DESCRIPTION The ZXLD1362 is a continuous mode inductive buck converter designed to efficiently drive single or multiple LEDs in series from a voltage source higher than the LED voltage. The device operates from an input supply between 6V and 60V and provides an externally adjustable output current up to 1A. Depending on the supply voltage and external components, this can provide up to 48 watts of output power. The ZXLD1362 includes an output switch and a high-side output current sensing circuit that uses an external resistor to set the nominal average output current. The output current can be adjusted above or below the set value by applying an external control signal to the "ADJ" pin.
The ADJ pin will accept a DC voltage or a pulse width modulated waveform. Depending on the control frequency, this will provide continuous (dim) or gated output current. Soft-start can be forced from the adjust pin to ground using an external capacitor. Applying 0.2V or less to the ADJ pin will turn off the output and switch the device to a low current standby state.
Features Simple low part count Internal 60V ndmos switch Up to 1A output current Single pin on/off and brightness control using DC voltage or PWM PWM resolution up to 3000 :1 Soft start capability High efficiency (up to 95% ) Wide Input Voltage Range: 6V to 60V 65V Transient Capability Low Power Shutdown Up to 1MHz Switching Frequency Intrinsic Open LED Protection Typical 4% Output Current Accuracy Low Voltage Halogen Replacement LED Automotive Lighting Low Voltage Industrial Lighting. LED Backlighting. Illuminated Signs .Emergency lighting.SELV lighting.LCD TV backlighting

Device Description The device together with the coil (l1) and the current sense resistor (rs) constitutes a self-oscillating continuous mode buck converter.
The operation of the device (refer to the block diagram and Figure 1 - Operational Waveforms) can be best understood by assuming that the ADJ pin of the device is not connected and the voltage on that pin (VADJ) appears directly at the (+) input of the comparator. When the input voltage vin is first applied, the initial current in l1 and rs is zero and there is no output from the current detection circuit. In this case, the comparator's (-) input is at ground and its output is high. This turns on MN and toggles the LX pin low, allowing current to flow from the VIN to ground through RS, L1, and the LED. The current rises at the rate determined by VIN and L1, creating a voltage ramp (VSENSE) across Rs. The supply reference voltage VSENSE is forced through the internal resistor R1 by the current sense circuit, and a proportional current is generated in the internal resistors R2 and R3. This produces a ground referenced rising voltage at the (-) input of the comparator. When the threshold voltage (vadj) is reached, the comparator output switches low and mn turns off. The comparator output also drives another NMOS switch, which bypasses internal resistor R3 to provide a controlled amount of hysteresis. Hysteresis is set by r3 to 15% of vadj. When MN is off, the current in L1 continues to flow back to Vin through D1 and LED. The current decays at a rate determined by the forward voltage of the LEDs and diode, producing a falling voltage at the input of the comparator. When this voltage returns to vadj, the comparator output switches high again. This event loop repeats and the comparator input ramps between the limits of VADJ ±15%. When vadj=vref, the ratio of r1, r2, and r3 defines the average vSense switching threshold of 100mV (measured relative to vin on the iSense pin). The average output current IOUTNOM is defined by this voltage and RS: IOUTNOM=100MV/RS rated ripple current ±15MV/RS adjusted output current. The device contains a low pass filter between the ADJ pin and the threshold comparator and an internal current limiting resistor ( 50K NOM) between ADJ and the internal reference. Voltage. This allows the ADJ pin to change the VSENSE switching threshold and adjust the output current with a DC or pulsed signal. Details of the different modes of adjusting the output current are given in the application section.

Output Shutdown The output low-pass filter drives the shutdown circuit. When the input voltage to this circuit is below a threshold (0.2V nominal), the internal regulator and output switch are turned off. The voltage reference remains powered during shutdown to provide bias current for the shutdown circuit. Quiescent supply current during shutdown is nominally 60A and switch leakage is less than 5A.

