TPS54350
SLVS456C − OCTOBER 2003 − REVISED OCTOBER 2004
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APPLICATION INFORMATION
FUNCTIONAL BLOCK DIAGRAM
BOOT
VIN
320 kΩ
Hiccup
UVLO
125 kΩ
1.2V
UVLO
Current Limit
PH
SYNC
RT
2x Oscillator
Bias + Drive
Regulator
PWM Ramp
(FeedFoward)
VBIAS
COMP
VSENSE
VBIAS2
Reference
System
5
µA
ENA
Hiccup
Timer
Hiccup
Error
Amplifier
PWM
Comparator
S
Q
Adaptive Deadtime
and
Control Logic
VBIAS
LSG
R
Thermal
Shutdown
PWRGD
UVLO
VSENSE
97% Ref
UVLO
Rising
Edge
Delay
TPS54350
POWERPAD
VBIAS
PGND
AGND
DETAILED DESCRIPTION
Undervoltage Lockout (UVLO)
The undervoltage lockout (UVLO) system has an internal
voltage divider from VIN to AGND. The defaults for the
start/stop values are labeled VIN and given in Table 1. The
internal UVLO threshold can be overridden by placing an
external resistor divider from VIN to ground. The internal
divider values are approximately 320 kΩ for the high side
resistor and 125 kΩ for the low side resistor. The divider
ratio (and therefore the default start/stop values) is quite
accurate, but the absolute values of the internal resistors
may vary as much as 15%. If high accuracy is required for
an externally adjusted UVLO threshold, select lower value
external resistors to set the UVLO threshold. Using a 1-kΩ
resistor for the low side resistor (R2 see Figure 1) is
recommended. Under no circumstances should the UVLO
pin be connected directly to VIN.
Table 1. Start/Stop Voltage Threshold
START VOLTAGE THRESHOLD
VIN (Default)
UVLO
4.49
1.24
STOP VOLTAGE THRESHOLD
3.69
1.02
6
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