Rainbow Electronics MAX5068 - Manual (Page 11)

MAX5068A/B/D/F UVLO with
Programmable Hysteresis
In addition to programmable undervoltage lockout dur-
ing startup, the MAX5068A/B/D/F incorporate a
UVLO/EN hysteresis that allows the user to set a volt-
age (V
OFF ) to disable the controller (see Figure 3).
At the beginning of the startup sequence, UVLO/EN is
below the 1.23V threshold, Q1 turns on connecting
R
HYST to GND (Figure 4). Once the UVLO 1.23V thresh-
old is crossed, Q1 turns off, resulting in the series com-
bination of R6, R
HYST , and R7, placing the MAX5068 in
normal operating condition.
Calculate the turn-on voltage (V ON ) by using the fol-
lowing formula:
where V ULR2 is the UVLO/EN’s 1.23V rising threshold.
Choose an R HYST value in the 20k
Ω
range.
The MAX5068 turns off when the MAX5068 UVLO/EN
falls below the 1.18V falling threshold. The turn-off volt-
age (V
OFF ) is then defined as:
where V ULF2 is the 1.18V UVLO/EN falling threshold.
Bootstrap Undervoltage Lockout
(MAX5068A/C/D Only)
In addition to the externally programmable UVLO func-
tion offered by the MAX5068, the MAX5068A/C/D fea-
ture an additional internal bootstrap UVLO for use in
high-voltage power supplies (see the Functional
Diagram). This allows the device to bootstrap itself dur-
ing initial power-up. The MAX5068A/C/D start when V
IN
exceeds the bootstrap UVLO threshold of 23.6V.
R R
V
V
R
OFF
ULF
HYST
7 6 1
2
/
=

 

− −
R
V
V
R
ON
ULR
HYST
6 1
2
=

 

× −
MAX5068
High-Frequency, Current-Mode PWM Controller
with Accurate Programmable Oscillator
______________________________________________________________________________________ 11
MAX5068C/E
1.23V
1.18V
UVLO/EN
R7
R6
V
IN
Figure 2. Setting the MAX5068C/E Undervoltage Lockout
Threshold
V
HYST
= V
ON
- V
OFF
V OFF V ON
Figure 3. MAX5068 Hysteresis
MAX5068A/B/D/F
1.23V
1.18V
UVLO/EN
HYST
R HYST
R6
R7
V IN
Q1
Figure 4. Setting the MAX5068A/B/D/F Turn-On/Turn-Off Voltages