Datasheet
V
OUTMIN
+ 0.12
ǒǒ
V
INMAX
* I
OMIN
0.110
Ǔ
) V
D
Ǔ
*
ǒ
I
OMIN
R
L
Ǔ
* V
D
(22)
Internal Compensation Network
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
(23)
Thermal Calculations
TPS5420-Q1
www.ti.com
................................................................................................................................................... SLVS752B – NOVEMBER 2007 – REVISED JUNE 2008
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:
Where:
V
INMAX
is the maximum input voltage.
I
OMIN
is the minimum load current.
V
D
is the catch diode forward voltage.
R
L
is the 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
checked to ensure proper functionality.
The design equations given in the example circuit can be used to generate circuits using the TPS5420. These
designs are based on certain assumptions, and 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
TPS5420. Equation 23 gives the nominal frequency response of the internal voltage-mode type-3 compensation
network:
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.
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
× Rds(on) × V
OUT
/V
IN
Switching Loss: Psw = V
IN
× I
OUT
× 0.01
Quiescent Current Loss: Pq = V
IN
× 0.01
Total Loss: Ptot = Pcon + Psw + Pq
Given T
A
=> Estimated Junction Temperature: T
J
= T
A
+ Rth × Ptot
Given T
JMAX
= 125 ° C => Estimated Maximum Ambient Temperature: T
AMAX
= T
JMAX
– Rth × Ptot
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