UCC28880 Datasheet, PDF (21/35 Pages) Texas Instruments – 700-V Lowest Quiescent Current Off-Line Switcher

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UCC28880
SLUSC05 – JULY 2014
8.2.2.2 Detailed Design Procedure
The low-side buck converter and high-side buck converter design procedures are very similar.
8.2.2.2.1 Feedback path (C_FB, R_FB1 and R_FB2) and Load Resistor (R_L)
In low-side buck converter the output voltage is always sensed by the FB pin and UCC28880 internal controller
can turn on the MOSFET on VOUT. In high-side buck converter applications the information on the output
voltage value is stored on C_FB capacitor. This information is not updated in real time. The information on C_FB
capacitor is updated just after MOSFET turn-off event. When the MOSFET is turned off, the inductor current
forces the freewheeling diode (D1 in Figure 17) to turn on and the GND pin of UCC28880 goes negative at -V_d1
(where V_d1 is the forward drop voltage of diode D1) with respect to the negative terminal of bulk capacitor (C1 in
Figure 17). When D1 is on, through diode D4, the C_FB capacitor is charged at V_OUT – V_d4 + V_d1. Set the output
voltage regulation level using Equation 18.
R_FB1
R_FB2
V_OUT(T)  V_d4  V_d1  V_{FB _ TH #} V_OUT(T)  V_{FB _ TH}
V_{FB _ TH}
V_{FB _ TH}
where
• V_{FB_TH} is the FB pin reference voltage.
• V_{OUT_T} is the target output voltage.
• R_FB1, R_FB2 is the resistance of the resistor divider connected with FB pin (see Figure 17)
• The capacitor C_FB after D1 is discharged with a time constant that is τfb = C_FB x (R_FB1 + R_FB2 ).
• Select the time constant τ_FB, given in Equation 19
(18)
  W_FB
C_FB u R_FB1  R_FB2
#
1
10
u
C_L
u
R_L
(19)
The time constant selection leads to a slight output-voltage increase in no-load or light-load conditions. In order
to reduce the output-voltage increase, increase τ_FB. The drawback of increasing τ_FB is t in high-load conditions
V_OUT could drop.
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