Datasheet LT3757, LT3757A (Analog Devices) - 10

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APPLICATIONS INFORMATION INTVCC Regulator Bypassing and Operation
The LT3757 uses packages with an Exposed Pad for An internal, low dropout (LDO) voltage regulator produces enhanced thermal conduction. With proper soldering to the 7.2V INTV the Exposed Pad on the underside of the package and a CC supply which powers the gate driver, as shown in Figure 1. If a low input voltage operation is full copper plane underneath the device, thermal resis- expected (e.g., supplying power from a lithium-ion battery tance (θJA) will be about 43°C/W for the DD package and or a 3.3V logic supply), low threshold MOSFETs should 40°C/W for the MSE package. For an ambient board tem- be used. The LT3757 contains an undervoltage lockout perature of TA = 70°C and maximum junction temperature comparator A8 and an overvoltage lockout compara- of 125°C, the maximum IDRIVE (IDRIVE(MAX)) of the DD tor A9 for the INTV package can be calculated as: CC supply. The INTVCC undervoltage (UV) threshold is 2.7V (typical), with 100mV hysteresis, (TJ − TA) 1.28W to ensure that the MOSFETs have sufficient gate drive I − I − 1.6mA DRIVE(MAX) = ( Q = θ V voltage before turning on. The logic circuitry within the JA • VIN) IN LT3757 is also powered from the internal INTV The LT3757 has an internal INTV CC supply. CC IDRIVE current limit function to protect the IC from excessive on-chip power The INTVCC overvoltage (OV) threshold is set to be 17.5V dissipation. The I (typical) to protect the gate of the power MOSFET. When DRIVE current limit decreases as the VIN increases (see the INTV INTV CC Minimum Output Current vs VIN CC is below the UV threshold, or above the OV thresh- graph in the Typical Performance Characteristics section). old, the GATE pin will be forced to GND and the soft-start If I operation will be triggered. DRIVE reaches the current limit, INTVCC voltage will fall and may trigger the soft-start. The INTVCC regulator must be bypassed to ground imme- Based on the preceding equation and the INTV diately adjacent to the IC pins with a minimum of 4.7µF CC Minimum Output Current vs V ceramic capacitor. Good bypassing is necessary to supply the IN graph, the user can calculate the maximum MOSFET gate charge the LT3757 can drive at high transient currents required by the MOSFET gate driver. a given VIN and switch frequency. A plot of the maximum In an actual application, most of the IC supply current is QG vs VIN at different frequencies to guarantee a minimum used to drive the gate capacitance of the power MOSFET. 4.5V INTVCC is shown in Figure 2. The on-chip power dissipation can be a significant concern As illustrated in Figure 2, a trade-off between the operat- when a large power MOSFET is being driven at a high fre- ing frequency and the size of the power MOSFET may quency and the VIN voltage is high. It is important to limit be needed in order to maintain a reliable IC junction the power dissipation through selection of MOSFET and/ or operating frequency so the LT3757 does not exceed its maximum junction temperature rating. The junction tem- 300 perature TJ can be estimated using the following equations: 250 T 300kHz J = TA + PIC • θJA 200 TA = ambient temperature (nC) 150 θJA = junction-to-ambient thermal resistance Q G P 100 IC = IC power consumption 1MHz = V 50 IN • (IQ + IDRIVE) IQ = VIN operation IQ = 1.6mA 0 0 5 10 15 20 25 30 35 40 IDRIVE = average gate drive current = f • QG VIN (V) 3757 F02 f = switching frequency
Figure 2. Recommended Maximum QG vs VIN at Different
Q
Frequencies to Ensure INTVCC Higher Than 4.5V
G = power MOSFET total gate charge Rev. F 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts