Datasheet
H(s) +
ǒ
1 )
s
2p Fz1
Ǔ
ǒ
1 )
s
2p Fz2
Ǔ
ǒ
s
2p Fp0
Ǔ
ǒ
1 )
s
2p Fp1
Ǔ
ǒ
1 )
s
2p Fp2
Ǔ
ǒ
1 )
s
2p Fp3
Ǔ
V
OUTMIN
+ 0.12
ǒǒ
V
INMAX
* I
OMIN
0.110
Ǔ
) V
D
Ǔ
*
ǒ
I
OMIN
R
L
Ǔ
* V
D
TPS5450
www.ti.com
SLVS757C –MARCH 2007–REVISED OCTOBER 2013
Where
V
INMIN
= minimum input voltage
I
OMAX
= maximum load current
V
D
= catch diode forward voltage.
R
L
= output inductor series resistance.
This equation assumes maximum on resistance for the internal high side FET.
The lower limit is constrained by the minimum controllable on time which may be as high as 200 ns. The
approximate minimum output voltage for a given input voltage and minimum load current is given by:
(14)
Where
V
INMAX
= maximum input voltage
I
OMIN
= minimum load current
V
D
= catch diode forward voltage.
R
L
= output inductor series resistance.
This equation assumes nominal on resistance for the high side FET and accounts for worst case variation of
operating frequency set point. Any design operating near the operational limits of the device should be
carefully checked to assure proper functionality.
Internal Compensation Network
The design equations given in the example circuit can be used to generate circuits using the TPS5450. These
designs are based on certain assumptions and will tend to always select output capacitors within a limited range
of ESR values. If a different capacitor type is desired, it may be possible to fit one to the internal compensation of
the TPS5450. Equation 15 gives the nominal frequency response of the internal voltage-mode type III
compensation network:
(15)
Where
Fp0 = 2165 Hz, Fz1 = 2170 Hz, Fz2 = 2590 Hz
Fp1 = 24 kHz, Fp2 = 54 kHz, Fp3 = 440 kHz
Fp3 represents the non-ideal parasitics effect.
Using this information along with the desired output voltage, feed forward gain and output filter characteristics,
the closed loop transfer function can be derived.
Thermal Calculations
The following formulas show how to estimate the device power dissipation under continuous conduction mode
operations. They should not be used if the device is working at light loads in the discontinuous conduction mode.
Conduction Loss: Pcon = I
OUT
2
x R
DS(on)
x V
OUT
/V
IN
Switching Loss: Psw = V
IN
x I
OUT
x 0.01
Quiescent Current Loss: Pq = V
IN
x 0.01
Total Loss: Ptot = Pcon + Psw + Pq
Given T
A
=> Estimated Junction Temperature: T
J
= T
A
+ Rth x Ptot
Given T
JMAX
= 125°C => Estimated Maximum Ambient Temperature: T
AMAX
= T
JMAX
– Rth x Ptot
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