LM3423MHBKBSTEV/NOPB National Semiconductor, LM3423MHBKBSTEV/NOPB Datasheet - Page 8

• Manufacturer Part Number: LM3423MHBKBSTEV/NOPB
• Description: BOARD EVAL BUCK BOOST LM3423
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3423MHBKBSTEV/NOPB

• Current - Output / Channel: 1A
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 35V
• Features: Dimmable
• Voltage - Input: 4.5 ~ 35V
• Utilized Ic / Part: LM3423
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM3423MHBKBSTEV

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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 sup-
ply startup and shutdown to prevent operation at voltages less
than the minimum operating input voltage. The UVLO thresh-
old is set up by the resister divider network of R13 and R25
(see figure below).
The UVLO threshold and hysteresis can be programmed
completely independent of each other. UVLO hysteresis is
accomplished with an internal 23 µA current source that is
switched on and off into the impedance of the UVLO set-point
resistor divider. When the UVLO pin exceeds 1.24V, the cur-
rent source is activated to instantly raise the voltage at the
UVLO pin. When the UVLO pin voltage falls below the 1.24V
threshold, the current source is turned off, causing the voltage
at the UVLO pin to fall. The UVLO hysteresis range can be
user adjusted using the gain resistor R26.
FIGURE 7. UVLO Circuit During Start-Up
30080816
30080815
8
Step 1: Choose V
(V
(V
Step 2: Solve for resistor value R13 with the following equa-
tion.
Solve for resistor value R25 wit the following equation:
Example Calculation of UVLO with Hysterisis:
•
•
•
•
•
•
If a small amount of hysteresis is desired and V
resistor R26 may need to be populated.
FIGURE 9. UVLO Circuit with R26 Populated for Small
UV_UP
UV_DN
V
V
I
V
R13
R25
FIGURE 8. UVLO Circuit During Normal Operation
UVLO
IN
IN
HYST
start-up = V
shut down = V
) and choose VIN voltage where converter shuts down
). V
≊
≊
= 23 µA
= 8.45V – 8.2 = 0.25V
10 kΩ
1.74 kΩ
HYST
Hysteresis and Large Input Voltage
= (V
IN
UV_UP
voltage where converter starts to operate
UVLO_UP
UV_DN
= 8.45V
= 8.2V
- V
UV_DN
)
30080820
30080816
IN
is large,