICATIONS INFORMATION Set the inductor value to give 40% ripple current at maxi- mum V using the adjusted operating frequency: IN ⎛ ⎞ ⎛ 1. 1– ⎝ ⎜ ⎠ ⎟ ⎝ ⎜ 350kHz • 40% • 15A ...

Page 41

APPLICATIONS INFORMATION These numbers show that careful attention should be paid to proper heat sinking when operating at higher ambient temperatures. Select the C capacitors to give ample capacitance and IN RMS ripple current rating. Consider worst-case duty cycles per ...

Page 42

LTC3838 APPLICATIONS INFORMATION DTR2 – SENSE2 R + FB2(2) SENSE2 V FB2 C SS2 R FB1(2) TRACK/SS2 R ITH2(2) C ITH1(2) R ITH1(2) ITH2 C ITH2(2) V RNG2 PHASMD MODE/PLLIN LOCALIZED CLKOUT SGND TRACE SGND RNG1 ...

Page 43

APPLICATIONS INFORMATION BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH. • The top N-channel MOSFETs of the two channels have to be located within a short distance from (preferably <1cm) ...

Page 44

LTC3838 APPLICATIONS INFORMATION • The path formed by the top and bottom N-channel MOSFETs, and the C capacitors should have short IN leads and PCB trace. The (–) terminal of output capaci- tors should be connected close to the (–) ...

Page 45

APPLICATIONS INFORMATION from cycle to cycle in a well designed, low noise PCB implementation. Variation in the phase of SW node pulse can suggest noise pickup at the current or voltage sensing inputs or inadequate loop compensation. Overcompensa- tion of ...

Page 46

LTC3838 TYPICAL APPLICATIONS V IN 4.5V TO 38V + C IN2 C IN1 10μF 100μF 3 3.57k MT1 L1 0.56μH V OUT1 1.2V 15A + C C OUT1 OUT2 100μF 330μ MB1 100 FORCED CONTINUOUS MODE DISCONTINUOUS MODE ...

Page 47

TYPICAL APPLICATIONS 26V + C IN2 C IN1 10μF 220μF 3 100Ω 100Ω MT1 0.47μH V 0.002Ω OUT1 1.2V 12A + C C OUT1 OUT2 100μF 330μ MB1 100k 60.4k 10k ...

Page 48

LTC3838 TYPICAL APPLICATIONS V IN 4.5V TO 14V + C IN2 C IN1 22μF 180μF 4 2.55k MT1 L1 0.36μH V OUT 1.2V 50A + C C OUT1 OUT2 100μF 330μ MB1 C : SANYO 16SVP180MX IN1 C ...

Page 49

TYPICAL APPLICATIONS V IN 6.5V TO 34V + C IN2 C IN1 10μF 220μF 3 20Ω 20Ω MT1 2.2μH V 0.002Ω OUT1 5V 12A + C OUT2 C OUT1 150μF 100μF 2 MB1 100k 100 90 80 ...

Page 50

LTC3838 TYPICAL APPLICATIONS 14V + C IN2 C IN1 10μF 2.2Ω 39μF 3 10Ω 10Ω 0.1μF MT1 0.8μH V 0.008Ω OUT1 5V 2.2Ω OUT1 47μF 2 MB1 73.2k 47pF 100k 2.5 ...

Page 51

PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ 5.50 0.05 4.10 0.05 3.00 REF 5.00 0.10 PIN 1 TOP MARK (SEE NOTE 6) 7.00 0.10 NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE M0-220 VARIATION WHKD 2. DRAWING NOT TO SCALE 3. ...

Page 52

LTC3838 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ 4.75 REF 6.60 ±0.10 4.50 REF SEE NOTE 4 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 0.50 – 0.75 (.020 – .030) (.0035 – .0079) NOTE: 1. ...

Page 53

REVISION HISTORY REV DATE DESCRIPTION A 6/12 Electrical specs clarification, 4.6V EXTV Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa- tion ...

Page 54

... LTC3880/LTC3880-1 Dual Output PolyPhase Step-Down DC/DC Controller with Digital Power System Management LTC3869/LTC3869-2 Dual Output, 2-Phase Synchronous Step-Down DC/DC Controller, with Accurate Current Sharing LTC3855 Dual Output, 2-phase, Synchronous Step-Down DC/DC Controller with Diff Amp and DCR Temperature Compensation LTC3856 Single Output 2-Channel Synchronous Step-Down DC/DC ...