7.3.1 Control Circuit Overview

The LM25010 employs a control scheme based on a comparator and a one-shot ON timer, with the output voltage feedback (FB) compared to an internal reference (2.5 V). If the FB voltage is below the reference the buck switch is turned on for a time period determined by the input voltage and a programming resistor (R_ON). Following the ON-time the switch remains off for a fixed 260-ns OFF-time, or until the FB voltage falls below the reference, whichever is longer. The buck switch then turns on for another ON-time period. Referring to the Functional Block Diagram, the output voltage is set by R1 and R2. The regulated output voltage is calculated with Equation 1.

The LM25010 requires a minimum of 25-mV of ripple voltage at the FB pin for stable fixed-frequency operation. If the output capacitor’s ESR is insufficient, additional series resistance may be required (R3 in the Functional Block Diagram).

The LM25010 operates in continuous conduction mode at heavy load currents, and discontinuous conduction mode at light load currents. In continuous conduction mode current always flows through the inductor, never decaying to zero during the OFF-time. In this mode the operating frequency remains relatively constant with load and line variations. The minimum load current for continuous conduction mode is one-half the inductor’s ripple current amplitude. Calculate the operating frequency in the continuous conduction mode with Equation 2.

Equation 2.

The buck switch duty cycle is equal to Equation 3.

Equation 3.

Under light load conditions, the LM25010 operates in discontinuous conduction mode, with zero current flowing through the inductor for a portion of the OFF-time. The operating frequency is always lower than that of the continuous conduction mode, and the switching frequency varies with load current. Conversion efficiency is maintained at a relatively high level at light loads because the switching losses diminish as the power delivered to the load is reduced. Calculate the approximate discontinuous mode operating frequency with Equation 4.

Equation 4.

where

• R_L = the load resistance

7.3.2 Start-Up Regulator (V_CC)

A high voltage bias regulator is integrated within the LM25010. The input pin (VIN) can be connected directly to line voltages between 6 V and 42 V. Referring to the Functional Block Diagram and the graph of V_CC vs V_IN, when V_IN is between 6 V and the bypass threshold (nominally 8.9 V), the bypass switch (Q2) is on, and V_CC tracks V_IN within 100 mV to 150 mV. The bypass switch on-resistance is approximately 50 Ω, with inherent current limiting at approximately 100 mA. When VIN is above the bypass threshold, Q2 is turned off, and V_CC is regulated at 7 V. The V_CC regulator output current is limited at approximately 15 mA. When the LM25010 is shutdown using the RON/SD pin, the V_CC bypass switch is shut off, regardless of the voltage at VIN.

When V_IN exceeds the bypass threshold, the time required for Q2 to shut off is approximately 2 µs to 3 µs. The capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at VCC from rising above its absolute maximum rating in response to a step input applied at VIN. C3 must be located as close as possible to the LM25010 pins.

In applications with a relatively high input voltage, power dissipation in the bias regulator is a concern. An auxiliary voltage of between 7.5 V and 14 V can be diode connected to the VCC pin (D2 in Figure 8) to shut off the VCC regulator, reducing internal power dissipation. The current required into the VCC pin is shown in the Typical Performance Characteristics. Internally a diode connects VCC to VIN requiring that the auxiliary voltage be less than V_IN.

The turn-on sequence is shown in Figure 1. When VCC exceeds the undervoltage lockout threshold (UVLO) of 5.25 V (t1 in Figure 1), the buck switch is enabled, and the SS pin is released to allow the softstart capacitor (C6) to charge up. The output voltage V_OUT is regulated at a reduced level which increases to the desired value as the softstart voltage increases (t2 in Figure 1).

Figure 8. Self-Biased Configuration

7.3.3 Regulation Comparator

The feedback voltage at the FB pin is compared to the voltage at the SS pin (2.5 V, ±2%). In normal operation an ON-time period is initiated when the voltage at FB falls below 2.5 V. The buck switch conducts for the ON-time programmed by R_ON, causing the FB voltage to rise above 2.5 V. After the ON-time period the buck switch remains off until the FB voltage falls below 2.5 V. Input bias current at the FB pin is less than 5 nA over temperature.

7.3.4 Overvoltage Comparator

The feedback voltage at FB is compared to an internal 2.9 V reference. If the voltage at FB rises above 2.9 V the ON-time is immediately terminated. This condition can occur if the input voltage, or the output load, changes suddenly. The buck switch remains off until the voltage at FB falls below 2.5 V.

7.3.5 ON-Time Control

The ON-time of the internal buck switch is determined by the R_ON resistor and the input voltage (V_IN), and is calculated with Equation 5.

Equation 5.

The R_ON resistor can be determined from the desired ON-time by re-arranging Equation 5 to Equation 6.

Equation 6.

