TPS40074 Datasheet, PDF (11/38 Pages) Texas Instruments – MIDRANGE INPUT SYNCHRONOUS BUCK CONTROLLER WITH VOLTAGE FEED-FORWARD

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APPLICATION INFORMATION (continued)
TPS40074
SLUS617 – APRIL 2005
UNDERVOLTAGE LOCKOUT THRESHOLDS
vs
FEED FORWARD IMPEDANCE
20
F_SW = 300 kHz
18
UNDERVOLTAGE LOCKOUT THRESHOLDS
vs
FEED FORWARD IMPEDANCE
20
F_SW = 500 kHz
18
UVLO V_ON
16
UVLO V_ON
16
14
14
12
12
10
10
8
8
6
6
4
UVLO V_OFF
4
UVLO V_OFF
2
90 120 150 180 210 240 270 300 330 360 390
R_KFF − Feed−Forward Impedance − kΩ
Figure 5.
2
0
30 60 90 120 150 180 210 240 270
R_KFF − Feed−Forward Impedance − kΩ
Figure 6.
UNDERVOLTAGE LOCKOUT THRESHOLDS
vs
FEED FORWARD IMPEDANCE
20
F_SW = 750 kHz
18
UVLO V_ON
16
14
12
10
8
6
4
UVLO V_OFF
2
30
60
90
120
150
180
R_KFF − Feed−Forward Impedance − kΩ
Figure 7.
The programmable UVLO circuit incorporates 20% hysteresis from the start voltage to the shutdown voltage. For
example, if the startup voltage is programmed to be 10 V, the controller starts when V_DD reaches 10 V and shuts
down when V_DD falls below 8 V. The maximum duty cycle begins to decrease as the input voltage rises to twice
the startup voltage. Below this point, the maximum duty cycle is as specified in the electrical table. Note that with
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