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NCP51530 Datasheet(PDF) 21 Page - ON Semiconductor

Part No. NCP51530
Description  High Frequency High and Low Side Driver
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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NCP51530 Datasheet(HTML) 21 Page - ON Semiconductor

 
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NCP51530
www.onsemi.com
21
Figure 34. Bootstrap Circuit
COMPONENT SELECTION
CBOOT CAPACITOR VALUE CALCULATION
NCP51530 has two independent drivers for driving high
side and low side external MOSFETs. The bias for the high
side driver is usually provided through a bootstrap circuit. A
typical bootstrap circuit is shown in the figure 8 below.
The high side driver is biased by the Cboot (bootstrap
capacitor). As can be seen in the circuit, Cboot will charge
only when HB goes to GND level. Low value of Cboot can
result in a little dip in the bias voltages VB. If the VB drops
below UVLO the power supply can shut down the high side
driver. Therefore choosing the right value of Cboot is very
important for a robust design.
An example design for Cboot is given below.
Qg + 30 nC, VCC + 15 V
(eq. 1)
Qb + IBQ *tdischarge + 81 mC*5 mS + 405 pC (eq. 2)
Qtot + Qg ) Qb + 30 nC ) 405p + 30.4 pC (eq. 3)
Cboot +
Qtot
Vripple
+
30.4 nC
150 mV
+ 203 nF
(eq. 4)
Qg is equivalent gate charge of the FET
IBQ is the boot quiescent current
tdishcharge is the discharge time for bootstrap capacitor
Vripple is the allowed ripple voltage in the bootstrap
capacitor
It is recommended to use a larger value so as to cover any
variations in the gate charge and voltage with temperature.
Rboot RESISTOR VALUE CALCULATION
Rboot resistor value is very important to ensure proper
function of the device. A high value of Rboot would slow
down the charging of the Cboot while too low a value would
push very high charging currents for Cboot. For NCP51530
a value between 2
W and 10 W is recommended for Rboot.
For example Rboot = 5 W
Iboot(pk) +
VCC * VD
Rboot
+
15 V * 1V
5 W
+ 2.8 A
(eq. 5)
Where VD is the bootstrap diode forward drop.
Thus, Rboot value of 5 W keeps the peak current below 2.8 A.
HIN AND LIN INPUT FILTER
For PWM connection on the LIN and HIN pin of the
NCP51530, a RC is recommended to filter high frequency
input noise.
This filter is particularly important in case of NCP51530B
where no internal filter is included.
The recommended value for RLIN/RHIN and CHIN/CLIN
are as below.
RLIN/RHIN = 100 W
CHIN/CLIN = 120 pF


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