ncp3127 ON Semiconductor, ncp3127 Datasheet - Page 13

• Manufacturer Part Number: ncp3127
• Description: Ncp3127 2 A Synchronous Pwm Switching Converter
• Manufacturer: ON Semiconductor

When calculating the rise time and fall time of the high side
MOSFET it is important to know the charge characteristic
shown in Figure 22.
I
Q
R
R
t
V
V
I
Q
R
R
t
V
V
by both the high−side and low−side MOSFETs, but are
dissipated only in the high−side MOSFET.
C
F
P
V
body diode in the low−side MOSFET is shown as follows:
Vth
RISE
FALL
G1
G2
t
SW
DS
HSPU
G
G
HSPD
OSS
GD
BST
TH
t
GD
BST
TH
IN
RISE
Next, the MOSFET output capacitance losses are caused
Finally, the loss due to the reverse recovery time of the
FALL
Figure 22. MOSFET Switching Characteristics
+
+
Q
Q
I
I
G1
G2
GD
GD
= Output current from the high−side gate
= MOSFET gate to drain gate charge
= Drive pull up resistance
= MOSFET gate resistance
= MOSFET rise time
= Boost voltage
= MOSFET gate threshold voltage
= Output current from the low−side gate drive
= MOSFET gate to drain gate charge
= MOSFET gate resistance
= Drive pull down resistance
= MOSFET fall time
= Boost voltage
= MOSFET gate threshold voltage
= MOSFET output capacitance at 0V
= Switching frequency
= MOSFET drain to source charge losses
= Input voltage
+
+
P
DS
drive
P
V
V
RR
+ 1
BST
BST
+ Q
2
* V
* V
@ C
RR
OSS
TH
TH
@ V
Q
Q
GD
@ V
GD
IN
R
R
HSPU
HSPD
IN
@ F
2
@ F
SW
) R
) R
SW
G
G
(eq. 26)
(eq. 27)
(eq. 28)
(eq. 29)
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13
F
P
Q
V
voltages so switching losses are negligible. The low−side
MOSFET’s power dissipation only consists of conduction
loss due to R
non−overlap periods.
P
P
P
follows:
I
R
P
D
I
I
ra
The body diode losses can be approximated as:
F
I
NOL
NOL
P
V
Control Dissipation
The control portion of the IC power dissipation is
determined by the formula below:
I
P
V
designer can calculate the required thermal impedance to
maintain a specified junction temperature at the worst case
RMS_LS
OUT
RMS_LS
OUT
CC
SW
RR
BODY
COND
D_LS
COND
SW
BODY
C
DS(on)_LS
P
RR
IN
FD
IN
The low−side MOSFET turns on into small negative
Conduction loss in the low−side MOSFET is described as
Once the IC power dissipations are determined, the
BODY
I
RMS_LS
HL
LH
+ V
P
COND
+ I
P
FD
= Switching frequency
= High side MOSFET reverse recovery losses
= Reverse recovery charge
= Input voltage
= Low side MOSFET body diode losses
= Low side MOSFET conduction losses
= Low side MOSFET losses
= RMS current in the low side
= Low−side MOSFET on resistance
= High side MOSFET conduction losses
= Duty ratio
= Load current
= RMS current in the low side
= Ripple current ratio
= Switching frequency
= Load current
= Dead time between the high−side
= Dead time between the low−side
= Low−side MOSFET body diode losses
= Body diode forward voltage drop
D_LS
DS(on)
OUT
@ I
+ I
MOSFET turning off and the low−side
MOSFET turning on, typically 50 ns
MOSFET turning off and the high−side
MOSFET turning on, typically 50 ns
= Control circuitry current draw
= Control power dissipation
= Input voltage
OUT
+ P
@
P
RMS_LS
C
@ F
and body diode loss during the
COND
+ I
( 1 * D ) @ 1 ) ra
SW
CC
@ NOL
2
) P
@ R
V
BODY
DS(on)_LS
IN
LH
) NOL
12
2
HL
(eq. 30)
(eq. 31)
(eq. 32)
(eq. 33)
(eq. 34)