IRU3007
T = 1 / Fsw
of the 1500µF, 6MV1500GX type Sanyo capacitors. With
Rs=5mΩ, the maximum ESR becomes 9.5mΩ which is
equivalent to » 4 caps. Another important consideration
is that if a trace is being used to implement the resistor,
the power dissipated by the trace increases the case
temperature of the output capacitors which could seri-
ously affect the life span of the output capacitors.
Vsw = Vsync = Io×RDS
D » (Vo + Vsync) / (VIN - Vsw + Vsync)
TON = D×T
TOFF = T - TON
∆Ir = (Vo + Vsync)×TOFF / L
∆Vo = ∆Ir×ESR
Output Inductor Selection
The output inductance must be selected such that un- In our example for Vo = 2.8V and 14.2 A load, assuming
der low line and the maximum output voltage condition, IRL3103 MOSFET for both switches with maximum on
the inductor current slope times the output capacitor resistanceof19mΩ, we have:
ESR is ramping up faster than the capacitor voltage is
T = 1 / 200000 = 5µs
drooping during a load current step. However, if the in-
Vsw = Vsync = 14.2×0.019 = 0.27V
D » (2.8 + 0.27) / (5 - 0.27 + 0.27) = 0.61
TON = 0.61×5 = 3.1µs
ductor is made too small, the output ripple current and
ripple voltage will become too large. One solution to bring
the ripple current down is to increase the switching fre-
quency, however that will be at the cost of reduced effi-
ciency and higher system cost. The following set of for-
mulas are derived to achieve optimum performance with-
out many design iterations.
TOFF = 5 - 3.1 = 1.9µs
∆Ir = (2.8 + 0.27)×1.9 / 3 = 1.94A
∆Vo = 1.94×0.006 = 0.011V = 11mV
The maximum output inductance is calculated using the Power Component Selection
following equation:
Vcore
Assuming IRL3103 MOSFETs as power components,
we will calculate the maximum power dissipation as fol-
lows:
(VIN(MIN) - Vo(MAX))
L = ESR × C ×
(2 × ∆I)
Where:
VIN(MIN) = Minimum input voltage
For Vo = 2.8V and ∆I = 14.2A, we get:
For high side switch the maximum power dissipation
happens at maximum Vo and maximum duty cycle.
(4.75 - 2.8)
(2 × 14.2)
L = 0.006 × 9000 ×
= 3.7µH
DMAX » (2.8 + 0.27) / (4.75 - 0.27 + 0.27) = 0.65
PDH = DMAX×Io2×RDS(MAX)
PDH = 0.65×14.22×0.029 = 3.8W
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:
RDS(MAX)=Maximum RDS(ON) of the MOSFET at 1258C
For synch MOSFET, maximum power dissipation hap-
The selected core material is Powder Iron, the selected pens at minimum Vo and minimum duty cycle.
core is T50-52D from Micro Metal wound with 8 turns of
DMIN » (2 + 0.27) / (5.25 - 0.27 + 0.27) = 0.43
PDS = (1 - DMIN)×Io2×RDS(MAX)
#16 AWG wire, resulting in 3µH inductance with » 3 mΩ
of DC resistance.
PDS = (1 - 0.43)×14.22 ×0.029 = 3.33W
Assuming L=3µH and Fsw=200KHz (switching fre-
quency), the inductor ripple current and the output ripple 3.3V Supply
voltage is calculated using the following set of equations: Again, for high side switch the maximum power dissipa-
tion happens at maximum Vo and maximum duty cycle.
T º Switching Period
D º Duty Cycle
Vsw º High-side MOSFET ON Voltage
RDS º MOSFET On-Resistance
The duty cycle equation for non synchronous replaces
the forward voltage of the diode with the Synch MOSFET
on voltage. In equations below:
Vsync º Synchronous MOSFET ON Voltage
Vf = 0.5V
∆Ir º Inductor Ripple Current
DMAX » (3.3 + 0.5) / (4.75 - 0.27 + 0.5) = 0.76
∆Vo º Output Ripple Voltage
Rev. 2.1
08/20/02
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