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4

OPERATING

CONSIDERATIONS

SA60

GENERAL

Please read Application Note 30 on "PWM Basics". Refer

to Application Note 1 "General Operating Considerations" for

helpful information regarding power supplies, heat sinking and

mounting. Visit www.apexmicrotech.com for design tools that

help automate pwm filter design; heat sink selection; Apex’s

complete Application Notes library; Technical Seminar Work-

book; and Evaluation Kits.

PWM OSCILLATOR – INTERNAL OR EXTERNAL

The SA60 contains an internal PWM oscillator whose

frequency is determined by an external capacitor connected

between pin 1 and pin 2. Maximum frequency is 125 kHz.

The user may also disregard the internal PWM oscillator and

supply the SA60 with an external TTL pulse generator up to

250KHZ.

PIN DESCRIPTION

VCC - is the low voltage supply for powering internal logic and

drivers for the lowside and highside MOSFETS. The supplies

for the highside drivers are derived from this voltage.

V

obtain the output current from this supply pin. The voltage on

this pin is limited to +80V by the drive IC. The MOSFETS are

rated at 100 volts.

ISENSE A AND B - These are tied to power gnd directly or

through sense resistors.

ANALOG GND - is the reference for the internal PWM

oscillator. Connect this pin to pin 6. Connect low side of Vcc

supply and any other supply used to generate analog input

signals to ANALOG GND.

ANALOG INPUT - is an analog input for controlling the

PWM pulse width of the bridge. A voltage higher than Vcc/2

will produce greater than 50% duty cycle pulses out of B OUT.

A voltage lower than Vcc/2 will produce greater than 50% duty

cycle pulses out of A OUT. If using in the digital mode, bias

this point at 1/2 the logic high level.

DISABLE - Is the connection for disabling all 4 output

switches. DISABLE high overrides all other inputs. When taken

low, everything functions normally. An internal pullup to Vcc

will keep DISABLE high if pin left open.

PWM INPUT - Is the TTL compatible digital input for control-

ling the PWM pulse width of the bridge. A duty cycle greater

than 50% will produce greater than 50% duty cycle pulses out

of the A out. A duty cycle less than 50% will produce greater

than 50% duty cycle from the B out. For analog inputs, the

integration capacitor for the internal clock must be connected

between this pin and analog ground. The internal switching

frequency is programmable up to 125 kHz by selection of the

integration capacitor. The formula is:

BYPASSING

Adequate bypassing of the power supplies is required for

proper operation. Failure to do so can cause erratic and low

efficiency operation as well as excessive ringing at the out-

puts. The Vs supply should be bypassed with at least a 1µF

ceramic capacitor in parallel with another low ESR capacitor

of at least 10µF per amp of output current. Capacitor types

rated for switching applications are the only types that should

be considered. The 1µF ceramic capacitor must be physically

connected directly to the Vs and POWER GND pins. Even one

inch of lead length will cause excessive ringing at the outputs.

This is due to the very fast switching times and the inductance

of the lead connection. The bypassing requirements of the Vcc

supply are less stringent, but still necessary. A .1µF to .47µF

ceramic capacitor connected directly to the Vcc and ANALOG

GND pins will suffice.

PCB LAYOUT

The designer needs to appreciate that the SA60 combines

in one circuit both high speed high power switching and low

level analog signals. Certain layout rules of thumb must be

considered when a circuit board layout is designed using the

SA60:

1. Bypassing of the power supplies is critical. Capacitors

must be connected directly to the power supply pins with

very short lead lengths (well under 1 inch). Ceramic chip

capacitors are best.

2. Connect ANALOG GND to POWER GND with a conductor

having no intermediate connections. Connect all Vs power

supply, filter and load related ground connections to POWER

GND keeping these conductors separate until reaching pin

6. Connect all Vcc power supply and input signal related

ground connections to ANALOG GND keeping conductors

separate until reaching pin 1. Do not allow ground loops to

form by making additional ground connections at the low

side of the physical power supplies. If ground plane is used

do not allow more than 1mA to flow through it.

3. Beware of capacitive coupling between output connections

and signal inputs through the parasitic capacitance between

layers in multilayer PCB designs.

4. Do not run small signal traces between the pins of the output

section (pins 8-12).

CURRENT SENSE

There are two load current sensing pins, I SENSE A and I

SENSE B. The two pins can be shorted to POWER GND in

the voltage mode connection but both must be used in the

current mode connection. It is recommended that R SENSE

resistors be non-inductive. Load current flows in the I SENSE

pins. The SA60 has no internal current limit.

TRANSIENT SUPPRESSION

An RC Network of A 100 PF Capacitor and a one ohm resis-

tor is required as shown in the external connection diagram on

page 1. This network assures proper operation under various

loads. Minimal power is dissipated in the resistor.

This data sheet has been carefully checked and is believed to be reliable, however, no responsibility is assumed for possible inaccuracies or omissions. All specifications are subject to change without notice.

SA60U REV J OCTOBER 2004 © 2004 Apex Microtechnology Corp.

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