7.3.1 Charge Pump

The input to the 1.5×/1× charge pump is connected to the V_IN pin, and the loosely regulated output of the charge pump is connected to the V_OUT pin. The recommended input voltage range of the LM27951 is 3 V to 5.5 V. The loosely regulated charge pump of the device has both open loop and closed loop modes of operation. When the device is in open loop, the voltage at V_OUT is equal to the gain times the voltage at the input. When the device is in closed loop, the voltage at V_OUT is loosely regulated to 4.5 V (typical). The charge pump gain transitions are actively selected to maintain regulation based on LED forward voltage and load requirements. This allows the charge pump to stay in the most efficient gain (1×) over as much of the input voltage range as possible, reducing the power consumed from the battery.

7.3.2 Soft Start

The LM27951 contains internal soft-start circuitry to limit input inrush currents when the part is enabled. Soft start is implemented internally with a controlled turnon of the internal voltage reference. Due to the soft-start circuitry, startup time of the LM27951 is approximately 330 µs (typical).

7.3.3 Thermal Protection

Internal thermal protection circuitry disables the LM27951 when the junction temperature exceeds 150°C (typical). This feature protects the device from being damaged by high die temperatures that might otherwise result from excessive power dissipation. The device recovers and operate normally when the junction temperature falls below 140°C (typical). It is important that the board layout provide good thermal conduction to keep the junction temperature within the specified operating ratings.

7.4.1 Enable and PWM Pins

The LM27951 has 2 logic control pins. Both pins are active-high logic (HIGH = ON). There is an internal pulldown resistor (150 kΩ typical) connected between the enable pin (EN) and GND. There is no pullup or pulldown connected to the pulse width modulated (PWM) pin.

The EN pin is the master enable pin for the part. When the voltage on this pin is low (< 0.4 V), the part is in shutdown mode. In this mode, all internal circuitry is OFF and the part consumes very little supply current (< 1 µA typical). When the voltage on the EN pin is high (> 1 V), the device activates the charge pump and regulate the output voltage to its nominal value.

The PWM pin serves as a dedicated logic input for LED brightness control. When the voltage on this pin is low (< 0.4 V), the current sources are turned off, and no current flows through the LEDs. When the voltage on this pin is high (> 1 V), the currents sources turn on and regulate to the current level set by the resistor connected to the I_SET pin.

7.4.2 Adjusting LED Brightness (PWM Control)

Perceived LED brightness can be adjusted using a PWM control signal on the LM27951 PWM logic input pin, turning the current sources ON and OFF at a rate faster than perceptible by the human eye. When this is done, the total brightness perceived is proportional to the duty cycle (D) of the PWM signal (D = the percentage of time that the LED is on in every PWM cycle). A simple example: if the LEDs are driven at 15 mA each with a PWM signal that has a 50% duty cycle, perceived LED brightness is about half as bright as compared to when the LEDs are driven continuously with 15 mA.

The minimum recommended PWM frequency is 100 Hz. Frequencies below this may be visible as flicker or blinking. The maximum recommended PWM frequency is 1 kHz. Frequencies above this may cause interference with internal current driver circuitry and/or noise in the audible range. Due to the regulation control loop, the maximum frequency and minimum duty cycle applied to the PWM pin must be chosen such that the minimum ON time is no less than 30 µs in duration. If a PWM signal is applied to the EN pin instead, the maximum frequency and minimum duty cycle should be chosen to accommodate both the LM27951 start-up time (330 µs typical) and the 30-µs control loop delay.

The preferred method to adjust brightness is to keep the master EN voltage ON continuously and apply a PWM signal to the dedicated PWM input pin. The benefit of this type of connection can be best understood with a contrary example. When a PWM signal is connected to the master enable (EN) pin, the charge pump repeatedly turns on and off. Every time the charge pump turns on, there is an inrush of current as the capacitors, both internal and external, are recharged. This inrush current results in a current spike and a voltage dip at the input of the part. By applying the PWM signal to PWM logic input pin, the charge pump remains active, resulting in much lower input noise.

When the PWM signal must be connected to the EN pin, measures can be taken to reduce the magnitude of the charge-pump turnon transient response. More input capacitance, series resistors, and/or ferrite beads may provide benefits. If the current spikes and voltage dips can be tolerated, connecting the PWM signal to the EN pin does provide a benefit of lower supply current consumption. When the PWM signal to the EN pin is low, the LM27951 is shut down, and the input current is only a few micro-amps. This results in a lower time-averaged input current than the prior suggestion, where EN is kept on continuously.