T8302 查看數據表（PDF） - Taiwan Memory Technology

零件编号

产品描述 (功能)

生产厂家

T8302

PWM Control 1A Step-Down Converter

Taiwan Memory Technology 

tm TE

CH

T8302

Functional Description

The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a selfoscillating

continuous-mode buck converter.

Device operation

Operation can be best understood by assuming that the ADJ pin of the device is unconnected and

the voltage on this pin (VADJ) appears directly at the (+) input of the comparator.

When input voltage VIN is first applied, the initial current in L1 and RS is zero and there is no

output from the current sense circuit. Under this condition, the (-) input to the comparator is at

ground and its output is high. This turns MN on and switches the LX pin low, causing current to

flow from VIN to ground, via RS, L1 and the LED(s). The current rises at a rate determined by VIN

and L1 to produce a voltage ramp (VSENSE) across RS. The supply referred voltage VSENSE is

forced across internal resistor R1 by the current sense circuit and produces a proportional current in

internal resistors R2 and R3. This produces a ground referred rising voltage at the (-) input of the

comparator. When this reaches the threshold voltage (VADJ), 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. The hysteresis is set by R3 to be

nominally 15% of VADJ.

When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to VIN. The

current decays at a rate determined by the LED(s) and diode forward voltages to produce a falling

voltage at the input of the comparator. When this voltage returns to VADJ, the comparator output

switches high again. This cycle of events repeats, with the comparator input ramping between limits

of VADJ ± 15%.

Switching thresholds

With VADJ = VREF, the ratios of R1, R2 and R3 define an average VSENSE switching threshold of

100mV (measured on the ISENSE pin with respect to VIN). The average output current IOUTnom is

then defined by this voltage and RS according to:

IOUTnom = 100mV/RS

Nominal ripple current is ±15mV/RS

Adjusting output current

The device contains a low pass filter between the ADJ pin and the threshold comparator and an

internal current limiting resistor (64kohn nom) between ADJ and the internal reference voltage.

This allows the ADJ pin to be overdriven with either DC or pulse signals to change the VSENSE

switching threshold and adjust the output current. The filter is third order, comprising three sections,

each with a cut-off frequency of nominally 4kHz. Details of the different modes of adjusting output

current are given in the applications section.

Output shutdown

The output of the low pass filter drives the shutdown circuit. When the input voltage to this circuit

falls below the threshold, the internal regulator and the output switch are turned off. The voltage

reference remains powered during shutdown to provide the bias current for the shutdown circuit.

Quiescent supply current during shutdown is nominally 70uA.

TM Technology, Inc. reserves the right

P. 5

to change products or specifications without notice.

Publication Date:April 2014

Revision: E

Share Link: