NCV8851
http://onsemi.com
16
Equations for placement of pole, zero and crossover frequency follow:
Current−loop Compensator Voltage−loop Compensator
w
iz
+
1
L @ C
Ǹ
w
vz
+
2
L @ C
Ǹ
w
ip
+
F
SW
@ p
4
w
vp
+
F
SW
@ p
4
w
i
+ 2 @ w
ip
w
v
+ 2 @ w
vp
The implementation of the above compensators is through
a resistance on the negative input (R
C2
, R
F1
), resistor (R
C1
,
R
V1
) and capacitor (C
C1
, C
V1
) in series in feedback and
another capacitor (C
C2
, C
V2
) in feedback of an opamp. The
feedback capacitors (C
C1
, C
V1
) in series with feedback
resistor are chosen, on the order of less than 3 nF. The values
are calculated as follows:
Current−loop Compensator Voltage−loop Compensator
R
C1
+
1
w
iz
@ C
C1
R
V1
+
1
w
vz
@ C
V1
C
CE
+
1
w
ip
@ R
C1
C
VE
+
1
w
vp
@ R
V1
C
C2
+
C
C1
C
C1
C
CE
* 1
C
V2
+
C
V1
C
V1
C
VE
* 1
R
C2
+
1
w
i
@ (C
C1
) C
C2
)
R
F1
+
1
w
v
@ (C
V1
) C
V2
)
The resistor divider on the negative input of the VEA also
sets the output voltage. This resistor divider is composed of
a resistor from the output voltage to the negative input of the
VEA (R
F1
) and a resistor from the negative input of the VEA
to ground (R
F0
). The bottom resistor value is calculated as
follows:
R
F0
+
R1 V
REF
V
OUT
* V
REF
Thermal Considerations
The power dissipation of the NCV8851 varies with the
MOSFETs used, V
IN
and the boost voltage (V
BST
). The
average MOSFET gate current typically dominates the
control IC power dissipation. The IC power dissipation can
be estimated as follows:
P
IC
+ V
IN
@ I
Q
) P
HS
) P
L
Where: P
IC
: control IC power dissipation
I
Q
: IC measured supply current (quiescent current)
P
TG
: high−side MOSFET gate driver losses
P
BG
: low−side MOSFET gate driver losses
The high−side switching MOSFET gate driver losses are:
P
TG
+ Q
TG
@ F
SW
@ V
BST
Where: Q
TG
: total high−side MOSFET gate charge at V
BST
V
BST
: BST pin voltage
The low−side synchronous rectifier MOSFET gate driver
losses are:
P
BG
+ Q
BG
@ F
SW
@ V
CC
Where: Q
BG
: total low−side MOSFET gate charge at V
IN
The junction temperature of the controller can then be
calculated as follows:
T
J
+ T
A
) P
IC
@ R
qJA
Where: T
J
= junction temperature of the IC
T
A
= ambient temperature
R
q
JA
= junction−to−ambient thermal resistance of
the IC package
The package thermal resistance (R
q
JA
) can be obtained
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.
