MAX13256 Datasheet, PDF(9/17 Page) Maxim Integrated Products – 36V H-Bridge Transformer Driver for Isolated Supplies Integrated System Protection

◁

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 tDEAD

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 VUVLO, 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 (RLIM) 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:

RLIM (I) = 650mV/IST1, ST2 (mA)

where IST1, ST2 is the desired current threshold in the

range of 215mA < IST1, ST2 < 650mA (typ). For example,

a 1kI resistor sets the current limit to 650mA. Use a 1%

resistor for RLIM 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

ILIM to 650mA. For an expected operating current of

100mA, set the ILIM 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 (TJ) 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 TJ falls below +150NC.

▷