IR3094M International Rectifier Corp., IR3094M Datasheet - Page 12

• Manufacturer Part Number: IR3094M
• Description: 3 Phase Pwm Controller For Point Of Load
• Manufacturer: International Rectifier Corp.

The RSFF is reset dominant allowing both phases to go to zero duty cycle within a few tens of nanoseconds in
response to a load step decrease. Phases can overlap and go to 100% duty cycle in response to a load step
increase with turn-on gated by the clock pulses. An Error Amplifier output voltage greater than the common mode
input range of the PWM comparator results in 100% duty cycle regardless of the voltage of the PWM ramp. This
arrangement guarantees the Error Amplifier is always in control and can demand 0 to 100% duty cycle as required.
It also favors response to a load step decrease which is appropriate given the low output to input voltage ratio of
most systems. The inductor current will increase much more rapidly than decrease in response to load transients.
This control method is designed to provide “single cycle transient response” where the inductor current changes in
response to load transients within a single switching cycle maximizing the effectiveness of the power train and
minimizing the output capacitor requirements.
Body Braking
In a conventional synchronous buck converter, the minimum time required to reduce the current in the inductor in
response to a load step decrease is;
The slew rate of the inductor current can be significantly increased by turning off the synchronous rectifier in
response to a load step decrease. The switch node voltage is then forced to decrease until conduction of the
synchronous rectifier’s body diode occurs. This increases the voltage across the inductor from Vout to Vout + V
DIODE
now;
. The minimum time required to reduce the current in the inductor in response to a load transient decrease is
Page 12 of 29
TM
CLK1
CLK2
CLK3
0.6V
EAOUT
50% INTERNAL OSCILLATOR RAMP
DUTY CYCLE
T
RAMP1 SLOPE = 50mV / % DC
SLEW
Figure 3 – 3 Phase Oscillator and PWM Waveforms
RAMP3
= [L x (I
T
SLEW
FIXED RAMP1
MAX
= [L x (I
- I
MIN
MAX
)] / (Vout + V
- I
MIN
RAMP3 MIN DUTY
CYCLE ADJUST
RAMP2
THE SHARE ADJUST ERROR AMPLIFIER CAN
CHANGE THE PULSE WIDTH OF RAMPS 2 & 3 FROM
0.5 x RAMP1 TO 2.0 X RAMP1 TO FORCE
CURRENT SHARING.
)] / Vout
BODY DIODE
)
RAMP3 MAX DUTY
CYCLE ADJUST
(1)
(2)
IR3094PBF
09/26/05
BODY