TPS40077_07 Datasheet, PDF (28/43 Pages) Texas Instruments – HIGH-EFFICIENCY, MIDRANGE-INPUT, SYNCHRONOUS BUCK CONTROLLER WITH VOLTAGE FEED-FORWARD

TPS40077
SLUS714 – JANUARY 2007
www.ti.com
P_CON + R_DS(on)
ƪ1
*
V_OUT
V_IN
*
ǒ_t1
)
t₂^Ǔ
Æ«f_S
ƪ ƫ I_out2
)
I _RIPPLE ²
12
(31)
P_BD + V_f I_OUT ^ǒt₁ ) t₂^Ǔ f_S
(32)
P_GATE + Q_g(TOTAL) V_g
f_S
(33)
where
P_BD = body diode losses
t₁ = body diode conduction prior to turnon of channel = 12 ns for PGD
t₂ = body diode conduction after turnoff of channel = 12 ns for PGD
V_f = body diode forward voltage
Estimating the body diode losses based on a forward voltage of 1 V gives 0.072 W. The gate losses are
unknown at this time, so assume 0.1-W gate losses. This leaves 0.428 W for conduction losses. Using this
figure, a target R_DS(on) of 5 mΩ was calculated.
The Si7336ADP from Vishay was chosen. Using the parameters from its data sheet, the actual expected power
losses are calculated. Conduction loss is 0.317 W, body diode loss is 0.072 W, and the gate loss is 0.136W.
This totals 0.525 W associated with the rectifier MOSFET.
Two other criteria should be verified before finalizing on the rectifier MOSFET. One is the requirement to ensure
that predictive gate drive functions correctly. The turnoff delay of the Si7336ADP is 97 ns. The minimum turnoff
delay of the Si7860DP is 25 ns. Together these devices meet the 130-ns requirement.
Secondly, the ratio between C_gs and C_gd should be greater than 1. The Si7336ADP easily meets this criterion.
This helps reduce the risk of dv/dt-induced turnon of the rectifier MOSFET. If this is likely to be a problem, a
small resistor may be added in series with the boost capacitor, CBOOST.
Component Selection for TPS40077
Timing Resistor, R_T
The timing resistor is calculated using the following equation.
R_T + _fS
1
17.82
10^{*6 * 23}
(34)
This gives a resistor value of 165 kΩ. The nominal frequency using this resistor is 300 kHz.
Feed-Forward and UVLO Resistor, R_KFF
A resistor connected to the KFF pin of the IC feeds into the ramp generator. This resistor provides current into
the ramp generator proportional to the input voltage. The ramp is then adjusted to compensate for different input
voltages. This provides the voltage feed-forward feature of the TPS40077.
The same resistor also sets the undervoltage lockout point. The input start voltage should be used to calculate a
value for R_KFF. For this module, the minimum input voltage is 8 V; however, due to tolerances in the IC, a start
voltage of 10% less than the minimum input voltage is selected. The start voltage for R_KFF calculation is 7.2 V.
Using Equation 35, R_KFF can be selected.
R_KFF + 0.131
R_T
V_UVLO(on) * 1.61
10^*3
V
2
UVLO(on)
)
1.886
V_UVLO * 1.363 * 0.02
R_T
* 4.87 10^*5 R_T2
(35)
where R_KFF and R_T are in kΩ.
This equation gives an R_KFF value of 156 kΩ. The closest lower standard value of 154 kΩ should be selected.
This gives a minimum start voltage of 7.1 V.
28
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