NX2601
pacitors supply current to the MOSFETs. Usually 1uF
ceramic capacitor is chosen to decouple the high fre-
quency noise. The bulk input capacitors are decided by
voltage rating and RMS current rating. The RMS current
in the input capacitor can be calculated
Output Voltage Calculation
Output voltage is set by reference voltage and
external voltage divider. The reference voltage is fixed
at 0.8V. The divider consists of two ratioed resistors
so that the output voltage applied at the Fb pin is 0.8V
when the output voltage is at the desired value. The
following equation and picture show the relationship
between VOUT , VREF and voltage divider.
IRMS = IOUT ´ D ´ 1-D
VOUT
D =
V
IN
...(19)
R 2 ´ VREF
R1=
VIN = 12V, VOUT=1.2V, IOUT=15A, using equation
(19), the result of input RMS current is 4.5A.
For higher efficiency, low ESR capacitors are
recommended.
...(18)
VOUT -VREF
where R2 is part of the compensator, and the
value of R1 value can be set by voltage divider.
Choose R2=10kW, to set the output voltage at
1.8V, the result of R1 is 8kW.
Two Sanyo OS-CON SVPA180M 16V 180uF
29mO with 3.4A RMS rating are chosen as input bulk
capacitors.
Vout
Power MOSFETs Selection
R2
The NX2601 requires two N-Channel power
Fb
MOSFETs. The selection of MOSFETs is based on
maximum drain source voltage, gate source voltage,
maximum current rating, MOSFET on resistance and
power dissipation. The main consideration is the power
loss contribution of MOSFETs to the overall converter
efficiency. In this design example, two IRFR3706 are
used. They have the following parameters: VDS=30V, ID
=75A,RDSON =9mW,QGATE =23nC.
R1
Vref
Voltage divider
Figure 17 - Voltage divider
In general, the minimum output load impedance
including the resistor divider should be less than 5kW to
prevent overcharge the output voltage by leakage cur-
rent (e.g. Error Amplifier feedback pin bias current). A
minimum load for 5kW less (<1/16w for most of applica-
tion) is recommended to put at the output. For example,
in this application,
There are three factors causing the MOSFET power
loss:conduction loss, switching loss and gate driver loss.
Gate driver loss is the loss generated by discharg-
ing the gate capacitor and is dissipated in driver circuits.
It is proportional to frequency and is defined as:
Pgate = (QHGATE ´ VHGS + QLGATE ´ VLGS )´ FS
...(20)
Vout=1.6V
where QHGATE is the high side MOSFETs gate
charge,QLGATE is the low side MOSFETs gate charge,VHGS
is the high side gate source voltage, and VLGS is
the low side gate source voltage.
The power loss is 1/16W less
RLOAD = 1.6V ´ 1.6V /(1/16W) = 40W
Select minimum load is 1kW should be good
enough.
According to equation (3), PGATE =0.07W. This
power dissipation should not exceed maximum power
dissipation of the driver device.
Input Capacitor Selection
Input capacitors are usually a mix of high frequency
ceramic capacitors and bulk capacitors. Ceramic ca-
pacitors bypass the high frequency noise, and bulk ca-
Conduction loss is simply defined as:
Rev. 2.3
12/01/06
18
