Datasheet
K =
P
O
P
O
+ P
TOTAL-LOSS
100 1k 10k 100k 1M
FREQUENCY (Hz)
-60
-40
-20
0
20
40
60
OVERALL LOOP GAIN (dB)
100 1k 10k 100k 1M
FREQUENCY (Hz)
-180
-160
-140
-120
-100
-80
-60
OVERALL LOOP PHASE (°)
LM3495
SNVS410F –FEBRUARY 2006–REVISED APRIL 2013
www.ti.com
Figure 39. Overall Loop Gain and Phase
The bandwidth of this example circuit is 49 kHz, with a phase margin of 46°.
Efficiency Calculations
A reasonable estimation for the efficiency, η, of a buck regulator controlled by the LM3495 can be obtained by
adding together the loss in each current carrying element, P
TOTAL-LOSS
, and using the equation:
(36)
The following shows an efficiency calculation to complement the Typical Application Circuit. Output power for this
circuit is P
O
= 1.2V x 10A = 12W. Input voltage is assumed to be 12V, and the calculations used assume that the
converter runs in CCM.
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
FETs. The gate driving loss term from the power FET section of Design Considerations is included in the chip
operating loss. For the LM3495, I
IN
is equal to the steady state operating current, I
Q
, plus the FET driving current,
I
GC
. Power is lost as this I
IN
passes through the internal linear regulator of the LM3495.
I
GC
= (Q
G-HI
+ Q
G-LO
) x f
OSC
(37)
I
GC
= (11nC + 33nC) x 500 kHz = 22 mA (38)
I
Q
is typically 1.8 mA, taken from the Electrical Characteristics table. Chip Operating Loss is then:
P
Q
= V
IN
x (I
Q
+ I
GC
) (39)
P
Q
= 12V x (1.8mA + 22mA) = 0.29W (40)
High-Side FET Switching Loss
P
SW
= 0.5 x V
IN
x I
O
x (t
R
+ t
F
) x f
SW
(41)
P
SW
= 0.5 x 12V x 10A x (5 ns + 8 ns) x 500 kHz = 0.39W (42)
FET Conduction Loss
P
C
= D (I
2
O
x R
DSON-HI
x 1.3) (43)
P
C-HI
= 0.1 x (100 x 0.013) = 0.13W (44)
P
C
= (1 - D) (I
2
O
x R
DSON-LO
x 1.3) (45)
P
C-LO
= 0.9 x (100 x 0.0044) = 0.40W (46)
R
SNS
Loss (if used)
P
SNS
= (1 - D) ((I
O
)
2
x R
SNS
) (47)
Not used in this example.
26 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LM3495