Application information – Diodes ZXSC410/ZXSC420/ZXSC440 User Manual
Page 5
ZXSC410/ZXSC420/ZXSC440
Document number: DS33618 Rev. 5 - 2
5 of 17
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
Depending on the input signal, the output is either “LOW” or “HIGH”. In the high state a 2.5mA current source (max drive voltage = V
CC
-0.4V)
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
resistor. This voltage is sensed by comparator, Comp2, at input I
SENSE
. Once the current sense voltage across the sensing resistor exceeds
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
regulation takes place, output EOR is “HIGH“ (high impedance, 100K to V
CC
). Short dips of the output voltage of less than 120μs are ignored. If
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.