LTC3878 Linear Technology Corporation, LTC3878 Datasheet
LTC3878
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LTC3878 Summary of contents
Page 1
... Soft-start capability for supply sequencing is accomplished through an external timing capacitor. The current limit is user programmable. The LTC3878 allows operation from 4V to 38V at the input and from 0.8V to 90% V available in a small 16-pin narrow SSOP package ...
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... LTC3878EGN#PBF LTC3878EGN#TRPBF LTC3878IGN#PBF LTC3878IGN#TRPBF Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi label on the shipping container. Consult LTC Marketing for information on non-standard lead based fi nish parts. For more information on lead free part marking, go to: For more information on tape and reel specifi ...
Page 3
... • 110°C/ Note 3: The LTC3878 is tested in a feedback loop that adjusts V achieve a specifi ed error amplifi er output voltage (I l The denotes the specifi cations which apply over the full operating = 25° 15V unless otherwise noted. A ...
Page 4
... LTC3878 TYPICAL PERFORMANCE CHARACTERISTICS Transient Response FCM (Forced Continuous Mode) www.datasheet4u.com V (AC) OUT 50mV/DIV I L 10A/DIV I LOAD 10A/DIV 50μs/DIV LOAD STEP 0A TO 10A 12V 1.2V OUT FCB = 0V SW FREQ = 400kHz FIGURE 7 CIRCUIT Transient Response DCM (Discontinuous Mode) V (AC) OUT ...
Page 5
... I CURRENT (A) ON 3878 G13 Maximum V Current Sense DS Threshold vs RUN/SS Voltage 140 120 100 1.5 2.0 2.5 2.0 RUN/SS VOLTAGE (V) 3878 G16 LTC3878 I Voltage vs Load Current TH 2.5 2.3 = 1.2V 2.1 1.9 1.7 1.5 1.3 1.1 0.9 CONTINUOUS MODE 0.7 DISCONTINUOUS MODE 0 LOAD CURRENT (A) 3878 G11 Current Limit Foldback 140 RNG ...
Page 6
... TEMPERATURE (°C) INTV Load Regulation CC 0 –0.2 –0.4 –0.6 –0.8 –1.0 –1.2 –1.4 –1.6 –1.8 –2 INTV LOAD CURRENT (mA) CC Effi ciency: LTC3878 vs LTC1778 LOAD CURRENT (A) 6 Shutdown Current vs Input Voltage ...
Page 7
... TG (Pin 15): Top Gate Drive. This pin drives the gate of the top N-channel power MOSFET between SW and BOOST. BOOST (Pin 16): Boosted Floating Driver Supply. The (+) terminal of the bootstrap capacitor, C node. This node swings from (INTV V + (INT IN LTC3878 . OUT . CC (Pin 11): Internal 5.3V Regulator Output. The . CC and the (– ...
Page 8
... LTC3878 FUNCTIONAL DIAGRAM 7 www.datasheet4u.com OST 0. CMP – 1.4V V RNG 3 V RNG 0.7V 3.3μA 1 240k NEG + CLMP – POS + CLMP – – – 1.16V 160μA 1.7mS EA + – 0. FCB 0.8V – (10pF FCNT R DSS 20k + I REV – ...
Page 9
... Pin 9 is EXTV , but on the LTC3878 connect. CC The other notable difference is that the shutdown latch- off timer is removed. The LTC3878 should be a drop in, pin-for-pin replacement in most applications that do not use EXTV . The LTC3878 should be tested and verifi each application without assuming compatibility ...
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... LTC3878 APPLICATIONS INFORMATION The basic LTC3878 application circuit is shown on the fi rst page of this data sheet. External component selection is largely determined by maximum load current and begins www.datasheet4u.com with the selection of sense resistance and power MOSFET switches. The LTC3878 uses the on-resistance of the syn- chronous power MOSFET to determine the inductor current ...
Page 11
... Conversely, raising the operating frequency degrades effi ciency but reduces component size. The operating frequency of LTC3878 applications is de- termined implicitly by the one-shot timer that controls the values. A on-time ...
Page 12
... Minimum Off-Time and Dropout Operation The minimum off-time, t OFF(MIN) required for the LTC3878 to turn on the bottom MOSFET, trip the current comparator and then turn off the bottom MOSFET. This time is typically about 220ns. The minimum off-time limit imposes a maximum duty cycle of t ...
