NX9548
by output capacitor's ESR must satisfy the requirement
as below:
Vout
F
1
SW
F
=
£
ESR
...(13)
R2
2´ p ´ ESR´ COUT
4
Fb
Besides that, ESR has to be bigger enough so
that the output voltage ripple can provide enough voltage
ramp to error amplifier through FB pin. If ESR is too
small, the error amplifier can not correctly dectect the
ramp, high side MOSFET will be only turned off for mini-
mum time 400nS. Double pulsing and bigger output ripple
will be observed. In summary, the ESR of output capaci-
tor has to be big enough to make the system stable, but
also has to be small enough to satify the transient and
DC ripple requirements.
R1
Vref
Figure 12 - Voltage Divider
R 2 ´ VREF
R1=
...(15)
VOUT -VREF
where R2 is part of the compensator, and the value
of R1 value can be set by voltage divider.
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-
pacitors supply switching current to the MOSFETs. Usu-
ally 1uF ceramic capacitor is chosen to decouple the
high frequency noise.The bulk input capacitors are de-
cided by voltage rating and RMS current rating. The RMS
current in the input capacitors can be calculated as:
Mode Selection
NX9548 can be operated in PFM mode, ultrasonic
PFM mode, CCM mode and shutdown mode by apply-
ing different voltage on ENSW/MODE pin.
When VCC applied to ENSW/MODE pin, NX9548
is In PFM mode. The low side MOSFET emulates the
function of diode when discontinuous continuous mode
happens, often in light load condition. During that time,
the inductor current crosses the zero ampere border and
becomes negative current. When the inductor current
reaches negative territory, the low side MOSFET is
turned off and it takes longer time for the output voltage
to drop, the high side MOSFET waits longer to be turned
on. At the same time, no matter light load and heavy
load, the on time of high side MOSFET keeps the same.
Therefore the lightier load, the lower the switching fre-
quency will be. In ultrosonic PFM mode, the lowest fre-
quency is set to be 25kHz to avoid audio frequency
modulation. This kind of reduction of frequency keeps
the system running at light light with high efficiency.
In CCM mode, inductor current zero-crossing sens-
ing is disabled, low side MOSFET keeps on even when
inductor current becomes negative. In this way the effi-
ciency is lower compared with PFM mode at light load,
but frequency will be kept constant.
IRMS = IOUT
´ D ´ 1-D
...(14)
D = TON ´ FS
When VIN = 22V, VOUT=1.5V, IOUT=9A, the result of
input RMS current is 2.3A.
For higher efficiency, low ESR capacitors are
recommended. One 10uF/X5R/25V and two 4.7uF/X5R
/25V ceramic capacitors are chosen as input capaci-
tors.
Output Voltage Calculation
Output voltage is set by reference voltage and ex-
ternal voltage divider. The reference voltage is fixed at
0.75V. The divider consists of two ratioed resistors so
that the output voltage applied at the Fb pin is 0.75V
when the output voltage is at the desired value.
The following equation applies to figure 12, which
shows the relationship between VOUT , VREF and volt-
age divider.
Rev.1.6
03/06/09
13
