TPS5430-Q1
SLVS751C–NOVEMBER 2007–REVISED JULY 2009................................................................................................................................................... www.ti.com
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:
ǒǒV
0.110Ǔ ) V Ǔ* ǒI
LǓ* V
D
V
+ 0.12
* I
R
OUTMIN
INMAX
OMIN
D
OMIN
(22)
Where:
VINMAX is the maximum input voltage.
IOMIN is the minimum load current.
VD is the catch diode forward voltage.
RL 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
carefully checked to ensure proper functionality.
Internal Compensation Network
The design equations given in the example circuit can be used to generate circuits using the TPS5430. 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 TPS5430. Equation 23 gives the nominal frequency response of the internal voltage-mode type-3
compensation network:
s
s
ǒ1 )
Ǔ ǒ
1 )
Ǔ
2p Fz1
2p Fz2
H(s) +
s
s
s
s
ǒ Ǔ ǒ
Ǔ ǒ
1 )
Ǔ ǒ
1 )
Ǔ
1 )
2p Fp0
2p Fp1
2p Fp2
2p Fp3
(23)
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 = IOUT2 × Rds(on) × VOUT/VIN
Switching loss: Psw = VIN × IOUT × 0.01
Quiescent current loss: Pq = VIN × 0.01
Total loss: Ptot = Pcon + Psw + Pq
Given TA ≥ Estimated junction temperature: TJ = TA + Rth × Ptot
Given TJMAX = 125°C ≥ Estimated maximum ambient temperature: TAMAX = TJMAX – Rth × Ptot
18
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