Page 13
... ESR in the range of 0.5Ω to 2Ω. High performance though-hole capacitors may also be used – 1 but an additional ceramic capacitor in parallel is recom- V OUT mended to reduce the effect of lead inductance. LTC3878 = 2V IN /2. This simple worst-case condition is com- is primarily determined by the ESR OUT is approximately bounded by: OUT ⎛ ...
Page 14
... Gate charge demands are greatest in high frequency low duty factor applications under high dI/dt load steps and at start-up. Setting Output Voltage The LTC3878 output voltage is set by an external feed- back resistive divider carefully placed across the output, as shown in Figure 5. The regulated output volt age is determined by: ⎛ ...
Page 15
... Soft-Start with the RUN/SS Pin The RUN/SS pin both enables the LTC3878 and provides a means of programmable current limited soft-start. Pulling the RUN/SS pin below 0.7V puts the LTC3878 into a low quiescent current shutdown (I pin allows an internal 1.2μA current source to charge up the external timing capacitor C pulled all the way to ground, there is a delay before start- ing ...
Page 16
... It is often useful to analyze individual losses to determine what is limiting the effi ciency and which change would produce the most improvement. Although all dissipative elements in the circuit produce losses, four main sources account for most of the losses in LTC3878 circuits losses. These arise from the resistances of the MOSFETs, inductor and PC board traces and cause the effi ...
Page 17
... CMDSH-3 16 BOOST C B LTC3878 0.22μ PGND VCC 4.7μF 11 INV Figure 7. Design Example: 1.2V/15A at 400kHz LTC3878 ⎛ ⎞ – ⎝ ⎜ ⎠ ⎟ 396 kHz • • ⎛ ⎞ ...
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... Board Layout Checklist The LTC3878 PC board layout can be designed with or is less than the without a ground plane. A ground plane is generally pre- J ferred based on performance and noise concerns. When using a ground plane, use a dedicated ground plane Ω = ...
Page 19
... TH C VCC 6 11 SGND INTV Figure 8. LTC3878 Layout Diagram Without Ground Plane LTC3878 , close to the IN decoupling capacitor C CC and PGND pins pin decoupling C closely to the – – ...
Page 20
... SW RNG 13 PGND VCC 4.7μF 11 INV 0.1μF 1Ω 4.5V to 24V Input, 1.8V/10A Output at 500kHz D B CMDSH-3 16 BOOST C B LTC3878 0.22μ PGND VCC 4.7μF 11 INV 0.1μF 2.2Ω 4.5V TO 14V C + IN1 ...
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... CMDSH-3 16 BOOST C B LTC3878 0.22μ PGND VCC 4.7μF 11 INV 0.1μF 2.2Ω LTC3878 V IN 4.5V TO 32V + C C IN1 IN2 4.7μF 22μF 50V 35V M1 L1 BSC093N04LS 2.2μH V OUT OUT2 OUT1 47μF 330μF BSC093N04LS 6.3V 2.5V ...
Page 22
... VCC 4.7μF 11 INV 0.1μF 2.2Ω 2.7M Positive-to-Negative Converter, –5V/5A at 300kHz D B CMDSH-3 16 BOOST C B LTC3878 0.22μ PGND VCC 4.7μF 11 INV 0.1μF 2.2Ω 13V TO 32V + ...
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... TYP .008 – .012 (0.203 – 0.305) TYP INCHES (MILLIMETERS) LTC3878 .189 – .196* (4.801 – 4.978) .009 (0.229 REF .150 – .157** (3.810 – 3.988) 1 ...
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... LTC3878 RELATED PARTS PART NUMBER DESCRIPTION LTC3608/LTC3609 8A/6A Monolithic Synchronous Step-Down DC/DC Converters Optimized for High Step-Down Ratios, V LTC3610/LTC3611 12A/10A Monolithic Synchronous Step-Down DC/DC www.datasheet4u.com Converters LTC3708 Dual, 2-Phase, Constant On-Time No R Step-Down Controller with Output Tracking LTC3728 2-Phase 550kHz, Dual Synchronous Step-Down Controller ...