MAX5068 Maxim Integrated Products, MAX5068 Datasheet - Page 14

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

Use the following formula to calculate the internal oscil-
lator frequency:
where f
resistor connected from RT to AGND.
Choose the appropriate resistor at RT to calculate the
desired output switching frequency (f
The MAX5068A/B and the MAX5068C/D/E/F have pro-
grammable output switching frequencies from 25kHz to
1.25MHz and 12.5kHz to 625kHz, respectively.
The MAX5068 programmable dead-time function
(Figure 7) allows additional flexibility in optimizing mag-
netics design and overcoming parasitic effects. The
MAX5068A/B and the MAX5068C/D/E/F have a maxi-
mum 50% and 75% duty cycle, respectively. In many
applications, the duty cycle of the external MOSFET
may need to be slightly decreased to prevent satura-
tion in the transformer’s primary. The dead time can be
configured from 30ns to 1 / (0.5 x f
ming the MAX5068. Connect a resistor between DT and
AGND to set the desired dead time using the following
formula:
where R
Connect DT to REG5 to remove the delay and achieve
the MAX5068 maximum duty cycles.
High-Frequency, Current-Mode PWM Controller
with Accurate Programmable Oscillator
Figure 7. MAX5068 NDRV Dead-Time Timing Diagram
14
DEAD TIME
NDRV
______________________________________________________________________________________
R
R
OSC
DT
RT
RT
is in kΩ and the dead time is in ns.
Dead time
=
=
is the oscillator frequency and R
< 50%
2
4
10
10
f
f
SW
SW
11
11
f
osc
for the MAX
for the MAX
=
=
t
DT
29 4
10
R
60
RT
.
11
Dead-Time Adjustment
×
R
5068
5068
SW
< 50%
DT
SW
) when program-
( )
ns
):
A B and
C D E F
/
/ / /
RT
is a
The MAX5068A/B/C/E can be synchronized using an
external clock at the SYNC input. For proper frequency
synchronization, the SYNC’s input frequency must be at
least 25% higher than the MAX5068A/B/C/E pro-
grammed internal oscillator frequency. Connect SYNC
to AGND when not using an external clock.
The integrating fault-protection feature allows transient
overcurrent conditions to be ignored for a programma-
ble amount of time, giving the power supply time to
behave like a current source to the load. For example,
this can occur under load current transients when the
control loop requests maximum current to keep the out-
put voltage from going out of regulation. Program the
fault-integration time by connecting an external suitably
sized capacitor to the FLTINT. Under sustained over-
current faults, the voltage across this capacitor ramps
up towards the FLTINT shutdown threshold (typically
2.8V). Once the threshold is reached, the power supply
shuts down. A high-value bleed resistor connected in
parallel with the FLTINT capacitor allows it to discharge
towards the restart threshold (typically 1.6V). Once this
threshold is reached, the supply restarts with a new
soft-start cycle.
Note that cycle-by-cycle current limiting is provided at
all times by CS with a threshold of 314mV (typ). The
fault-integration circuit forces a 60µA current onto
FLTINT each time that the current-limit comparator is
tripped (see the Functional Diagram). Use the following
formula to calculate the value of the capacitor neces-
sary for the desired shutdown time of the circuit:
Figure 8. External Synchronization of the MAX5068A/B/C/E
C
FLTINT
Integrating Fault Protection
SYNC
RT
AGND
≅
MAX5068A/B/C/E
External Synchronization
I
FLTINT
2 8
.
(MAX5068A/B/C/E)
V
x t
SH