The ZXSC400 is a voltage mode boost converter in a SOT2 3-6 package. Its low feedback voltage allows the current in the LED chain to be set and precisely monitored through a single resistor with minimal losses. Its excellent load and line regulation means that the variation in LED current is typically less than 1% over the full supply range of the Li-Ion battery. Using high-efficiency Zetex switching transistors rated at 20V and higher, many LEDs can be connected in series for optimal LED current matching.
feature
1.8V to 8V Supply Range
1% Typical Output Regulation Over 80% Typical Efficiency
4.5A Typical Shutdown Current for Final LED Current Matching Series Connection Applications White LED Backlight for Color LCD Panels General Purpose LED Backlight High Performance White LED Torches Battery-Powered General Purpose LEDs

IC Operation Description Block Diagram

Bandgap Reference Source All threshold voltages and internal currents come from a temperature-compensated bandgap reference circuit with a reference voltage of 1.22V.
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 enabled with a short transient current to rapidly discharge the base or gate of the switching transistor.
S switch circuit The switch circuit is composed 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 increases in inductors, switching transistors, and external current sense resistors. This voltage is sensed at the input ense by the comparator comp2. Once the current sensed voltage across the sense resistor exceeds 30mV, the comparator comp2 triggers a retriggerable monostable through gate u1 and turns off the output driver stage for 2s. The inductor discharges to the application load. After 2s, a new charge cycle starts, which increases the output voltage. When the output voltage reaches the nominal value and FB gets an input voltage greater than 300MV, the monostable is forced "on" from COMP1 through 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, bipolar transistors with low vce(s at) and high gain are required. The vceo of the switching transistor is also an important parameter because it sees the full output voltage when the transistor is off. Zetex Supers OT™ transistors are ideal for this application.
Schottky Diode Selection Like switching transistors, Schottky rectifier diodes have a large impact on the efficiency of the converter. 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 fulfills these needs.
Combination Device To reduce 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.

The ZXSC400 offers a shutdown mode that consumes less than 5A of standby current. When the voltage on the STDN pin is between 1V and 8V (and open), the ZXSC400 is enabled and the driver is in normal operation. When the voltage on the STDN pin is 0.7V or lower, the ZXSC400 is disabled and the driver is in shutdown mode. The SHDN input is a 1A type high impedance current source. The driver can be an open collector, open drain or logic output with a maximum "high" voltage of 5V. The device shutdown current depends on the supply voltage, see Typical Characteristics Diagram. Open Circuit Protection For applications where the LED chain may open, a Zener diode can be connected across 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:

Dimming Control There are four types of dimming control that can be achieved by ZX 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 and led brightness can be adjusted.
To implement this method of brightness control on the zxsc400, apply a pwm signal with an amplitude between 0.7v and vcc at 120hz or higher (to eliminate led flicker). The LED current and 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 that do not have a shutdown pin, a DC voltage can be used to control dimming. The led current can be adjusted from 100% to 0% by adding resistors r2 and r3 and applying a DC voltage. 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 smaller than the LED current and much larger than the feedback current. The component values in the graph below represent 0% to 100% dimming control from 0 to 2V DC voltage.

Layout Considerations Layout is critical for a circuit to function in the most efficient manner in terms of electrical efficiency, thermal considerations, and noise.
The "boost converter" has four main current loops: the input loop, the power switch loop, the rectifier loop, and the output loop. The power supply charging the input capacitor forms the input loop. When q1 is "on", the power switch loop is defined and current flows from the input through the inductor q1, rs ense and to 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 turned off, the output capacitor supplies power to the load, forming an output loop.
To optimize best performance, these loops are separated from each other and interconnected with short, thick traces to minimize parasitic inductance, capacitance, and resistance. Also, the RS ENS E resistor should be connected with a minimum trace length between Q1's emitter lead and ground, again reducing stray parasitics.
reference signal

Li-ion-2led converter