LTC3709 Linear Technology, LTC3709 Datasheet
LTC3709
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LTC3709 Summary of contents
Page 1
... A TRACK pin is provided for tracking or sequencing the output voltage among several LTC3709 chips or an LTC3709 and other DC/DC regulators. Soft-start is ac- complished using an external timing capacitor. Fault protection is provided by an output overvoltage com- parator and an optional short-circuit shutdown timer. The current limit level is user programmable ...
Page 2
... OS DIFFOUT PACKAGE 32-LEAD (5mm × 5mm) PLASTIC QFN EXPOSED PAD IS SGND (PIN 33) MUST BE SOLDERED TO PCB T = 125°C, θ = 34°C/ W JMAX JA UH PART MARKING LTC3709EUH = 5V, unless otherwise noted. CC MIN TYP 2.4 25 – 35 ● 0.594 0.600 0.02 – 0.12 1.3 1.45 90 116 ...
Page 3
... Measured with a AC Pulse at FCB Pin I = 180µ 1.8V ON1 EXTPLL I = 180µ 0.6V ON1 EXTPLL I = 180µ 1.8V ON2 INTPLL I = 180µ 0.6V ON2 INTPLL LTC3709 MIN TYP MAX UNITS 124 144 166 mV 86 101 119 mV 177 202 234 mV – – – ...
Page 4
... T J dissipation P as follows: D LTC3709EUH • 34°C/ Note 3: The LTC3709 is tested in a feedback loop that adjusts V achieve a specified error amplifier output voltage (I 4 ● The denotes specifications which apply over the full operating = 25° DRV = 5V, unless otherwise noted ...
Page 5
... Efficiency vs Load Current 100 V = 12V 1.5V OUT f = 220kHz 3709 G05 100 1000 LOAD (mA) Quiescent Current at V 3.0 2.8 2.6 2.4 2.2 2.0 –40 – TEMPERATURE (°C) 3709 G08 LTC3709 3709 G03 10000 100000 3709 G06 = 3709 G09 3709f 5 ...
Page 6
... LTC3709 TYPICAL PERFOR A CE CHARACTERISTICS Shutdown Current –40 – TEMPERATURE (°C) V Pin Input Current FB –20 –25 –30 –35 –40 –45 –50 40 –40 – TEMPERATURE (°C) UVLO Threshold 4.5 4.3 4.1 3.9 3.7 3.5 –40 – TEMPERATURE (° ...
Page 7
... DIFFOUT (Pin 8): The Output of the Differential Amplifier EXTLPF (Pin 10): Filter Connection for the PLL. This PLL is used to synchronize the LTC3709 with an external clock. INTLPF (Pin 11): Filter Connection for the PLL. This PLL is use to phase shift the second channel to the first channel by 180°. ...
Page 8
... FCB (Pin 31): Forced Continuous and External Clock Input. Tie this pin to ground to force continuous synchro- nous operation operation at light load. Feeding an external clock signal into this pin will synchronize the LTC3709 to the external clock and enable forced continuous mode. V RNG this pin is ten times the nominal sense voltage at maximum output current and can be programmed from 0 ...
Page 9
... 20k + I REV – SHDN OV SHDN TO CHANNEL 2 SWITCH LOGIC 3.3µA – SHED RUN SHDN I THB + – 0. REF 1.4V RUN/SS + 1.4V – LTC3709 0.6V REF + C VCC BOOST SWITCH + SENSE B LOGIC 5V DRV CC C DRVCC BG M2 PGND – SENSE – ...
Page 10
... MOSFET and bottom MOSFET. Releasing the pin allows an internal 1.2µA current source to charge an external soft-start capacitor C voltage reaches 1.4V, the LTC3709 turns on and begins operating with a clamp on the noninverting input of the error amplifier. This input is also the reference input of the error amplifier ...
Page 11
... Dual Phase Operation An internal phase-lock loop (PLL1) ensures that channel 2 operates exactly at the same frequency as channel 1 and is also phase shifted by 180°, enabling the LTC3709 to operate optimally as a dual phase controller. The loop filter . This capacitor is normally B connected to the INTLPF pin provides stability to the PLL ...
