ZXSC410/ZXSC420/ZXSC440

Document number: DS33618 Rev. 5 - 2

5 of 17

www.diodes.com

March 2013

© Diodes Incorporated

ZXSC410/ZXSC420/ZXSC440

Application Information

Functional Blocks

Bandgap Reference

All threshold voltages and internal currents are derived from a temperature compensated bandgap reference circuit with a reference voltage of

1.22V nominal.

Dynamic Drive Output

drives the base or gate of the external transistor. In order to operate the external switching transistor at optimum efficiency, both output states

are initiated with a short transient current in order to quickly discharge the base or the gate of the switching transistor.

Switching Circuit

The switching circuit consists of two comparators, Comp1 and Comp2, a gate U1, a monostable and the drive output. Normally the DRIVE output

is “HIGH”; the external switching transistor is turned on. Current ramps up in the inductor, the switching transistor and external current sensing

20mV, comparator Comp2 through gate U1 triggers a re-triggerable monostable and turns off the output drive stage for 2μs. The inductor

discharges to the load of the application. After 2μs a new charge cycle begins, thus ramping the output voltage. When the output voltage reaches

the nominal value and VFB gets an input voltage of more than 300mV, the monostable is forced “on” from Comp1 through gate U1, until the

feedback voltage falls below 300mV. The above action continues to maintain regulation.

EOR, End of Regulation Detector (ZXSC420/440)

The EOR circuit is a retriggerable 120μs monostable, which is re-triggered by every down regulating action of comparator Comp1. As long as

the output voltage falls below the nominal value for more than 120μs, output EOR goes ”LOW”. The reason for this to happen is usually a slowly

progressing drop of input voltage from the discharging battery. Therefore the output voltage will also start to drop slowly. With the EOR detector,

batteries can be used to the ultimate end of discharge, with enough time left for a safe shutdown. It can also be used in high-voltage photoflash

with the ZXSC440 to show when the capacitor is fully charged.

Shutdown Control

The ZXSC410/440 offers a shutdown mode that consumes a standby current of less than 5µA. The ZXSC410/440 is enabled, and is in normal

operation, when the voltage at the STDN pin is between 1V and 8V (and also open circuit). The ZXSC410/440 is shutdown with the driver

disabled when the voltage at the STDN pin is 0.7V or lower. The STDN input is a high impedance current source of 1µA typ. The driving device

can be an open-collector or -drain or a logic output with a “High” voltage of 5V max. The device shutdown current depends on the supply voltage,

see typical characteristics graph.The ZXSC440 with its STDN pin and EOR pins can be used as a camera flash driver.

The STDN pin is used to initiate the high voltage capacitor charge cycle. The EOR pin is used as flag to show when the capacitor has been

charged to the appropriate level.

A transformer is used to boost the voltage. If designing a transformer, bear in mind that the primary current may be over an amp and, if this flows

through 10 turns, the primary flux will be 10 Amp. Small number of turns and small cores will need an air gap to cope with this value without

saturation. Secondary winding capacitance should not be too high as this is working at 300V and could soon cause excessive losses.