NCP81245
www.onsemi.com
16
pull−up resistor from 5 V V
CC
can be used to offset the
IOUT signal positive if needed.
R
IOUT
+
2V*R
LIMIT
10 *
Rcs2)
Rcs1*Rth
Rcs1)Rth
Rph
*
ǒ
Iout
ICC_MAX
* DCR
Ǔ
(eq. 8)
Programming ICC_MAX (multiphase)
A resistor to ground on the IMAX pin programs these
registers at the time the part is enabled. 10 mA is sourced
from these pins to generate a voltage on the program resistor.
The resistor value should be no less than 10k.
ICC_MAX
21k
+
R*10mA * 256 A
2V
(eq. 9)
Programming TSENSE
A temperature sense inputs are provided. A precision
current is sourced out the output of the TSENSE pin to
generate a voltage on the temperature sense network. The
voltage on the temperature sense input is sampled by the
internal A/D converter. A 100k NTC similar to the VISHAY
ERT−J1VS104JA should be used. Rcomp1 is mainly used
for noise. See the specification table for the thermal sensing
voltage thresholds and source current.
Rcomp2
8.2K
RNTC
100K
Cfilter
0.1uF
AGNDAGND
Rcomp1
0.0
TSENSE
Figure 9.
Precision Oscillator
A programmable precision oscillator is provided. The
clock oscillator serves as the master clock to the ramp
generator circuit. This oscillator is programmed by a resistor
to ground on the ROSC pin. The oscillator frequency range
is between 300 kHz/phase to 1.2 MHz/phase. The ROSC
pin provides approximately 2 V out and the source current
is mirrored into the internal ramp oscillator. The oscillator
frequency is approximately proportional to the current
flowing in the ROSC resistor.
Table 8. 3 PHASE / 1 PHASE FSW V ROSC (PIN21 / PIN22)
Resistor Per phase Fsw MPH_A Per phase Fsw MPH_B Per phase Fsw 1PH
6.19 kW
1.2 MHz 1.2 MHz 1.2 MHz
14.7 kW
1.1 MHz 1.1 MHz 1.1 MHz
24.9 kW
1.0 MHz 1.0 MHz 1.0 MHz
37.4 kW
900 kHz 900 kHz 900 kHz
53.6 kW
800 kHz 800 kHz 800 kHz
73.2 kW
700 kHz 700 kHz 700 kHz
97.6 kW
600 kHz 600 kHz 600 kHz
130 kW
500 kHz 500 kHz 500 kHz
169 kW
400 kHz 400 kHz 400 kHz
215 kW
300 kHz 300 kHz 300 kHz
The oscillator generates triangle ramps that are 0.5~2.5 V
in amplitude depending on the VRMP pin voltage to provide
input voltage feed forward compensation. The ramps are
equally spaced out of phase with respect to each other and
the single phase rail is set half way between phases 1 and 2
of the multi phase rail for minimum input ripple current.
For use with ON Semiconductor’s phase doubler, the
NCP81245 offers the user the ability to multiply the
frequency of multiphase rail A. On the NCP81245, the
switching frequency is increased by a factor of 2 when the
phase doubler configuration is used.
Programming the Ramp Feed−Forward Circuit
The ramp generator circuit provides the ramp used by the
PWM comparators. The ramp generator provides voltage
feed−forward control by varying the ramp magnitude with
respect to the VRMP pin voltage. The VRMP pin also has
a 4 V UVLO function. The VRMP UVLO is only active
after the controller is enabled. The VRMP pin is high
impedance input when the controller is disabled.
The PWM ramp time is changed according to the
following,
V
RAMPpk+pkPP
+ 0.1 * V
VRMP
(eq. 10)