Page 12
... The maximum allowed sense volt- age is about 1.3 times this nominal value. + Connecting the SENSE and SENSE The LTC3709 provides the user with an optional method to sense current through a sense resistor instead of using the R of the synchronous MOSFET. When using a sense DS(ON) resistor placed between the source of the synchro- nous MOSFET and ground ...
Page 13
... R losses and the top while the second PLL is for locking onto an external clock. Since the LTC3709 is a constant on-time architecture, the error signal generated by the phase detector of the PLL the top MOSFET switch. The on-time is set by ON ...
Page 14
... If an external clock is present, then channel 1’s on-time will be varied and channel 2 will follow suit. Both PLLs are set up with the same capture range and the fre- quency range that the LTC3709 can be externally synchro- nized to is between 2 • f and 0.5 • frequency setting of the two channels ...
Page 15
... OUT ⎝ ⎠ OUT increases with input voltage, the output ripple connected to the BOOST B is approximately DRV . When the top MOSFET B CC and the BOOST pin IN + DRV . The boost capacitor IN CC LTC3709 from DRV B CC 3709f 15 ...
Page 16
... SENSE Minimum Off-Time and Dropout Operation The minimum off-time t time that the LTC3709 is capable of turning on the bottom MOSFET, tripping the current comparator and turning the MOSFET back off. This time is generally about 250ns. The minimum off-time limit imposes a maximum duty cycle of ...
Page 17
... Figure 3. Maximum Switching Frequency vs Duty Cycle Soft-Start and Latchoff with the RUN/SS Pin The RUN/SS pin provides a means to shut down the LTC3709 as well as a timer for soft-start and overcurrent latchoff. Pulling the RUN/SS pin below 1.4V puts the LTC3709 into a low quiescent current shutdown (I the pin allows an internal 1.2µ ...
Page 18
... TRACK pin is greater than V at the end of the tracking interval. REF The LTC3709 allows the user to set up start-up sequenc- ing among different supplies in either coincident tracking or ratiometric tracking as shown in Figure 6. To implement the coincident tracking, connect an extra resistor divider to the output of supply 1 ...
Page 19
... Although all dissipative elements in the circuit produce losses, four main sources account for most of the losses in LTC3709 circuits losses. These arise from the resistances of the MOSFETs, inductor and PC board traces and cause the efficiency to drop at high output currents ...
Page 20
... MOSFETs, D1, D2 and inductors all in IN OUT one compact area. It may help to have some compo- nents on the bottom side of the board. ground plane including SGND and PGND of LTC3709. Use several larger vias for power components. down. and V to maintain good voltage ...
Page 21
... PC trace spacing? The filter capacitor be- – + tween SENSE and SENSE (C ) should be as close SENSE as possible to the IC. Ensure accurate current sensing with Kelvin connections at the sense resistor as shown in Figure 8. R SENSE + – SENSE SENSE (8b) Sensing a Resistor LTC3709 closely VCC closely to the B closely to F 3709 F08 3709f 21 ...
Page 22
... B1 SGND BOOST1 C 0.22µ SGND TG1 7 26 – V SW1 DIFFOUT SENSE1 100pF – V SENSE1 OS LTC3709EUH 10 23 EXTLPF PGND1 1µ INTLPF BG1 DRV CC 1µ BOOST2 BG2 14 19 TG2 PGND2 15 18 – SW2 SENSE2 10Ω ...
Page 23
... 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 represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. LTC3709 BOTTOM VIEW—EXPOSED PAD R = 0.115 ...
Page 24
... FCB TH R 2.86M 100k 4 29 PGOOD TRACK PGOOD D CMDSH SGND BOOST1 C 0.22µ SGND TG1 LTC3709 7 26 – V SW1 DIFFOUT SENSE1 100pF – V SENSE1 OS 23 EXTLPF PGND1 1µF 22 INTLPF BG1 21 NC DRV CC 20 BOOST2 BG2 1µ ...