8.3.1 Input Range (VIN)

The LM25145 operational input voltage range is from 6 V to 42 V. The device is intended for step-down conversions from 12-V, 24-V, 28-V and 36-V unregulated, semiregulated, and fully-regulated supply rails. The application circuit of Figure 31 shows all the necessary components to implement an LM25145-based wide-V_IN step-down regulator using a single supply. The LM25145 uses an internal LDO subregulator to provide a 7.5-V VCC bias rail for the gate drive and control circuits (assuming the input voltage is higher than 7.5 V plus the necessary subregulator dropout specification).

In high voltage applications, take extra care to ensure the VIN pin does not exceed the absolute maximum voltage rating of 55 V during line or load transient events. Voltage ringing on the VIN pin that exceeds the Absolute Maximum Ratings can damage the IC. Use high-quality ceramic input capacitors to minimize ringing. An RC filter from the input rail to the VIN pin (for example, 4.7 Ω and 0.1 µF) provides supplementary filtering at the VIN pin.

8.3.2 Output Voltage Setpoint and Accuracy (FB)

The reference voltage at the FB pin is set at 0.8 V with a feedback system accuracy over the full junction temperature range of ±1%. Junction temperature range for the device is –40°C to +125°C. While dependent on switching frequency and load current levels, the LM25145 is generally capable of providing output voltages in the range of 0.8 V to a maximum of slightly less than VIN. The DC output voltage setpoint during normal operation is set by the feedback resistor network, R_FB1 and R_FB2, connected to the output.

8.3.3 High-Voltage Bias Supply Regulator (VCC)

The LM25145 contains an internal high-voltage VCC regulator that provides a bias supply for the PWM controller and its gate drivers for the external MOSFETs. The input pin (VIN) can be connected directly to an input voltage source up to 42 V. The output of the VCC regulator is set to 7.5 V. However, when the input voltage is below the VCC setpoint level, the VCC output tracks V_IN with a small voltage drop. Connect a ceramic decoupling capacitor between 1 µF and 5 µF from VCC to AGND for stability.

The VCC regulator output has a current limit of 40 mA (minimum). At power up, the regulator sources current into the capacitor connected to the VCC pin. When the VCC voltage exceeds its rising UVLO threshold of 4.93 V, the output is enabled (if EN/UVLO is above 1.2 V) and the soft-start sequence begins. The output remain active until the VCC voltage falls below its falling UVLO threshold of 4.67 V (typical) or if EN/UVLO goes to a standby or shutdown state.

Internal power dissipation of the VCC regulator can be minimized by connecting the output voltage or an auxiliary bias supply rail (up to 13 V) to VCC using a diode D_VCC as shown in Figure 32. A diode in series with the input prevents reverse current flow from VCC to VIN if the input voltage falls below the external VCC rail.

Note that a finite bias supply regulator dropout voltage exists and is manifested to a larger extent when driving high gate charge (Q_G) power MOSFETs at elevated switching frequencies. For example, at V_VIN = 6 V, the VCC voltage is 5.8 V with a DC operating current, I_VCC, of 20 mA. Such a low gate drive voltage may be insufficient to fully enhance the power MOSFETs. At the very least, MOSFET on-state resistance, R_DS(ON), may increase at such low gate drive voltage.

Here are the main considerations when operating at input voltages below 7.5 V:

• Increased MOSFET R_DS(on) at lower V_GS, leading to Increased conduction losses and reduced OCP setpoint.
• Increased switching losses given the slower switching times when operating at lower gate voltages.
• Restricted range of suitable power MOSFETs to choose from (MOSFETs with R_DS(on) rated at V_GS = 4.5 V become mandatory).

8.3.4 Precision Enable (EN/UVLO)

The EN/UVLO input supports adjustable input undervoltage lockout (UVLO) with hysteresis programmed by the resistor values for application specific power-up and power-down requirements. EN/UVLO connects to a comparator-based input referenced to a 1.2-V bandgap voltage. An external logic signal can be used to drive the EN/UVLO input to toggle the output ON and OFF and for system sequencing or protection. The simplest way to enable the operation of the LM25145 is to connect EN/UVLO directly to VIN. This allows self start-up of the LM25145 when V_CC is within its valid operating range. However, many applications benefit from using a resistor divider R_UV1 and R_UV2 as shown in Figure 33 to establish a precision UVLO level.

Use Equation 1 and Equation 2 to calculate the UVLO resistors given the required input turnon and turnoff voltages.

The LM25145 enters a low I_Q shutdown mode when EN/UVLO is pulled below approximately 0.4 V. The internal LDO regulator powers off and the internal bias supply rail collapses, shutting down the bias currents of the LM25145. The LM25145 operates in standby mode when the EN/UVLO voltage is between the hard shutdown and precision enable (standby) thresholds.

