Datasheet

R22 =
R20 x 1.24V
V
OVP_UP
± 1.24V
©
§
¹
·
R20 =
V
HYST
23 PA
+
-
-+
1.24V
7
OVP
R20
R22
C18
Q3
RPD
V
O
V
IN
OVP
COMP
Board Features www.ti.com
Figure 8. OVP Circuit
Calculating OVP hysterisis and set points:
Step 1: Determine V
HYST
, V
HYST
= (V
OVP_UP
– V
OVP_DN
)
Step 2: Calculate R20
(15)
The V
O
OVP release point (which includes the OVP hysteresis) is described by the equation:
(16)
The evaluation board is already configured with OVP, and the V
O
OVP threshold is programmed on the
evaluation board to 55V with 13V of hysteresis. OVP will therefore release when V
O
reaches 42V (R20 =
562 kΩ, R22 = 12.4 kΩ).
5.11 Under-Voltage Protection
The LM3423 can be configured for under-voltage lockout (UVLO) protection with hysteresis using the
dimming input nDIM (pin 8) and a resistor divider from input voltage to ground. UVLO protects the power
devices during power supply startup and shutdown to prevent operation at voltages less than the minimum
operating input voltage. The UVLO threshold is set up by the resister divider network of R13 and R25 (see
Figure 9).
10
AN-1907 LM3423 Buck-Boost Configuration Evaluation Board SNVA376A–December 2008–Revised May 2013
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