NCV8851B ON Semiconductor, NCV8851B Datasheet - Page 16

• Manufacturer Part Number: NCV8851B
• Description: Automotive Grade Synchronous Buck Controller
• Manufacturer: ON Semiconductor

a resistance on the negative input (R
R
another capacitor (C
sets the output voltage. This resistance comprises a resistor
divider with a resistor from the output voltage to the negative
input of the VEA (R
of the VEA to ground (R
calculated as follows:
Thermal Considerations
MOSFETs used, V
average MOSFET gate current typically dominates the
control IC power dissipation. The IC power dissipation can
be estimated as follows:
Where: P
V1
Equations for placement of pole, zero and crossover frequency follow:
The implementation of the above compensators is through
The resistance on the negative input of the VEA (R
The power dissipation of the NCV8851B varies with the
The high−side switching MOSFET gate driver losses are:
) and capacitor (C
I
P
P
Q
IC
TG
BG
: IC measured supply current (quiescent current)
: control IC power dissipation
: high−side MOSFET gate driver losses
: low−side MOSFET gate driver losses
P
IC
Current−loop Compensator
Current−loop Compensator
R
F1
+ V
IN
R
R
C2
C2
) and a resistor from the negative input
F1
F0
C
, C
R
and the boost voltage (V
C1
+
C
w
IN
CE
C1
+
+
w
w
C2
, C
ip
V2
w
@ I
iz
i
+
+
V
V
R
+
+ 2 @ w
R
) in feedback of an opamp. The
+
i
+
V1
Q
@ (C
F0
OUT
REF
F1
V2
w
F
w
) P
C
C
) in series in feedback and
). The resistor values are
* R
SW
iz
ip
C1
CE
@ R
L @ C
C
C1
@ R
1
4
@ C
@ R
1
1
1
C1
* 1
HS
@ p
ip
F1
) C
C2
V2
V2
C1
C1
) P
, R
C2
V2
L
)
), resistor (R
BST
V2
http://onsemi.com
). The
) also
NCV8851B
C1
,
16
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feedback capacitors (C
resistor are chosen, on the order of less than 3 nF. The values
are calculated as follows:
Where: Q
losses are:
Where: Q
calculated as follows:
Where: T
from the specifications section of this data sheet and a
calculation can be made to determine the IC junction
temperature. It should be noted that the physical layout of
the board, the proximity of other heat sources such as
MOSFETs and inductors and the amount of metal connected
to the IC impact the temperature of the device. Use these
calculations as a guide, but measurements should be taken
in the actual application.
The low−side synchronous rectifier MOSFET gate driver
The junction temperature of the controller can then be
The package thermal resistance (R
V
T
R
J
A
TG
BG
qJA
BST
= junction temperature of the IC
the IC package
= ambient temperature
: total high−side MOSFET gate charge at V
: total low−side MOSFET gate charge at V
: BST pin voltage
= junction−to−ambient thermal resistance of
Voltage−loop Compensator
Voltage−loop Compensator
R
P
P
V2
TG
T
BG
C
R
J
+
w
C
+ Q
VE
+ T
V1
w
w
+ Q
vp
V2
vz
w
v
C1
+
+
+ 2 @ w
v
+
+
+
A
TG
@ (C
, C
BG
w
w
) P
F
C
C
vz
vp
@ F
SW
V1
VE
@ F
L @ C
V1
C
V1
2
@ C
4
@ R
1
1
1
V1
IC
* 1
SW
) in series with feedback
@ p
SW
vp
) C
@ R
V1
V1
@ V
@ V
qJA
V2
qJA
BST
CC
)
) can be obtained
BST
IN
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