IR3088A(2004) View Datasheet(PDF) - International Rectifier
Part Name
Description
MFG CO.
IR3088A Datasheet PDF : 34 Pages
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IR3088A
DESIGN PROCEDURES - IR3081 AND IR3088A CHIPSET
IR3081 EXTERNAL COMPONENTS
Oscillator Resistor Rosc
The oscillator of IR3081 generates a triangle waveform to synchronize the phase ICs, and the switching frequency
of the each phase converter equals the oscillator frequency, which is set by the external resistor ROSC according to
the curve in Figure 13 of IR3081 Data Sheet.
Soft Start Capacitor CSS/DEL and Resistor RSS/DEL
Because the capacitor CSS/DEL programs four different time parameters, i.e. soft start delay time, soft start time,
over-current latch delay time, and power good delay time, they should be considered together while choosing
CSS/DEL.
The SS/DEL pin voltage controls the slew rate of the converter output voltage, as shown in Figure 10 of IR3081
Data Sheet. After the ENABLE pin voltage rises above 0.6V, there is a soft-start delay time tSSDEL, after which the
error amplifier output is released to allow the soft start. The soft start time tSS represents the time during which
converter voltage rises from zero to VO. tSS can be programmed by an external capacitor, which is determined by
Equation (1).
CSS / DEL
=
I CHG * t SS
VO
= 70 *10 −6 * t SS
VO
(1)
Once CSS/DEL is chosen, the soft start delay time tSSDEL, the over-current fault latch delay time tOCDEL, and the
delay time tVccPG from output voltage (VO) in regulation to Power Good are fixed and shown in Equations (2), (3)
and (4) respectively.
t SSDEL
=
CSS / DEL *1.3
ICHG
=
CSS / DEL *1.3
70 *10−6
(2)
t OCDEL
=
C SS / DEL * 0.09
I DISCHG
=
C SS / DEL * 0.09
6 *10 −6
(3)
tVccPG
=
CSS
/ DEL
* (3.91 −VO
ICHG
− 1.3)
=
CSS
/ DEL
* (3.91−VO
70 *10−6
− 1.3)
(4)
If faster over-current protection is required, a resistor in series with the soft start capacitor CSS/DEL can be used to
reduce the over-current fault latch delay time tOCDEL, and the resistor RSS/DEL is determined by Equation (5).
Equation (1) for soft start capacitor CSS/DEL and Equation (4) for power good delay time tVccPG are unchanged,
while the equation for soft start delay time tSS/DEL (Equation 2) is changed to Equation (6). Considering the worst
case values of charge and discharge current, RSS/DEL should be no grater than 10kΩ.
0.09 − tOCDEL * IDISCHG 0.09 − tOCDEL ∗ 6 *10−6
RSS / DEL =
CSS / DEL
=
I DISCHG
CSS / DEL
6 *10−6
(5)
t SSDEL
=
CSS / DEL * (1.3 − RSS / DEL
ICHG
∗ ICHG )
=
CSS / DEL
* (1.3 − RSS / DEL * 70 *10−6 )
70 *10−6
(6)
VDAC Slew Rate Programming Capacitor CVDAC and Resistor RVDAC
The slew rate of VDAC down-slope SRDOWN can be programmed by the external capacitor CVDAC as defined in
Equation (7), where ISINK is the sink current of VDAC pin as shown in Figure 15 of IR3081 Data Sheet. The resistor
RVDAC is used to compensate VDAC circuit and is determined by Equation (8). The slew rate of VDAC up-slope
Page 14 of 34
9/30/04
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