IR3502A Datasheet, PDF (11/38 Pages) International Rectifier – XPHASE3TM CONTROL IC

IR3502A
PHASE IC
CLOCK
PULSE
EAIN
PWMRMP
VDAC
GATEH
GATEL
STEADY-STATE
OPERATION
DUTY CYCLE INCREASE
DUE TO LOAD
INCREASE
DUTY CYCLE DECREASE
DUE TO VIN INCREASE
(FEED-FORWARD)
DUTY CYCLE DECREASE DUE TO LOAD
DECREASE (BODY BRAKING) OR FAULT
(VCCLUV, OCP, VID=11111X)
STEADY-STATE
OPERATION
Body Braking^TM
Figure 5 PWM Operating Waveforms
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;
T_SLEW

L * (I_MAX  I_MIN )
V_O
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_BODYDIODE. The
minimum time required to reduce the current in the inductor in response to a load transient decrease is now;
T_SLEW

L * (I_MAX  I_MIN )
V_O  V_BODYDIODE
Since the voltage drop in the body diode is often comparable to the output voltage, the inductor current slew rate can
be increased significantly. This patented technique is referred to as “body braking” and is accomplished through the
“body braking comparator” located in the phase IC. If the error amplifier’s output voltage drops below the output voltage
of the share adjust amplifier in the phase IC, this comparator turns off the low side gate driver, enabling the bottom
FET body diode to take over. There is 100mV upslope and 200mV down slope hysteresis for the body braking
comparator.
Lossless Average Inductor Current Sensing
Inductor current can be sensed by connecting a series resistor and a capacitor network in parallel with the inductor and
measuring the voltage across the capacitor, as shown in Figure 6. The equation of the sensing network is,
v_C
( s)

v_L
(s)
1

1
sR_CS C_CS

i_L
(s)
1
R_L  sL
 sR_CSC_CS
Usually the resistor Rcs and capacitor Ccs are chosen, such that, the time constant of Rcs and Ccs equals the time
constant of the inductor, which is the inductance L over the inductor DCR RL. If the two time constants match, the
voltage across Ccs is proportional to the current through L, and the sense circuit can be treated as if only a sense
Page 11 of 38
December 17, 2009