illustrate
The ZXS C400 is a voltage mode boost converter in a SOT23-6 package. Its low feedback voltage enables the current in the LED chain to be precisely set. Monitor with a minimum loss resistor. Its excellent load and line regulation means that the LED current variation is typically less than 1% over the full supply range of the Li-Ion battery. Using high-efficiency Zetex switches with transistors rated at 20V and above allows many LEDs to be connected in series.
for optimal LED current matching.
feature
1% Typical Output Regulation from 1.8V to 8V Supply Range
Typical efficiency over 80%
4.5a typical shutdown current for series application with final LED current matching white LED backlight for color LCD panel common LED backlight high performance white LED flashlight common LED battery drive

IC Operation Description Block Diagram Bandgap Reference Source All threshold voltages and internal currents are derived from a temperature compensated bandgap reference circuit with a reference voltage of 1.22V, rated value.
The dynamic drive output is either "low" or "high" depending on the input signal. In the high state, a 2.5mA current source (maximum drive voltage = VCC - 0.4V) drives the base or gate of an external transistor. In order for the external switching transistor to operate at optimum efficiency, both output states are started with a short transient current to rapidly discharge the base or gate of the switching transistor.
S switch circuit The switch circuit consists of two comparators COMP1 and COMP2, U1 gate, monostable and drive output. Normally the drive output is "high"; the external switching transistor is on. Current rises in the inductor, switching transistor, and external current sense resistor. This voltage is sensed at the input by the comparator comp2. Once the current sensed voltage across the sense resistor exceeds 30 mV, the comparator comp2 triggers a retriggerable monostable through the U1 gate and turns off the output driver stage for 2 seconds. The inductor discharges the load of the application. After 2s, a new charge cycle starts, which increases the output voltage. When the output voltage reaches the rated value and FB gets an input voltage greater than 300mV, the monostable is forced "on" from COMP1 to the U1 gate until the feedback voltage drops below 300mV. The above actions continue to maintain regulation.

Application Information Switching Transistor Selection Switching transistor selection has a large impact on the efficiency of the converter. For best performance, low VCE and high gain bipolar transistors are required. The VCEO of the switching transistor is also an important parameter because when the transistor is off, the VCEO can see the full output voltage. Zetex Supers OT 8482 ; transistors are ideal for this application.
Schottky Diode Selection Like switching transistors, Schottky rectifier diodes have a significant impact on converter efficiency. This application should use a Schottky diode with low forward voltage and fast recovery time.
The diodes should be chosen such that the maximum forward current rating is greater than or equal to the maximum peak current in the inductor and the maximum reverse voltage is greater than or equal to the output voltage. The Zetex ZHCS series meets these needs.
Combination Setup To minimize the number of external components, Zetex recommends combining the Zx3CDBS1M832 of npn transistors and Schottky diodes in a 3mm x 2mm MLP package. It is recommended to use this device in applications using 1 to 4 white LEDs.

Integrated circuits are also capable of driving MOSFETs. Zetex recommends combining the Zxmns3bm832 low threshold voltage N-channel MOSFET and Schottky diode in a 3mm x 2mm MLP package. It is recommended to use this device in applications using 1 to 8 white LEDs.
Capacitor selection requires a small value, low ESR ceramic capacitor to filter the output, typically 1F to 4.7F.
Input capacitors are usually not required, but a small ceramic capacitor can be added to assist EMC, typically 1F to 4.7F.
Inductor Selection Inductor values must be selected to meet the performance, cost and size requirements of the overall solution.
The choice of inductance has an important influence on the performance of the converter. For applications where efficiency is critical, an inductor with a series resistance of 250 mΩ or less should be used.
Peak Current Definition In general, the IPK value must be chosen to ensure that switching transistor Q1 is fully saturated at maximum output power conditions, assuming worse input voltage and transistor gain at all operating temperature extremes.
Sensor Resistor Setting the peak current requires a low value sense resistor. Due to the low induced voltage threshold, the power of this resistor is negligible. There is a table of recommended sense resistors at the bottom of the page.
Output Power Calculation By making the above assumptions for inductance and peak current, the output power can be determined by:

