MAX5068 Datasheet, PDF (11/20 Pages) Maxim Integrated Products – High-Frequency, Current-Mode PWM Controller with Accurate Programmable Oscillator

High-Frequency, Current-Mode PWM Controller
with Accurate Programmable Oscillator
MAX5068C/E
V_IN
R6
UVLO/EN
R7
1.23V
1.18V
V_HYST = V_ON - V_OFF
V_OFF
V_ON
Figure 2. Setting the MAX5068C/E Undervoltage Lockout
Threshold
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:
R6
=
⎛ V_ON
⎝⎜ V_ULR2
⎞
− 1_⎠⎟ ×
R_HYST
where V_ULR2 is the UVLO/EN’s 1.23V rising threshold.
Choose an R_HYST value in the 20kΩ 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:
R7 =
R6 /
⎛
⎝⎜
V_OFF
V_ULF2
⎞
− 1_⎠⎟ − R_HYST
where V_ULF2 is the 1.18V UVLO/EN falling threshold.
Figure 3. MAX5068 Hysteresis
MAX5068A/B/D/F
V_IN
R6
UVLO/EN
1.23V
1.18V
HYST
Q1
R_HYST
R7
Figure 4. Setting the MAX5068A/B/D/F Turn-On/Turn-Off Voltages
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.
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