Application Notes Set the nominal average output current with an external resistor, RS. The nominal average output current in the LED(S) is determined by the value of the external current sense resistor (RS) connected between VIN and ISCONSE and given by IOutnOM = 0.1/RS [0.1 for RS]. The table below gives several nominal average output current values. Preferred values for the current setting resistor (RS) in the typical application circuit shown on page 1: The above values assume the ADJ pin is floating and at the nominal voltage of VREF (=1.25V). Note that under these conditions RS=0.1 is the minimum allowable value of the sense resistor to keep the switch current below the specified maximum value. Different RS values can be used if the ADJ pin is driven by an external voltage.
Adjustment by External DC Control Voltage The output current adjust pin can be driven by an external DC voltage (VADJ) as shown to adjust the output current above or below the nominal average value defined by RS.
In this case, the nominal average output current is given by: ioutdc = (vadj/1.25) x 100mv x rs [for 0.3

Adjust the output current with pwm control to directly drive the ADJ input. A pulse width modulation (PWM) signal with duty cycle DPWM can be applied to the ADJ pin as shown below to adjust the output current above or below the resistor RS setting. The nominal average value determined by:
Driving the ADJ input via an open collector transistor The recommended way to drive the ADJ pin and control the amplitude of the PWM waveform is to use a small NPN switching transistor as shown below: This scheme uses a 50K resistor between the ADJ pin and the internal voltage reference as an external Pull-up resistors for transistors.
Another possibility to drive the ADJ input from the microcontroller is to drive the device from the microcontroller's open drain output. The diagram below shows one way to do this: If the NMOS transistors inside the microcontroller have high drain/source capacitance, this arrangement can inject negative spikes into the ADJ input of the 1362 and cause instability operation, but the addition of a Schottky clamp diode (cathode to ADJ) to ground and the addition of a series resistor (3.3K) will prevent this from happening.

Shutdown mode taking the voltage of the ADJ pin below 0.2V for more than about 100 seconds will turn off the output and the supply current will drop to a low standby level of 60µA nominal. Note that the ADJ pin is not a logic input. Setting the ADJ pin to a voltage higher than VREF will cause the output current to be higher than 100% of the nominal average value.
A soft-start external capacitor from the ADJ pin to ground will provide a soft-start delay by increasing the time for the voltage on this pin to rise to the turn-on threshold and by reducing the rate of rise of the control voltage at the input of the comparator. Adding capacitance increases this delay by about 0.2MS/NF. The graph below shows the change in soft-start time for different capacitor values.
Actual working waveform [vin=24v, rs=0.1, l=68μh, 22nf on adj] soft start operation. Output current (ch2) and lx voltage (ch1)

Sensor selection
The recommended inductance value for the ZXLD1362 is between 68h and 220h. Higher inductor values are recommended at higher supply voltages to minimize errors due to switching delays, which will result in increased ripple and reduced efficiency. The higher the inductance value, the smaller the change in output current over the supply voltage range. (see chart). The sensor should be mounted as close to the device as possible with low resistance connections to the LX and VIN pins. The selected coil should have a saturation current above the peak output current and a continuous current rating above the desired average output current. Suitable coils for use with the ZXLD1362 can be selected from the MSS series manufactured by Coilcraft or the NPIS series manufactured by NIC components.
The inductor value should be chosen to maintain the duty cycle of operation and the on/off time within the specified range over the supply voltage and load current range.

For maximum efficiency and performance of diode selection, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at maximum operating voltage and temperature. They also offer better efficiency than silicon diodes due to a combination of low forward voltage and reduced recovery time. It is important to choose a part with a peak current rating higher than the peak coil current and continuous current rating higher than the maximum output load current. It is important to consider diode reverse leakage when operating above 85°C. Excessive leakage increases power losses in the device and can create thermal runaway conditions if close to the load. Forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the LX output. If silicon diodes are used, care should be taken to ensure that the total voltage present at the LX pin, including power supply ripple, does not exceed the specified maximum value.

