US3007
Output Inductor Selection
In our example for Vo = 2.8V and 14.2 A load , Assum-
ing IRL3103 MOSFET for both switches with maximum
onresistanceof19mW, we have :
T = 1 / 200000 = 5 uSec
Vsw =Vsync= 14.2*0.019=0.27 V
D » ( 2.8 + 0.27 ) / ( 5 - 0.27 + 0.27 ) = 0.61
Ton = 0.61 * 5 = 3.1 uSec
Toff = 5 - 3.1 = 1.9 uSec
DIr = ( 2.8 + 0.27 ) * 1.9 / 3 = 1.94 A
DVo = 1.94 * .006 = .011 V = 11 mV
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.
Power Component Selection
Vcore
Assuming IRL3103 MOSFETs as power components,
we will calculate the maximum power dissipation as fol-
lows:
The maximum output inductance is calculated using the
following equation :
L = ESR * C * ( Vinmin - Vomax ) / ( 2* DI )
Where :
Vinmin = Minimum input voltage
For high side switch the maximum power dissipation
happens at maximum Vo and maximum duty cycle.
Dmax » ( 2.8 + 0.27 ) / ( 4.75 - 0.27 + 0.27 ) = 0.65
Pdh = Dmax * Io^2*Rds(max)
For Vo = 2.8 V , DI = 14.2 A
L =0.006 * 9000 * ( 4.75 - 2.8) / (2 * 14.2) = 3.7 uH
Assuming that the programmed switching frequency is
set at 200 KHZ , 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 3 uH
inductance with » 3 mW of DC resistance.
Assuming L = 3 uH and the switching frequency ; Fsw =
200 KHZ , the inductor ripple current and the output
ripple voltage is calculated using the following set of
equations :
Pdh= 0.65*14.2^2*0.029=3.8 W
Rds(max)=Maximum Rds-on of the MOSFET at 125°C
For synch MOSFET, maximum power dissipation hap-
pens at minimum Vo and minimum duty cycle.
Dmin » ( 2 + 0.27 ) / ( 5.25 - 0.27 + 0.27 ) = 0.43
Pds = (1-Dmin)*Io^2*Rds(max)
Pds=(1 - 0.43) * 14.2^2 * 0.029 = 3.33 W
3.3V Supply
Again,for high side switch the maximum power dissipa-
tion happens at maximum Vo and maximum duty cycle.
The duty cycle equation for non synchronous replaces
the forward voltage of the diode with the Synch MOSFET
on voltage. In equation below, Vf=0.5V
Dmax » ( 3.3 + 0.5 ) / ( 4.75 - 0.27 + 0.5 ) = 0.76
Pdh = Dmax * Io^2*Rds(max)
T = 1/Fsw
T º Switching Period
Pdh= 0.76*10^2*0.029=2.21 W
D » ( Vo + Vsync ) / ( Vin - Vsw + Vsync )
D º Duty Cycle
Ton = D * T
Rds(max)=Maximum Rds-on of the MOSFET at 125°C
For diode, the maximum power dissipation happens at
minimum Vo and minimum duty cycle.
Vsw º High side Mosfet ON Voltage = Io * Rds
Rds º Mosfet On Resistance
Toff = T - Ton
Dmin » ( 3.3 + 0.5 ) / ( 5.25 - 0.27 + 0.5 ) = 0.69
Pdd = (1-Dmin)*Io*Vf=(1 - 0.69) * 10 * 0.5 = 1.55 W
Vsync º Synchronous MOSFET ON Voltage=Io * Rds
DIr = ( Vo + Vsync ) * Toff /L
DIr º Inductor Ripple Current
DVo = DIr * ESR
Switcher Current Limit Protection
The US3007 uses the MOSFET Rds-on as the sensing
resistor to sense the MOSFET current and compares to
a programmed voltage which is set externally via a re-
sistor (Rcs) placed between the drain of the MOSFET
and the “CS+” terminal of the IC as shown in the appli-
cation circuit.
DVo º Output Ripple Voltage
Rev. 1.8
12/8/00
4-12
UNISEM [ UNISEM ]
