Datasheet

K =
P
O
P
O
+ P
total-loss
100 1k 10k 100k 1M
FREQUENCY (Hz)
-180
-120
-60
0
60
120
180
OVERALL LOOP PHASE (°)
100 1k 10k 100k 1M
FREQUENCY (Hz)
-60
-40
-20
0
20
40
60
OVERALL LOOP GAIN (dB)
LM5022
SNVS480G –JANUARY 2007–REVISED DECEMBER 2013 www.ti.com
Figure 23. Overall Loop Gain and Phase Figure 24. Overall Loop Gain and Phase
The bandwidth of this example circuit at V
IN
= 16V is 10.5 kHz, with a phase margin of 66°.
13. Re-evaluate at the corners of input voltage and output current: Boost converters exhibit significant
change in their loop response when V
IN
and I
O
change. With the compensation fixed, the total control loop gain
and phase should be checked to ensure a minimum phase margin of 45° over both line and load.
Efficiency Calculations
A reasonable estimation for the efficiency of a boost regulator controlled by the LM5022 can be obtained by
adding together the loss is each current carrying element and using the equation:
(53)
The following shows an efficiency calculation to complement the circuit design from Design Considerations.
Output power for this circuit is 40V x 0.5A = 20W. Input voltage is assumed to be 13.8V, and the calculations
used assume that the converter runs in CCM. Duty cycle for V
IN
= 13.8V is 66%, and the average inductor
current is 1.5A.
CHIP OPERATING LOSS
This term accounts for the current drawn at the VIN pin. This current, I
IN
, drives the logic circuitry and the power
MOSFETs. The gate driving loss term from MOSFET is included in the chip operating loss. For the LM5022, I
IN
is
equal to the steady state operating current, I
CC
, plus the MOSFET driving current, I
GC
. Power is lost as this
current passes through the internal linear regulator of the LM5022.
I
GC
= Q
G
X f
SW
I
GC
= 27 nC x 500 kHz = 13.5 mA (54)
I
CC
is typically 3.5 mA, taken from the Electrical Characteristics table. Chip Operating Loss is then:
P
Q
= V
IN
X (I
Q
+ I
GC
) P
Q
= 13.8 X (3.5m + 13.5m) = 235 mW (55)
MOSFET SWITCHING LOSS
P
SW
= 0.5 x V
IN
x I
L
x (t
R
+ t
F
) x f
SW
P
SW
= 0.5 x 13.8 x 1.5 x (10 ns + 12 ns) x 5 x 10
5
= 114 mW (56)
MOSFET AND R
SNS
CONDUCTION LOSS
P
C
= D x (I
L
2
x (R
DSON
x 1.3 + R
SNS
)) P
C
= 0.66 x (1.5
2
x (0.029 + 0.1)) = 192 mW (57)
OUTPUT DIODE LOSS
The average output diode current is equal to I
O
, or 0.5A. The estimated forward drop, V
D
, is 0.5V. The output
diode loss is therefore:
P
D1
= I
O
x V
D
P
D1
= 0.5 x 0.5 = 0.25W (58)
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