iru3011 International Rectifier Corp., iru3011 Datasheet - Page 9

• Manufacturer Part Number: iru3011
• Description: 5-bit Programmable Synchronous Buck Controller Ic -
• Manufacturer: International Rectifier Corp.

Output Inductor Selection
The output inductance must be selected such that un-
der low line and the maximum output voltage condition,
the inductor current slope times the output capacitor
ESR is ramping up faster than the capacitor voltage is
drooping during a load current step. However, if the in-
ductor is too small, the output ripple current and ripple
voltage become too large. One solution to bring the ripple
current down is to increase the switching frequency,
however, that will be at the cost of reduced efficiency
and higher system cost. The following set of formulas
are derived to achieve the optimum performance without
many design iterations.
The maximum output inductance is calculated using the
following equation:
Assuming that the programmed switching frequency is
set at 200KHz, an inductor is designed using the
Micrometals’ powder iron core material. The summary
of the design is outlined below:
The selected core material is Powder Iron, the selected
core is T50-52D from Micro Metal wounded with 8 turns
of #16 AWG wire, resulting in 3mH inductance with
3mV of DC resistance.
Assuming L=3mH and Fsw=200KHz(switching fre-
quency), the inductor ripple current and the output ripple
voltage is calculated using the following set of equations:
Rev. 1.6
08/20/02
L = ESR3C3(V
Where:
V
For Vo=2.8V and DI=14.2A
L = 0.006390003(4.75 - 2.8) / (2314.2) = 3.7mH
T
D
Vsw
R
Vsync
DIr
DVo
T = 1/Fsw
Vsw = Vsync = Io3R
D
T
T
DIr = (Vo + Vsync)3T
DVo = DIr3ESR
ON
OFF
IN(MIN)
DS
Switching Period
= D3T
Duty Cycle
(Vo + Vsync) / (V
= T - T
Inductor Ripple Current
MOSFET On-Resistance
Output Ripple Voltage
High-side MOSFET ON Voltage
= Minimum input voltage
Synchronous MOSFET ON Voltage
ON
IN(MIN)
IN
DS
OFF
- Vo
- Vsw + Vsync)
/ L
(MAX)
) / (23DI)
www.irf.com
In our example for Vo=2.8V and 14.2A load, assuming
IRL3103 MOSFET for both switches with maximum on
resistance 0f 19mV, we have:
Power Component Selection
Assuming IRL3103 MOSFETs as power components,
we will calculate the maximum power dissipation as fol-
lows:
For high-side switch the maximum power dissipation
happens at maximum Vo and maximum duty cycle.
R
For synch MOSFET, maximum power dissipation hap-
pens at minimum Vo and minimum duty cycle.
Heat Sink Selection
Selection of the heat sink is based on the maximum
allowable junction temperature of the MOSFETS. Since
we previously selected the maximum R
then we must keep the junction below this temperature.
Selecting TO-220 package gives u
venders’ data sheet) and assuming that the selected
heat sink is black anodized, the heat-sink-to-case ther-
mal resistance is ucs=0.058C/W, the maximum heat sink
temperature is then calculated as:
With the maximum heat sink temperature calculated in
the previous step, the heat-sink-to-air thermal resistance
(u
Assuming T
DS(MAX)
SA
) is calculated as follows:
T = 1 / 200000 = 5ms
Vsw = Vsync = 14.230.019 = 0.27V
D
T
T
DIr = (2.8 + 0.27)31.9 / 3 = 1.94A
DVo = 1.9430.006 = 0.011V = 11mV
D
P
P
D
P
P
Ts = T
Ts = 125 - 3.823(1.8 + 0.05) = 1188C
DT = Ts - T
Temperature Rise Above Ambient
u
SA
ON
OFF
MAX
DH
DH
MIN
DS
DS
= DT / P
= Maximum R
= 0.6135 = 3.1ms
= (1 - D
= (1 - 0.43)314.2
(2.8 + 0.27) / (5 - 0.27 + 0.27) = 0.61
= D
= 0.65314.2
= 5 - 3.1 = 1.9ms
J
(2 + 0.27) / (5.25 - 0.27 + 0.27) = 0.43
(2.8 + 0.27) / (4.75 - 0.27 + 0.27) = 0.65
- P
MAX
A
= 358C:
D
3Io
A
3(u
MIN
D
= 118 - 35 = 838C
= 83 / 3.82 = 228C/W
)3Io
2
3R
JC
2
30.029 = 3.8W
DS(ON)
+ ucs)
2
DS(MAX)
3R
2
30.029 = 3.33W
DS(MAX)
of the MOSFET at 1258C
JC
=1.88C/W (From the
IRU3011
DS(ON)
at 1258C,
9