MAX256ASA-T Maxim Integrated Products, MAX256ASA-T Datasheet - Page 6

• Manufacturer Part Number: MAX256ASA-T
• Description: MOSFET & Power Driver ICs
• Manufacturer: Maxim Integrated Products

Specifications of MAX256ASA-T

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

The MAX256 is an integrated primary-side controller
and H-bridge driver for isolated power-supply circuits.
The device contains an on-board oscillator, protection
circuitry, and internal FET drivers to provide up to 3W of
power to the primary winding of a transformer. The
MAX256 can be operated using the internal program-
mable oscillator, or can be driven by an external clock
for improved EMI performance. Regardless of the clock
source being used, an internal flip-flop stage guaran-
tees a fixed 50% duty cycle to prevent DC current flow
in the transformer.
The MAX256 operates from a single-supply voltage of
+5V or +3.3V, and includes undervoltage lockout for
controlled startup. The device prevents cross-conduc-
tion of the H-bridge MOSFETs by implementing break-
before-make switching. Thermal shutdown circuitry
provides additional protection against damage due to
overtemperature conditions.
The MAX256 is driven by the internal programmable
oscillator or an external clock. The logic state of MODE
determines the clock source (see Table 1). Drive
MODE high to select the internal resistor programmable
oscillator. Drive MODE low to operate the MAX256 with
an external clock signal on CK_RS.
The MAX256 includes a 100kHz to 1MHz programma-
ble oscillator. Set the oscillator frequency by connect-
ing CK_RS to ground with a 10kΩ or larger resistor.
Leave CK_RS unconnected to set the oscillator to the
minimum default frequency of 100kHz. CK_RS is inter-
nally pulled to ground with a 165kΩ resistor.
The MAX256 provides an external clock mode. When
operating in external clock mode, 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 MAX256 outputs switch at one half the
external clock frequency. The device switches on the
rising edge of the external clock signal.
When the MAX256 is operating in external clock mode,
a stalled clock could cause excessive DC current to
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3W Primary-Side Transformer H-Bridge Driver
Detailed Description
Internal Oscillator Mode
External Clock Mode
Oscillator Modes
Watchdog
flow through the primary winding of the transformer.
The MAX256 features an internal watchdog circuit to
prevent damage from this condition. The MAX256 is
disabled when the external clock signal on CK_RS
remains at the same logic level for longer than 55µs
(max). The device resumes normal operation upon the
next rising edge on CK_RS.
When using the internal oscillator, drive MODE low to
disable the MAX256. The device is disabled within
55µs after MODE goes low. When operating in external
clock mode, suspend the clock signal for longer than
55µs to disable the MAX256. The device resumes nor-
mal operation when MODE is driven high or when the
external clock signal resumes.
The MAX256 provides an undervoltage lockout feature
to 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 1.8V), the undervoltage lockout
disables the device.
The MAX256 is protected from overtemperature dam-
age by a thermal shutdown circuit. When the junction
temperature (T
abled. The device resumes normal operation when T
falls below +155°C.
As with all Maxim devices, ESD-protection structures
are incorporated on all pins to protect against electro-
static discharges encountered during handling and
assembly.
ESD performance depends on a variety of conditions.
Please contact Maxim for a reliability report document-
ing test setup, methodology, and results.
for Isolated Supplies
Power-Up and Undervoltage Lockout
J
) exceeds +165°C, the device is dis-
ESD Test Conditions
Thermal Shutdown
ESD Protection
Disable Mode
J