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BD63731EFV Datasheet(PDF) 10 Page - Rohm |
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BD63731EFV Datasheet(HTML) 10 Page - Rohm |
10 / 31 page 10/28 BD63731EFV TSZ02201-0P1P0C702140-1-2 © 2019 ROHM Co., Ltd. All rights reserved. 21.Jun.2019 Rev.001 www.rohm.com TSZ22111 • 15 • 001 PWM Constant Current Control 1 Current control operation When the output transistor is turned on, the output current increases. The output current is converted to voltage due to the connected external resistance to the RNFX (Note 1) pin. When the voltage on the RNF X pin reaches the voltage value set by the VREF input voltage, the current limit comparator operates and enters current decay mode. Output turns on again after changing this pin to the high voltage from the low voltage. The process repeats itself with chopping period (tCHOP). (Note 1) x=1 or 2 2 Noise-masking function In order to avoid misdetection of current detection comparator due to RNF spike noise that may occur when the output turns ON, the IC has the minimum ON time tONMIN (Blank time). The current detection is invalid from the output transistor turned on to tONMIN. This allows for constant-current drive without the need for an external filter. 3 CR Timer The external capacitor and resistor connected to the CR pin is repeatedly charged and discharged between the VCRH and VCRL levels. The CR pin voltage decides in IC and it is VCRL=0.4 V, VCRH=1.0 V respectively. The output of the current detection comparator is masked while charging from VCRL to VCRH. As mentioned above, this period defines the minimum ON-time. The CR pin begins discharging once the voltage reaches VCRH. When the output current reaches the current limit during this period, then the IC enters decay mode. The CR continues to discharge during this period until it reaches VCRL, at which point the IC output is switched back ON. The current output and the CR pin begin charging simultaneously. The CR charge time (tONMIN) and discharge time (tDISCHARGE) are set by external components, according to the following formulas. The total of tONMIN and tDISCHARGE yield the chopping period, tCHOP. ������������������������ ≒ ���� × ����′×���� ����′+���� × �������� ( ������������−0.4 ������������−1.0 ) [s] Where: tONMIN : is the minimum ON-time. C : is the capacitance of the CR Pin. R : is the resistance of the CR Pin. R’ : is the CR Pin internal impedance 5 k Ω(Typ) VCR : is the CR Pin voltage. ������������ = ���� × ���� ����′+���� [V] Where: V : is the internal regulator voltage 5 V(Typ). ���������������������������������������� ≒ ���� × ���� × �������� ( 1+���� 0.4 ) [s] Where: tDISCHARGE : is the CR discharge time. α : Refer to the right graph. �������������������� = ������������������������ + ���������������������������������������� [s] Where: tCHOP : is the chopping period. Figure 1. CR Coefficient for calculation of discharge time 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0 500 1000 1500 2000 C[pF] |
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