Rainbow Electronics MAX5069 - Manual (Page 14)

MAX5069
Oscillator/Switching Frequency
Use an external resistor at RT to program the MAX5069
internal oscillator frequency from 50kHz to 2.5MHz. The
MAX5069 NDRVA/NDRVB switching frequency is one
half of the programmed oscillator frequency with a
maximum 50% duty cycle.
Use the following formula to calculate the internal oscil-
lator frequency:
where f
OSC
is the oscillator frequency and R
RT
is a
resistor connected from RT to AGND.
Choose the appropriate resistor at RT to calculate the
desired switching frequency (f
SW ):
For the maximum 50% duty cycle at NDRVA/NDRVB,
connect DT to REG5.
Dual N-Channel MOSFET Switch Driver
The MAX5069 drives two external N-channel MOSFETs
in push-pull isolated power supplies. Each MOSFET
driver operates with a maximum 50% duty cycle. The
NDRV_ outputs are supplied by the internal regulator
(V
CC ), which is internally set to approximately 9.5V. For
the universal input voltage range, the MOSFETs used
must be able to withstand at least twice the DC level of
the high-line input voltage. Both NDRVA and NDRVB
can source and sink in excess of 650mA and 1000mA
peak current, respectively.
Dead-Time Control
In typical push-pull designs, it is desirable to add some
extra delay between the turning off of one MOSFET and
the turning on of the next MOSFET (Figure 7). The extra
time ensures that the first MOSFET is fully off when the
other MOSFET starts to turn on. This prevents both
MOSFETs from being on simultaneously, thus avoiding
shorting out the transformer’s primary. The MAX5069
allows the dead-time delay required to turn on the
NDRVB FET after the NDRVA FET turns off. The dead
time can be programmed to a minimum of 30ns to 1 / (0.5
x f
SW ). Connect a resistor between DT and AGND to set
the desired dead time. Calculate the dead time using the
following formula:
where R
DT
is in kΩ and the dead time is in ns.
External Synchronization (MAX5069A/D)
The MAX5069A/D can be synchronized using an exter-
nal clock at the SYNC input. For proper frequency syn-
chronization, the SYNC’s input frequency must be at
least 25% higher than the MAX5069A/D programmed
internal oscillator frequency. Connect SYNC to AGND
when not using an external clock.
Integrating Fault Protection
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.
Dead time R ns
DT
.
( ) = ×
60
29 4
RT
SW
R
f
=
10
2
11
f
R
osc
RT
=
10
11
High-Frequency, Current-Mode PWM Controller
with Accurate Oscillator and Dual FET Drivers
14 ______________________________________________________________________________________
DEAD TIME
NDRVA PWM
PWM
<50%
<50% NDRVB
t
DT
Figure 7. MAX5069 Dead-Time Timing Diagram
MAX5069A/D
AGND
RT
SYNC
Figure 8. External Synchronization of the MAX5069A/D