MAX5068 Maxim Integrated Products, MAX5068 Datasheet - Page 11

• Manufacturer Part Number: MAX5068
• Description: Current-Mode PWM Controller
• Manufacturer: Maxim Integrated Products

Figure 2. Setting the MAX5068C/E Undervoltage Lockout
Threshold
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
At the beginning of the startup sequence, UVLO/EN is
below the 1.23V threshold, Q1 turns on connecting
R
old is crossed, Q1 turns off, resulting in the series com-
bination of R6, R
normal operating condition.
Calculate the turn-on voltage (V
lowing formula:
where V
Choose an R
The MAX5068 turns off when the MAX5068 UVLO/EN
falls below the 1.18V falling threshold. The turn-off volt-
age (V
where V
HYST
High-Frequency, Current-Mode PWM Controller
OFF
OFF
to GND (Figure 4). Once the UVLO 1.23V thresh-
ULR2
ULF2
R
) to disable the controller (see
) is then defined as:
7
HYST
is the 1.18V UVLO/EN falling threshold.
is the UVLO/EN’s 1.23V rising threshold.
=
R
6
HYST
R
MAX5068C/E
6
______________________________________________________________________________________
=
value in the 20kΩ range.
with Accurate Programmable Oscillator
/
⎛
⎜
⎝
⎛
⎜
⎝
, and R7, placing the MAX5068 in
V
V
V
V
ULR
ULF
OFF
ON
MAX5068A/B/D/F UVLO with
2
2
Programmable Hysteresis
1.23V
1.18V
−
−
1
1
⎞
⎟
⎠
⎞
⎟ ×
⎠
ON
UVLO/EN
−
) by using the fol-
R
R
HYST
HYST
Figure 3).
V
IN
R6
R7
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
exceeds the bootstrap UVLO threshold of 23.6V.
Figure 3. MAX5068 Hysteresis
Figure 4. Setting the MAX5068A/B/D/F Turn-On/Turn-Off Voltages
Bootstrap Undervoltage Lockout
MAX5068A/B/D/F
V
OFF
V
HYST
= V
1.23V
1.18V
ON
(MAX5068A/C/D Only)
- V
OFF
Q1
V
UVLO/EN
HYST
ON
V
IN
R6
R
R7
HYST
11
IN