8.3.5 Power Good Monitor (PGOOD)

The LM25145 provides a PGOOD flag pin to indicate when the output voltage is within a regulation window. Use the PGOOD signal as shown in Figure 34 for start-up sequencing of downstream converters, fault protection, and output monitoring. PGOOD is an open-drain output that requires a pullup resistor to a DC supply not greater than 13 V. The typical range of pullup resistance is 10 kΩ to 100 kΩ. If necessary, use a resistor divider to decrease the voltage from a higher voltage pullup rail.

When the FB voltage exceeds 94% of the internal reference V_REF, the internal PGOOD switch turns off and PGOOD can be pulled high by the external pullup. If the FB voltage falls below 92% of V_REF, the internal PGOOD switch turns on, and PGOOD is pulled low to indicate that the output voltage is out of regulation. Similarly, when the FB voltage exceeds 108% of V_REF, the internal PGOOD switch turns on, pulling PGOOD low. If the FB voltage subsequently falls below 105% of V_REF, the PGOOD switch is turned off and PGOOD is pulled high. PGOOD has a built-in deglitch delay of 25 µs.

8.3.6 Switching Frequency (RT, SYNCIN)

There are two options for setting the switching frequency, F_SW, of the LM25145, thus providing a power supply designer with a level of flexibility when choosing external components for various applications. To adjust the frequency, use a resistor from the RT pin to AGND, or synchronize the LM25145 to an external clock signal through the SYNCIN pin.

8.3.7 Configurable Soft-Start (SS/TRK)

After the EN/UVLO pin exceeds its rising threshold of 1.2 V, the LM25145 begins charging the output to the DC level dictated by the feedback resistor network. The LM25145 features an adjustable soft-start (set by a capacitor from the SS/TRK pin to GND) that determines the charging time of the output. A 10-µA current source charges this soft-start capacitor. Soft-start limits inrush current as a result of high output capacitance to avoid an overcurrent condition. Stress on the input supply rail is also reduced. The soft-start time, t_SS, for the output voltage to ramp to its nominal level is set by Equation 4.

More simply, calculate C_SS using Equation 5.

The SS/TRK pin is internally clamped to V_FB + 115 mV to allow a soft-start recovery from an overload event. The clamp circuit requires a soft-start capacitance greater than 2 nF for stability and has a current limit of approximately 2 mA.

8.3.8 Voltage-Mode Control (COMP)

The LM25145 incorporates a voltage-mode control loop implementation with input voltage feedforward to eliminate the input voltage dependence of the PWM modulator gain. This configuration allows the controller to maintain stability throughout the entire input voltage operating range and provides for optimal response to input voltage transient disturbances. The constant gain provided by the controller greatly simplifies loop compensation design because the loop characteristics remain constant as the input voltage changes, unlike a buck converter without voltage feedforward. An increase in input voltage is matched by a concomitant increase in ramp voltage amplitude to maintain constant modulator gain. The input voltage feedforward gain, k_FF, is 15, equivalent to the input voltage divided by the ramp amplitude, V_IN/V_RAMP. See Control Loop Compensation for more detail.

8.3.9 Gate Drivers (LO, HO)

The LM25145 gate driver impedances are low enough to perform effectively in high output current applications where large die-size or paralleled MOSFETs with correspondingly large gate charge, Q_G, are used. Measured at V_VCC = 7.5 V, the low-side driver of the LM25145 has a low impedance pulldown path of 0.9 Ω to minimize the effect of dv/dt induced turnon, particularly with low gate-threshold voltage MOSFETs. Similarly, the high-side driver has 1.5-Ω and 0.9-Ω pullup and pulldown impedances, respectively, for faster switching transition times, lower switching loss, and greater efficiency.

The high-side gate driver works in conjunction with an integrated bootstrap diode and external bootstrap capacitor, C_BST. When the low-side MOSFET conducts, the SW voltage is approximately at 0 V and C_BST is charged from VCC through the integrated boot diode. Connect a 0.1-μF or larger ceramic capacitor close to the BST and SW pins.

Furthermore, there is a proprietary adaptive dead-time control on both switching edges to prevent shoot-through and cross-conduction, minimize body diode conduction time, and reduce body diode reverse recovery losses.

8.3.10 Current Sensing and Overcurrent Protection (ILIM)

The LM25145 implements a lossless current sense scheme designed to limit the inductor current during an overload or short-circuit condition. Figure 38 portrays the popular current sense method using the on-state resistance of the low-side MOSFET. Meanwhile, Figure 39 shows an alternative implementation with current shunt resistor, R_S. The LM25145 senses the inductor current during the PWM off-time (when LO is high).

The ILIM pin of the LM25145 sources a reference current that flows in an external resistor, designated R_ILIM, to program of the current limit threshold. A current limit comparator on the ILIM pin prevents further SW pulses if the ILIM pin voltage goes below GND. Figure 40 shows the implementation.

