Rainbow Electronics MAX13256 - Manual (Page 9)

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MAX13256
36V H-Bridge Transformer
Driver for Isolated Supplies
Clock Source
Either the internal oscillator or an external clock provides
the switching signal for the MAX13256. Connect CLK to
ground to select the internal oscillator. Provide a clock
signal to CLK to automatically select external clocking.
Internal Oscillator Mode
The MAX13256 includes an internal oscillator that drives
the H-bridge when a watchdog timeout is detected on
CLK. The outputs switch at 425kHz (typ) with a guaran-
teed 50% duty cycle in the internal oscillator mode.
External Clock Mode
The MAX13256 provides an external clock mode. When
an external clock source is applied to CLK, the external
clock drives the H-bridge. An internal flip-flop divides the
external clock by two in order to generate a switching
signal with a guaranteed 50% duty cycle. As a result, the
device outputs switch at one-half of the external clock
frequency. The device switches on the rising edge of the
external clock signal.
Watchdog
A stalled clock could cause excessive DC current to
flow through the primary winding of the transformer. The
MAX13256 features an internal watchdog circuit to pre-
vent damage from this condition. The internal oscillator
provides the switching signal to the H-bridge whenever
the period between edges on CLK exceeds the watch-
dog timeout period of 20Fs (min).
Transients on ST1/ST2 During t
DEAD
When the MAX13256 switches, there is a period of time
when both ST1 and ST2 are high impedance to ensure
that there are no shoot-through currents in the H-bridge.
During this dead time, the voltage at these pins may
temporarily exceed the absolute maximum ratings due
to the inductive load presented by the transformer. This
transient voltage will not damage the device.
Disable Mode
The MAX13256 provides a disable mode to reduce cur-
rent consumption. The ST1 and ST2 outputs are high
impedance in disable mode.
Power-Up and Undervoltage Lockout
The MAX13256 provides an undervoltage-lockout feature
to both ensure a controlled power-up state and prevent
operation before the oscillator has stabilized. On power-
up and during normal operation if the supply voltage
drops below V
UVLO
, the undervoltage-lockout circuit
forces the device into disable mode. The ST1 and ST2
outputs are high impedance in disable mode.
Overcurrent Limiting
The MAX13256 limits the ST1/ST2 output current. Connect
an external resistor (R
LIM
) to ITH to set the current limit.
When the current reaches the limit for longer than the
blanking time of 1.2ms (typ), the drivers are disabled and
FAULT is asserted low. The drivers are reenabled after
the autoretry time of 38.4ms (typ). If a continuous fault
condition is present, the duty cycle of the fault current is
approximately 3%.
To set the current-limit threshold, use the following equation:
( )
( ) LIM
LIM
650
R k
I mA
Ω =
where I
LIM
is the desired current threshold in the range of
215mA < I
LIM
< 650mA (typ). For example, a 1kI resis-
tor sets the current limit to 650mA. Use a 1% resistor for
R
LIM
for increased accuracy.
Ensure that the overcurrent threshold is set to at least
twice the expected maximum operating current. For an
expected maximum operating current of 300mA, set the
I
LIM
to 650mA. For an expected operating current of
100mA, set the I
LIM
to 215mA.
FAULT Output
The FAULT output is asserted low whenever the device
is disabled due to a fault condition. FAULT is automati-
cally deasserted when the device is enabled after the
autoretry time following an overcurrent fault, resulting in
FAULT toggling during a continuous overcurrent condi-
tion. FAULT is asserted for the entire duration of an over-
temperature fault. FAULT is an open-drain output.
Thermal Shutdown
The MAX13256 is protected from overtemperature dam-
age by a thermal-shutdown circuit. When the junction
temperature (T
J
) exceeds +160NC, the device is disabled
and FAULT is asserted low. FAULT stays low for the dura-
tion of an overtemperature fault. The device resumes
normal operation when T
J
falls below +150NC.