Programming the LED Current Once the desired output power is determined, the LED current can be programmed by adding a resistor to the LED chain. The resistance value is determined by the following factors:

ZXSC400 provides shutdown mode and consumes less than 5a of standby current. When the voltage on the STDN pin is between 1V and 8V (open circuit at the same time), the ZXSC400 is enabled and the driver is in normal operation. When the voltage at the stdn pin is 0.7V or lower, the ZXSC400 is disabled and the driver is in shutdown mode. The SHDN input is a typical 1A high impedance current source. The drive can be open collector or open drain, or a logic output with a maximum "high" voltage of 5 volts. The turn-off current of the device depends on the supply voltage, see Typical Characteristics Diagram Open Circuit Protection For applications where the LED chain may be open circuited, a Zener diode can be connected through the LED chain to prevent overvoltage and possible damage to the main switching transistor. The Zener diode should be chosen so that its voltage rating is higher than the combined forward voltage of the LED chain. Under open circuit conditions, the current in the Zener diode defines the output current as:

The circuit example below gives an open circuit output current of 300A.
Dimming Control There are four dimming controls available for the ZX S C400.
Dimming control using the shutdown pin The first method uses the shutdown pin. By injecting a PWM waveform on this pin and changing the duty cycle, the LED current can be adjusted and thus the LED brightness can be adjusted.
To implement this brightness control method on the ZXSC400, at 120Hz or above, apply a PWM signal with an amplitude between 0.7V and VCC (to eliminate LED flickering). The LED current and resulting LED brightness scale linearly with the duty cycle, so for brightness control, adjust the duty cycle as needed. For example, a 10% duty cycle equals 10% of full LED brightness.

Dimming Control Using DC Voltage For applications without a shutdown pin, dimming can be controlled using a DC voltage. By adding resistors r2 and r3 and applying a DC voltage, the LED current can be adjusted from 100% to 0%. As the DC voltage increases, the voltage drop across R2 increases and the voltage drop across R1 decreases, reducing the current through the LED. The selection of R2 and R3 should ensure that the current from the DC voltage is much less than the LED current and much greater than the feedback current. The component values in the figure below represent 0% to 100% dimming control from 0 to 2V DC voltage.
Using the filtered pulse width modulation signal for dimming control The filtered pulse width modulation signal can be used as an adjustable DC voltage with an RC filter. The values shown in the figure below are configured to provide 0% to 100% dimming at 2V amplitude with a PWM signal of 1KHz to 100KHz. For example, a 50% duty cycle will produce 50% dimming.

Dimming Control Using Logic Signals For applications that require discrete steps to adjust the LED current, a logic signal can be applied, as shown in the figure below. R1 sets the minimum LED current when Q1 OS is "off". When q1 is "on", r2 sets the LED current that will be added to the minimum LED current.
Layout is critical for the circuit to operate in the most efficient manner in terms of electrical efficiency, thermal considerations, and noise.
For a "boost converter", there are four main current loops: the input loop, the power switch loop, the rectifier loop, and the output loop. The power supply that charges the input capacitors forms the input loop. When q1 is "on" defining the power switch loop, current flows from the input through the inductor q1, rs ense and ground. When q1 is "off", the energy stored in the inductor is transferred to the output capacitor and the load through d1, forming a rectifier loop. When Q1 is open, the output capacitor provides the load, forming the output loop.
To optimize best performance, each of these loops is kept separate from each other and interconnected with short, thick traces that minimize parasitic inductance, capacitance, and resistance. Additionally, the RS ENS E resistor should be connected with a minimum trace length between Q1's emitter lead and ground, again to minimize parasitics.
Reference Design Li-Ion to 2 LED Converter