Resistor R_ILIM is tied to SW to use the R_DS(on) of the low-side MOSFET as a sensing element (termed R_DS-ON mode). Alternatively, R_ILIM is tied to a shunt resistor connected at the source of the low-side MOSFET (termed R_SENSE mode). The LM25145 detects the appropriate mode at start-up and sets the source current amplitude and temperature coefficient (TC) accordingly.

The ILIM current with R_DS-ON sensing is 200 µA at 27°C junction temperature and incorporates a TC of +4500 ppm/°C to generally track the R_DS(on) temperature variation of the low-side MOSFET. Conversely, the ILIM current is a constant 100 µA in R_SENSE mode. This controls the valley of the inductor current during a steady-state overload at the output. Depending on the chosen mode, select the resistance of R_ILIM using Equation 6.

Given the large voltage swings of ILIM in R_DS-ON mode, a capacitor designated C_ILIM connected from ILIM to PGND is essential to the operation of the valley current limit circuit. Choose this capacitance such that the time constant R_ILIM · C_ILIM is approximately 6 ns.

Note that current sensing with a shunt component is typically implemented at lower output current levels to provide accurate overcurrent protection. Burdened by the unavoidable efficiency penalty, PCB layout, and additional cost implications, this configuration is not usually implemented in high-current applications (except where OCP setpoint accuracy and stability over the operating temperature range are critical specifications).

8.3.11 OCP Duty Cycle Limiter

In addition to valley current limiting, the LM25145 uses a proprietary duty-cycle limiter circuit to reduce the PWM on-time during an overcurrent condition. As shown in Figure 40, an auxiliary PWM comparator along with a modulated CLAMP voltage limits how quickly the on-time increases in response to a large step in the COMP voltage that typically occurs with a voltage-mode control loop architecture.

As depicted in Figure 41, the CLAMP voltage, V_CLAMP, is normally regulated above the COMP voltage to provide adequate headroom during a response to a load-on transient. If the COMP voltage rises quickly during an overloaded or shorted output condition, the on-time pulse terminates thereby limiting the on-time and peak inductor current. Moreover, the CLAMP voltage is reduced if additional valley current limit events occur, further reducing the average output current.

If the overcurrent condition exists for 128 continuous clock cycles, a hiccup event is triggered and SS is pulled low for 8192 clock cycles before a soft-start sequence is initiated.

8.4.1 Shutdown Mode

The EN/UVLO pin provides ON / OFF control for the LM25145. When the EN/UVLO voltage is below 0.37 V (typical), the device is in shutdown mode. Both the internal bias supply LDO and the switching regulator are off. The quiescent current in shutdown mode drops to 13.5 μA (typical) at V_IN = 24 V. The LM25145 also includes undervoltage protection of the internal bias LDO. If the internal bias supply voltage is below its UVLO threshold level, the switching regulator remains off.

8.4.2 Standby Mode

The internal bias supply LDO has a lower enable threshold than the switching regulator. When the EN/UVLO voltage exceeds 0.42 V (typical) and is below the precision enable threshold (1.2 V typically), the internal LDO is on and regulating. Switching action and output voltage regulation are disabled in standby mode.

8.4.3 Active Mode

The LM25145 is in active mode when the VCC voltage is above its rising UVLO threshold of 5 V and the EN/UVLO voltage is above the precision EN threshold of 1.2 V. The simplest way to enable the LM25145 is to tie EN/UVLO to VIN. This allows self start-up of the LM25145 when the input voltage exceeds the VCC threshold plus the LDO dropout voltage from VIN to VCC.

8.4.4 Diode Emulation Mode

The LM25145 provides a diode emulation feature that can be enabled to prevent reverse (drain-to-source) current flow in the low-side MOSFET. When configured for diode emulation, the low-side MOSFET is switched off when reverse current flow is detected by sensing of the SW voltage using a zero-cross comparator. The benefit of this configuration is lower power loss at no-load and light-load conditions, the disadvantage being slower light-load transient response.

The diode emulation feature is configured with the SYNCIN pin. To enable diode emulation and thus achieve discontinuous conduction mode (DCM) operation at light loads, connect the SYNCIN pin to AGND or leave SYNCIN floating. If forced PWM (FPWM) continuous conduction mode (CCM) operation is desired, tie SYNCIN to VCC either directly or using a pullup resistor. Note that diode emulation mode is automatically engaged to prevent reverse current flow during a prebias start-up. A gradual change from DCM to CCM operation provides monotonic start-up performance.

8.4.5 Thermal Shutdown

The LM25145 includes an internal junction temperature monitor. If the temperature exceeds 175°C (typical), thermal shutdown occurs.

When entering thermal shutdown, the device:

1. Turns off the low-side and high-side MOSFETs;
2. Pulls SS/TRK and PGOOD low;
3. Initiates a soft-start sequence when the die temperature decreases by the thermal shutdown hysteresis of 20°C (typical).

This is a non-latching protection, and, as such, the device will cycle into and out of thermal shutdown if the fault persists.