To set a specific continuous conduction mode switching frequency (Fs), the R_ON resistor is determined from Equation 7.

Equation 7.

In high frequency applications the minimum value for t_ON is limited by the maximum duty cycle required for regulation and the minimum OFF-time of the LM25010 (260 ns, ±15%). The fixed OFF-time limits the maximum duty cycle achievable with a low voltage at VIN. The minimum allowed ON-time to regulate the desired V_OUT at the minimum V_IN is determined from Equation 8.

Equation 8.

7.3.6 Current Limit

Current limit detection occurs during the OFF-time by monitoring the recirculating current through the internal current sense resistor (R_SENSE). The detection threshold is 1.25 A, ±0.25 A. Referring to the Functional Block Diagram, if the current into SGND during the OFF-time exceeds the threshold level the current limit comparator delays the start of the next ON-time period. The next ON-time starts when the current into SGND is below the threshold and the voltage at FB is below 2.5 V. Figure 9 illustrates the inductor current waveform during normal operation and during current limit. The output current I_O is the average of the inductor ripple current waveform. The Low Load Current waveform illustrates continuous conduction mode operation with peak and valley inductor currents below the current limit threshold. When the load current is increased (High Load Current), the ripple waveform maintains the same amplitude and frequency since the current falls below the current limit threshold at the valley of the ripple waveform. Note the average current in the High Load Current portion of Figure 9 is above the current limit threshold. Since the current reduces below the threshold in the normal OFF-time each cycle, the start of each ON-time is not delayed, and the circuit’s output voltage is regulated at the correct value. When the load current is further increased such that the lower peak would be above the threshold, the OFF-time is lengthened to allow the current to decrease to the threshold before the next ON-time begins (Current Limited portion of Figure 9). Both V_OUT and the switching frequency are reduced as the circuit operates in a constant current mode. The load current (I_OCL) is equal to the current limit threshold plus half the ripple current (ΔI/2). The ripple amplitude (ΔI) is calculated from Equation 9.

Equation 9.

The current limit threshold can be increased by connecting an external resistor (R_CL) between SGND and ISEN. R_CL typically is less than 1 Ω, and the calculation of its value is explained in Application and Implementation. If the current limit threshold is increased by adding R_CL, the maximum continuous load current should not exceed 1.5 A, and the peak current out of the SW pin should not exceed 2 A.

Figure 9. Inductor Current - Current Limit Operation

7.3.7 Soft Start

The soft-start feature allows the regulator to gradually reach a steady-state operating point, thereby reducing start-up stresses and current surges. At turnon, while V_CC is below the undervoltage threshold (t1 in Figure 1), the SS pin is internally grounded, and V_OUT is held at 0 V. When V_CC exceeds the undervoltage threshold (UVLO) an internal 11.5-µA current source charges the external capacitor (C6) at the SS pin to 2.5 V (t2 in Figure 1). The increasing SS voltage at the non-inverting input of the regulation comparator gradually increases the output voltage from zero to the desired value. The soft-start feature keeps the load inductor current from reaching the current limit threshold during start-up, thereby reducing inrush currents.

An internal switch grounds the SS pin if V_CC is below the undervoltage lockout threshold, or if the circuit is shutdown using the RON/SD pin.

7.3.8 N-Channel Buck Switch and Driver

The LM25010 integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak current through the buck switch should not exceed 2A, and the load current should not exceed 1.5A. The gate driver circuit is powered by the external bootstrap capacitor between BST and SW (C4), which is recharged each OFF-time from V_CC through the internal high voltage diode. The minimum OFF-time, nominally 260 ns, ensures sufficient time during each cycle to recharge the bootstrap capacitor. A 0.022 µF ceramic capacitor is recommended for C4.

7.3.9 Thermal Shutdown

The LM25010 should be operated below the maximum operating junction temperature rating. If the junction temperature increases during a fault or abnormal operating condition, the internal Thermal Shutdown circuit activates typically at 175°C. The Thermal Shutdown circuit reduces power dissipation by disabling the buck switch and the ON timer. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature reduces below approximately 155°C (20°C typical hysteresis), normal operation resumes.

7.4.1 Shutdown

The LM25010 can be remotely shut down by forcing the RON/SD pin below 0.7 V with a switch or open drain device. See Figure 10. In the shutdown mode the SS pin is internally grounded, the ON-time one-shot is disabled, the input current at VIN is reduced, and the V_CC bypass switch is turned off. The V_CC regulator is not disabled in the shutdown mode. Releasing the RON/SD pin allows normal operation to resume. The nominal voltage at RON/SD is shown in Figure 6. When switching the RON/SD pin, the transition time should be faster than one to two cycles of the regulator’s nominal switching frequency.

Figure 10. Shutdown Implementation