A value of 1F will reduce power supply ripple current by a factor of three (approx.). Higher capacitance values can achieve correspondingly lower ripple. Note that the capacitor will not affect the operating frequency or efficiency, but it will increase the startup delay by reducing the rate of rise of the LED voltage. By adding this capacitor, the current waveform through the LED(S) changes from a triangular ramp to a more sinusoidal version without changing the average current value.
Drivers that operate output transistors at low supply voltages below the undervoltage lockout threshold (VSD) are turned off to prevent the device from operating with excessive on-resistance of the output transistors. The output transistors are not fully boosted until the supply voltage exceeds about 17V. A supply voltage between VSD and 17V must be taken to avoid excessive power dissipation due to on-resistance. If the power supply voltage is always less than 30V continuous (or less than 40V continuously for less than 0.5s), the alternative device ZXLD1360 can be used. Note that when driving a load of two or more LEDs, the forward dip is usually sufficient to prevent the device from switching below about 6V, which will minimize the risk of damage to the device.
When operating devices at high ambient temperatures, or when driving maximum load currents, thermal considerations must be taken to avoid exceeding package power dissipation limits. The figure below gives the details of power derating. This assumes the device is mounted on a 25mm square PCB with 1oz copper in still air.

Note that at the minimum supply voltage, the device power dissipation is usually the maximum. It also increases if the efficiency of the circuit is low. This may be due to the use of an unsuitable coil, or excessive parasitic output capacitance on the switch output.
Thermal compensation of high brightness LED output current usually requires a temperature compensation current to maintain stable and reliable operation at all drive levels. The LEDs are usually mounted remotely from the unit, so the temperature coefficient for the ZXLD1362's internal circuitry has been optimized to minimize output current variations when compensation is not used. If output current compensation is required, an external temperature sensing network can be used - usually a negative temperature coefficient (NTC) thermistor and/or diode, mounted very close to the LED. The output of the sensing network can be used to drive the ADJ pin, thereby reducing the output current with increasing temperature.

Layout Considerations
The lx pin of the lx pin device is a fast switching node, so the traces of the pcb should be as short as possible. To minimize ground "bounce", the device's ground pins should be soldered directly to the ground plane.
Coil and decoupling capacitors and current sense resistors, it is especially important to mount the coil and input decoupling capacitors as close as possible to the device pins to minimize parasitic resistance and inductance, which will reduce efficiency. It is also important to minimize any track resistance in series with the current sense resistor Rs. It is best to connect VIN directly to one end of RS, and directly to the opposite end of RS, with no other current flowing in those rails. It is important that the cathode current of the Schottky diode does not flow in the rail between RS and V, as this may give a higher current measurement than the rail resistance actually.
ADJ Pin The ADJ pin is a high impedance input voltage up to 1.35V, so when floating left, the PCB trace for this pin should be as short as possible to reduce noise pickup. Under these conditions, a 100nF capacitor from the ADJ pin to ground will reduce the frequency modulation of the output. Additional 3.3K series resistors can also be used when driving the ADJ pin from an external circuit (see below). This resistor will provide filtering of low frequency noise and provide protection against high voltage transients.
High Voltage Rails Avoid running any high voltage rails close to the L pin to reduce the risk of leakage currents due to board contamination. At voltages above 1.35V, the ADJ pin is soft-clamped to make it insensitive to leakage that could raise the ADJ pin voltage and cause excessive output current. However, under these conditions, it is recommended to place a ground ring around the ADJ pin to minimize output current variation.
Evaluation PCB The ZXLD1362 evaluation board is available upon request. These boards contain LEDs that allow quick testing of the 1362 device. Additional terminals allow connection to the customer's own led products.

Dimming the output current with pwm in low frequency PWM mode When driving the ADJ pin with a low frequency PWM signal (such as 100Hz), in the case of a high level voltage VADJ and a low level zero, the output of the internal low pass filter will swing between 0V and VADJ, causing the input to the shutdown circuit to be below its shutdown threshold (200mV nominal) when the ADJ pin is low. This will cause the output current to turn on and off at the pwm frequency, resulting in an average output current ioutavg proportional to the pwm duty cycle.
Low Frequency PWM Operating Waveform The average value of the output current in this mode is given by: IOTAvvg 0.1dpWM/RS (for DPWM > 0.001), this mode is preferred if optimum LED "whiteness" is required. It will also provide the widest possible dimming range (about 1000:1) and higher efficiency at the expense of larger output ripple.