BD9422EFV Datasheet, PDF (11/28 Pages) Rohm – White LED Driver for large LCD panel

BD9422EFV
Datasheet
* For the adjustment of LED current with analog dimming by VREF, note that the output voltage of the DC/DC converter
largely changes accompanied by LED VF changes if the VREF voltage is changed rapidly. In particularly, when the VREF
voltages become high to low, it makes the LED terminal voltage seem higher transiently, which may influence application
such as activation of the LED short circuit protection. It needs to be adequately verified with an actual device when analog
dimming is used.
31pin. CP
Terminal which sets the time from detection of abnormality until shutdown (Timer latch). When the LED short protection,
LED open protection or SCP is detected, it perform s constant current charge of 1.0uA (typ.) to external capacitor. When the
CP terminal voltage reaches 2.0V (typ.), the IC is latched and FAIL terminal operates (at FAIL_MODE = L).
32pin. SS
Terminal which sets soft-start time of DC/DC converter: it performs constant current charge of 1.0uA to the external
capacitor connected with SS terminal, which enables soft-start of DC/DC converter.
Since the LED protection function (OPEN/SHORT detection) works when the SS terminal voltage reaches 3.0 V (typ.) or
higher, it must be set to bring stability to conditions such as DC/DC output voltage and LED constant current drive operation,
etc. before the voltage of 3.0 V is detected.
33pin. FB
Output terminal of the error amplifier of DC/DC converter which controls current mode:
The voltage of LED terminal which is the highest VF voltage among 6 LED strings and the voltage of LED_LV terminal
become input of the error amplifier. The DC/DC output voltage is kept constant to control the duty of the output N terminal
by adjusting the FB voltage.
The voltage of other LED terminals is, as a result, higher by the variation of Vf. Phase compensation setting is separately
described in ●3.7 How to set phase compensation.
A resistor and a capacitor need to be connected in series nearest to the terminal between FB and AGND.
The state in which all PWM signals are in LOW state brings high Impedance, keeping FB voltage. This action removes the
time of charge to the specified voltage, which results in speed-up in DC/DC conversion.
34pin. RT
RT sets charge/discharge current determining frequency inside IC.
Only a resistor connected to RT determines the drive frequency inside IC, the relationship has the following equation: FCT
is 500 kHz at RT= 30 kohm.
FCT vs RT(measurement data)
10000
1000
100
10
1
10
100
RT [kOhm]
1000
Figure 16.
Voltage between LEDx to Sx vs LED
Current (Tj=25, 85deg.)
1.2
85℃
25℃
1
0.8
0.6
0.4
0.2
0
0
50
100
150
200
250
300
350
400
LED Current Setting
Figure 17.
36pin. LED_LV
LED_LV terminal sets the reference voltage error amplifier. LED_LV terminal is assumed that it is set by dividing the
resistance with a high degree of accuracy, LED_LV terminal inside the IC is in open state (High Impedance). It is necessary
to input voltage to divide the resistance from the output of REG9V or use external power source. Using the terminal in open
state needs to be avoided.
According to output current, lowering LED_LV voltage can reduce the loss and heat generation inside IC. However, it is
necessary to ensure the voltage between drain and source of FET inside IC, so LED_LV voltage has restriction on the
following equation.
VLED_LV ≧ (LED-S terminal voltage) + 0.2×VREF [V]
For example, at ILED = 100mA setting by VREF = 1V, from figure the voltage between LED and S terminal is required 0.27
V at Tj = 85°C, so LED_LV voltage must be at least a minimum 0.47V.
Note: Rises in VLED_LV voltage and LED current accelerate heat generation of IC. Adequate consideration needs to be
taken to thermal design in use.
Note: LED_LV voltage is not allowed setting below 0.3V.
Note: LED current by raising LED_LV voltage can flow to MAX 400mA, use with care in the dissipation of the package.
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved.
TSZ22111・15・001
11/25
TSZ02201-0F1F0C100210-1-2
31.May.2013 Rev.003