MAX5069A Maxim Integrated, MAX5069A Datasheet - Page 4

• Manufacturer Part Number: MAX5069A
• Description: MAX5069A Evaluation Kit
• Manufacturer: Maxim Integrated

The MAX5069A EV kit is a 120W, isolated, push-pull
DC-DC converter that provides +12V at up to 10A out-
put. The circuit can be powered from a 36V to 72V DC
source. The user should supply an additional 330µF
bulk storage capacitor between the input terminals
(+VIN, -VIN). This capacitor should be rated for 100V
and be able to carry approximately 2A of ripple current.
Lower ripple-current-rated capacitors should be
acceptable for short-term operation
The push-pull converter achieves high efficiency by
using two external n-channel MOSFETs on the primary
side that can switch up to 90% combined duty cycle
that results in lower primary RMS currents through the
MOSFET switches resulting in improved efficiency.
Cycle-by-cycle current limiting prevents the primary
current from reaching destructive levels, whereas the
fault integration feature provides hiccup fault protec-
tion. For a continuous short circuit at the output, the
MAX5069’s fault integration feature provides hiccup
fault protection. This greatly minimizes destructive tem-
perature rise during persistent overload conditions. Pin
8 of the MAX5069A, capacitor C11, and resistor R9
form the fault integration circuit. Current-sense resistors
R14 and R15 sense the current through the primary of
transformer T1 and turn off the respective MOSFET (N1
or N2) that is switching when the trip level of 314mV
(typ) is reached.
The planar surface-mount transformer features a bias
winding that, along with diode-pair D2, D3, and inductor
L2, provide full-wave rectification for the bias voltage
powering the MAX5069A once the input voltage is stable.
Upon initial input-voltage application, bootstrap resistor
R10 and reservoir capacitor C9 enable the MAX5069A to
startup within approximately 200ms. The transformer uses
a secondary-side center tap and dual Schottky diode D1
to provide full-wave rectification for reduced output-volt-
age ripple. When either MOSFET is turned off and the
alternate turned on, both D1 diodes are conducting. The
transformer provides galvanic isolation up to 500V.
Resistor R21 and capacitor C24 form a snubber network
that suppresses transient overvoltage ringing at diode D1
caused by transformer T1 leakage inductance and diode
D1 junction capacitance.
On the transformer’s secondary side, a 0.6V shunt reg-
ulator (MAX8515, U3) along with feedback resistors R1
and R2 provide secondary-side voltage feedback
through optocoupler U2 to the primary side. The
MAX5069A receives the voltage feedback signal on the
primary side from biasing resistors R16, R17, R18, and
optocoupler U2.
MAX5069A Evaluation Kit
4
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Detailed Description
The MAX5069A controller’s internal oscillator clock
switches at 500kHz and the frequency is programmed
by resistor R3. The duty cycle of both MOSFETs is var-
ied to control energy transfer to the output. The
MAX5069A EV kit’s maximum duty cycle is 90% as con-
figured. Resistor R4 sets the dead time for both switch-
es and has an impact on the maximum duty cycle.
The MAX5069A brownout and input-supply startup
UVLO threshold voltage is set by resistors R5 and R6.
This prevents the power supply from starting up or
operating below the programmed input supply voltage.
Test point TP1 is provided to connect an external clock
to the SYNC pin on the MAX5069A.
The EV kit’s PC board layout is a 4-layer 2oz copper
PC board.
The MAX5069A EV kit’s output (V
feedback resistors R1 and R2. To generate other output
voltages in the vicinity of +12V (from +8V to +15V, limit-
ed by the output capacitor and diode D1 voltage rat-
ing), select different voltage-divider resistors (R1, R2).
Resistor R2 is typically chosen to be less than 2kΩ.
Using the desired output voltage, resistor R1 is then
found by the following equation:
where V
The maximum output current should be limited to less
than 10A. The usable output voltage range for the EV kit
is +8V to +15V. Additionally, ICs U3, U2, and resistor R19
limit the minimum output voltage (V
voltages outside the above range, a different turns ratio
transformer may be required.
The EV kit features cycle-by-cycle current limiting of the
transformer primary current. Current-sense resistors
R14 and R15 sense the current through the primary of
transformer T1 and turn off MOSFET N1 or N2 when the
trip level of 314mV (typ) is reached. Current-sense
resistors R14 and R15 limit the peak primary current to
approximately 5.3A (314mV / 0.060Ω ≈ 5.3A). This will
limit short-circuit current on the secondary output
(VOUT) to 20A typically. To evaluate lower current lim-
its, current-sense resistors R14 and R15 must be
replaced with different value surface-mount resistors
(2010 size) as determined by the following equation:
REF
Evaluating Other Output Voltages,
= 0.6V
R
1
=
R
2



Current Limits, UVLOs



V
V
OUT
REF
V
OUT



OUT
−
OUT
1
) is set to +12V by



Current Limiting
Output Voltage
) to +2.6V. For