BD9470AEFV Datasheet, PDF (10/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
2. Understanding BD9470AEFV・BD9470AFM
●Pin Functions
○ISET （HTSSOP-B28:8PIN/HSOP-M28：1PIN）
The ISET pin is a resister value of output current setting. The output current ILED vary in inverse proportion to resister
value. The relation of the output current ILED and ISET pin connecting resistor RISET are as bellow.
R_ISET
=
3000
I _LED [mA]
[kΩ]　　
However, current setting range is from 40mA to 150mA.
And the setting of ISET resistor is bellow at using 150mA to 250mA.
ILED(mA)
RSET(kohm)
150
20.00
R_ISET = 2653 × ( I _LED[mA] )^−0.9753
160 170 180 190 200
18.80 17.72 16.76 15.90 15.12
[kΩ]　　
210 220
14.42 13.78
230
13.19
240
12.66
250
12.16
For a setting example, please refer to ‘3.1 application explanation / LED current setting’.
When the RISET is shorted and the ISET pin is grand shorted, the LED current is OFF and the FAIL=OPEN(abnormal
signal) to prevent flowing a large current to LED pin when it becomes less than VISET×0.90V(typ).
When the ISET pin back to normal state the LED current return to former system, too and the FAIL=GND(normal signal).
It prepare automatically to suitable LED feedback voltage that can output LED current set by ISET pin.
In short LED feedback voltage is dropped when the LED current is small and the IC heating is held automatically.
In case of a large current is needed, raise the LED pin feedback voltage. And it adjust automatically to LED pin voltage that
can be flow large LED current.
The calculation is as below.
VLED = 3.462 × I [ A] [V ]　　
LED
The LED feedback voltage (VLED) is clamped to 0.4V(typ.) when the LED current (ILED) is less than 115.6mA.
○PWM1-6 （HTSSOP-B28:9,10,11,12,13,14PIN / HSOP-M28：2,3,4,5,6,7PIN）
The ON/OFF pin for LED driver. Light can be modulated by changing the duty cycle through the direct input of a PWM
light modulation signal in each PWM pin.
The high and low voltage levels of PWM_x pins are as listed in the table below.
State
PWMxvoltage
LED ON state
PWMx=1.5V~15.0V
LED OFF state
PWMx=‐0.3V～0.8V
The sequence of STB/PWM for start-up, please input PWM signal before STB or the same timing STB=PWM=ON.
○GND （HTSSOP-B28:15PIN / HSOP-M28：8PIN）
IC internal analog GND pin.
○FAIL （HTSSOP-B28:16PIN / HSOP-M28：9PIN）
FAIL signal output pin （OPEN DRAIN）.Internal NMOS will become OPEN while abnormal is detected.
State
Normal
Abnormal（After Timer Latch）
FAILoutput
GND
OPEN Level
○OVP （HTSSOP-B28:17PIN / HSOP-M28：10PIN）
The OVP pin is an input pin for overvoltage protection and short circuit protection of DC/DC output voltage. If over voltage
is detected, the OVP pin will stop the DC/DC converter conducting step-up operation. If Vout was increased by abnormality,
timer is set while OVP＞2.9V(typ.).when it comes to OVP＞3.0V, timer will ON at the same time and to stop DCDC.
Although Counter will be stopped when OVP＜2.9V during counting time, in the state of OVP>2.9V, when internal counter
completed 2¹⁸count （262152 count）, the system will be latched.
When the short circuit protection (SCP) function is activated, the DC/DC converter will stop operation, and then the timer
will start counting, after 2¹⁶ count（65536 count）, DCDC and LED driver will stop and latch.
The OVP pin is of the high impedance type and involves no pull-down resistor, resulting in unstable potential in the
open-circuit state. To avoid this problem, be sure to make input voltage setting with the use of a resistive divider or
otherwise. OVP pin will be feedback pin when PWM=L. Also, this pin will hold OVP voltage at that time when switch PWM
= H to L.
For setting example, refer to information in“3.4 Selection of External Components-OVP/SCP setting procedure
OVP Voltage keep internal IC with PWM=Low timing, and VOUT voltage can hold by using copied OVP voltage while
PWM=Low.（The OVP keep voltage range is 0~3V, 30steps）.For setting example, refer to information in “3.2 Selection of
External Components”, “Explanation of VOUT（OVP） voltage holding function when PWM=Low”
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TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003