TPS53353_17 Datasheet, PDF (22/37 Pages) Texas Instruments – High-Efficiency 20-A Synchronous Buck SWIFT Converter With Eco-mode

TPS53353
SLUSAK2C – AUGUST 2011 – REVISED FEBRUARY 2016
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4. CHOOSE THE OUTPUT CAPACITORS
When organic semiconductor capacitors or specialty polymer capacitors are used, for loop stability,
capacitance and ESR should satisfy Equation 6. For jitter performance, Equation 10 is a good starting point
to determine ESR.
ESR = ^{VOUT ´10mV ´ (1- D)} = ^{10mV ´ L ´ fSW} = ^{L ´ fSW} (W)
0.6 V ´ I_IND(ripple)
0.6 V
60
where
• D is the duty factor.
• The required output ripple slope is approximately 10 mV per t_SW (switching period) in terms of VFB terminal
voltage.
(10)
5. DETERMINE THE VALUE OF R1 AND R2
The output voltage is programmed by the voltage-divider resistor, R1 and R2 shown in Figure 37. R1 is
connected between VFB pin and the output, and R2 is connected between the VFB pin and GND.
Recommended R2 value is from 1 kΩ to 20 kΩ. Determine R1 using Equation 11.
R1 =
V_OUT
-
I_IND(ripple) ´ ESR
2
- 0.6
´ R2
0.6
(11)
6. CHOOSE THE OVERCURRENT SETTING RESISTOR
The overcurrent setting resistor, R_TRIP, can be determined by .
R_TRIP (kW)
=
æ
ççèI_OCP
-
æ
ç
è
1
2 ´ L ´ f_SW
ö
÷
´
ø
(V_IN - V_OUT )´
V_IN
I_TRIP (mA)
V_OUT
ö
÷÷ø ´ 32 ´ R_DS(on)
(mW)
where
• I_TRIP is the TRIP pin sourcing current (10 µA).
• R_DS(on) is the thermally compensated on-time resistance value of the low-side MOSFET.
(12)
Use an R_DS(on) value of 1.6 mΩ for an overcurrent level of approximately 20 A. Use an R_DS(on) value of
1.7 mΩ for overcurrent level of approximately 10 A.
8.2.1.3 Application Curves
Figure 39. Enable Turnon
Figure 40. Enable Turnoff
22
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