LTC3615E

Part: LTC3615 / LTC3615-1 — Linear Technology (Analog Devices)

Type: Dual Synchronous Step-Down DC/DC Converter

Description: 2.25–5.5 V input, dual 3 A synchronous step-down regulator with up to 94% efficiency, 0.6 V minimum output, switching frequency up to 4 MHz and 130 μA Burst Mode quiescent current.

Operating Conditions:

• Supply voltage: 2.25–5.5 V
• Operating junction temperature: -40 to 125 °C (E/I grades; H grade -40 to 150 °C, MP grade -55 to 150 °C — grade-dependent, see Order Information)
• Switching frequency: adjustable up to 4 MHz; synchronization range 0.4–4 MHz
• Output voltage: down to 0.6 V

Absolute Maximum Ratings:

• SVIN voltage: -0.3 to 6 V
• PVIN1, PVIN2 voltage: -0.3 V to SVIN + 0.3 V
• PGOOD1, PGOOD2 voltage: -0.3 to 6 V
• Operating junction temperature: -55 to 150 °C
• Storage temperature: -65 to 150 °C

Key Specs:

• Feedback voltage (internal reference): 0.6 V typ; 0.592–0.608 V (0 to 85 °C), 0.590–0.610 V (-40 to 125 °C)
• Line regulation: 0.2 %/V max (SVIN = PVIN = 2.25–5.5 V)
• Sleep-mode supply current: 95 μA typ, 130 μA max (one channel, internal compensation)
• Shutdown current: 0.1 μA typ, 1 μA max
• Top/bottom switch on-resistance: 75 mΩ / 55 mΩ typ (PVIN = 3.3 V)
• Top switch current limit: 6 A typ (4.5–7.5 A, duty cycle <35%)
• Internal oscillator: 2.25 MHz typ (1.85–2.65 MHz with RRT = 178k)
• Internal soft-start time: 1.1 ms typ (0.65–1.7 ms)
• Power-good window: ±6% typ entering, ±9% typ leaving; blanking 105 μs typ

Features:

• Dual outputs with 2 × 3 A capability
• ±1% output voltage accuracy
• Burst Mode operation, pulse-skipping or forced continuous mode with adjustable burst clamp
• 100% duty cycle for low dropout operation
• Programmable slew rate at switch pins
• Internal or external compensation; optional active voltage positioning (AVP)
• Selectable phase shift: 0°/90°/180° (LTC3615) or 140°/180° (LTC3615-1)
• Fixed internal and programmable external soft-start with tracking
• DDR memory mode with IOUT = ±1.5 A

Applications:

• Point-of-load supplies
• Distributed power supplies
• Portable computer systems
• DDR memory termination
• Handheld devices

Package:

• 24-lead TSSOP (thermally enhanced)
• 24-lead 4 mm × 4 mm QFN

• n High Efficiency: Up to 94%
• n Dual Outputs with 2 × 3A Output Current Capability
• n Low Output Ripple Burst Mode ® Operation: I Q = 130μA
• n 2.25V to 5.5V Input Voltage Range
• n ±1% Output Voltage Accuracy
• n Output Voltages Down to 0.6V
• n Programmable Slew Rate at Switch Pins
• n Low Dropout Operation: 100% Duty Cycle
• n Shutdown Current ≤1μA
• n Adjustable Switching Frequency Up to 4MHz
• n Internal or External Compensation
• n Selectable Pulse-Skipping/Forced Continuous/ Burst Mode Operation with Adjustable Burst Clamp
• n Optional Active Voltage Positioning (AVP) with Internal Compensation
• n Selectable 0°/90°/180° (LTC3615) or selectable 140°/180° (LTC3615-1) Phase Shift Between Channels
• n Fixed Internal and Programmable External Soft-Start
• n Accurate Start-Up T racking Capability
• n DDR Memory Mode I OUT = ±1.5A
• n Available in 4mm × 4mm QFN-24 and TSSOP-24 Packages

• n Point-of-Load Supplies
• n Distributed Power Supplies
• n Portable Computer Systems
• n DDR Memory Termination
• n Handheld Devices

The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at T A = 25°C (Note 2), SV IN = PV INx = 3.3V, R T = 178k, R SRLIM = 40.2k, unless otherwise specified.

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In most applications, the LTC3615 does not dissipate much heat due to its high efficiency. However, in applications where the LTC3615 is running at high ambient

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temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 170°C, all four power switches will be turned off and the SW node will become high impedance.

To prevent the LTC3615 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. To determine whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise is given by:

$$T_{RISE} = P_D \bullet \theta_{JA}$$

where P D is the power dissipated by the regulator, and θ JA is the thermal resistance from the junction of the die to the ambient temperature. The junction temperature, T J , is given by:

$$T_J = T_A + T_{RISE}$$

where T A is the ambient temperature.

As an example, consider this case: the LTC3615 is in dropout at an input voltage of 3.3V with a load current for each channel of 2A at an ambient temperature of 70°C. Assuming a 20°C rise in junction temperature, to 90°C, results in an R DS(ON) of 0.086mΩ (see the graph in the Typical Performance Characteristics section). Therefore, the power dissipated by the part is:

$$P_D = (I_1^2 + I_2^2) \bullet R_{DS(ON)} = 0.69W$$

For the QFN package, the θ JA is 37°C/W.

Therefore, the junction temperature of the regulator operating at 70°C ambient temperature is approximately:

$$T_J = 0.69W \bullet 37°C/W + 70°C = 95°C$$

Note that for very low input voltage, the junction temperature will be higher due to increased switch resistance RDS(ON) . It is not recommended to use full load current at high ambient temperature and low input voltage.

To maximize the thermal performance of the LTC3615, the Exposed Pad should be soldered to a ground plane. See the PC Board Layout Checklist.

External Compensation, Forced Continuous Operation, In-Phase Switching, Slew Rate Limit, Common PGOOD Output

VOUT1 Waveform

VOUT2 Waveform