Electronic Components Datasheet Search
  English  ▼

Delete All
ON OFF
ALLDATASHEET.COM

X  

Preview PDF Download HTML

ADP3208D Datasheet(PDF) 30 Page - ON Semiconductor

Part No. ADP3208D
Description  7-Bit, Programmable, Dual-Phase, Mobile, CPU, Synchronous Buck Controller
Download  37 Pages
Scroll/Zoom Zoom In 100% Zoom Out
Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
Logo 

ADP3208D Datasheet(HTML) 30 Page - ON Semiconductor

Zoom Inzoom in Zoom Outzoom out
 30 / 37 page
background image
ADP3208D
http://onsemi.com
30
LX is about 150 pH for the six SP capacitors, which is low
enough to avoid ringing during a load change. If the LX of
the chosen bulk capacitor bank is too large, the number of
ceramic capacitors may need to be increased to prevent
excessive ringing.
For this multi−mode control technique, an all ceramic
capacitor design can be used if the conditions of Equations
16, 17, and 18 are satisfied.
Power MOSFETs
For typical 20 A per phase applications, the N−channel
power MOSFETs are selected for two high−side switches
and two or three low−side switches per phase. The main
selection parameters for the power MOSFETs are VGS(TH),
QG, CISS, CRSS, and RDS(ON). Because the voltage of the
gate driver is 5.0 V, logic−level threshold MOSFETs must be
used.
The maximum output current, IO, determines the RDS(ON)
requirement for the low−side (synchronous) MOSFETs. In
the ADP3208D, currents are balanced between phases; the
current in each low−side MOSFET is the output current
divided by the total number of MOSFETs (nSF). With
conduction losses being dominant, the following expression
shows the total power that is dissipated in each synchronous
MOSFET in terms of the ripple current per phase (IR) and
the average total output current (IO):
PSF + (1 * D)
IO
nSF
2
) 1
12
n
IR
nSF
2
RDS(SF)
(eq. 19)
where:
D is the duty cycle and is approximately the output voltage
divided by the input voltage.
IR is the inductor peak−to−peak ripple current and is
approximately:
IR +
(1 * D)
VOUT
L
fSW
(eq. 20)
Knowing the maximum output current and the maximum
allowed power dissipation, the user can calculate the required
RDS(ON) for the MOSFET. For 8−lead SOIC or 8−lead
SOIC−compatible MOSFETs, the junction−to−ambient
(PCB) thermal impedance is 50
°C/W. In the worst case, the
PCB temperature is 70
°C to 80°C during heavy load
operation of the notebook, and a safe limit for PSF is about 0.8
W to 1.0 W at 120
°C junction temperature. Therefore, for this
example (40 A maximum), the RDS(SF) per MOSFET is less
than 8.5 m
W for two pieces of low−side MOSFETs. This
RDS(SF) is also at a junction temperature of about 120°C;
therefore, the RDS(SF) per MOSFET should be less than 6 mW
at room temperature, or 8.5 m
W at high temperature.
Another important factor for the synchronous MOSFET
is the input capacitance and feedback capacitance. The ratio
of the feedback to input must be small (less than 10% is
recommended) to prevent accidentally turning on the
synchronous MOSFETs when the switch node goes high.
The high−side (main) MOSFET must be able to handle
two main power dissipation components: conduction losses
and switching losses. Switching loss is related to the time for
the main MOSFET to turn on and off and to the current and
voltage that are being switched. Basing the switching speed
on the rise and fall times of the gate driver impedance and
MOSFET input capacitance, the following expression
provides an approximate value for the switching loss per
main MOSFET:
RRPM +
2
RT
VVID 1.0 V
AR (1 * D) VVID
RR CR fSW
* 0.5 kW
(eq. 21)
where:
nMF is the total number of main MOSFETs.
RG is the total gate resistance.
CISS is the input capacitance of the main MOSFET.
The most effective way to reduce switching loss is to use
lower gate capacitance devices.
The conduction loss of the main MOSFET is given by the
following equation:
RRPM +
2
280 kW
1.150 V ) 1.0 V
0.5
(1 * 0.061)
1.150
462 kW
5pF
300 kHz
(eq. 22)
* 500 kW + 202 kW
where RDS(MF) is the on resistance of the MOSFET.
Typically, a user wants the highest speed (low CISS)
device for a main MOSFET, but such a device usually has
higher on resistance. Therefore, the user must select a device
that meets the total power dissipation (about 0.8 W to 1.0 W
for an 8−lead SOIC) when combining the switching and
conduction losses.
For example, an IRF7821 device can be selected as the
main MOSFET (four in total; that is, nMF = 4), with
approximately CISS = 1010 pF (max) and RDS(MF) = 18 mW
(max at TJ = 120°C), and an IR7832 device can be selected
as the synchronous MOSFET (four in total; that is, nSF = 4),
with RDS(SF) = 6.7 mW (max at TJ = 120°C). Solving for the
power dissipation per MOSFET at IO = 40 A and IR = 9.0 A
yields 630 mW for each synchronous MOSFET and
590 mW for each main MOSFET. A third synchronous
MOSFET is an option to further increase the conversion
efficiency and reduce thermal stress.
Finally, consider the power dissipation in the driver for
each phase. This is best described in terms of the QG for the
MOSFETs and is given by the following equation:
PDRV +
(eq. 23)
fSW
2
n
nMF QGMF ) nSF QQSF ) ICC
VCC
where QGMF is the total gate charge for each main MOSFET,
and QGSF is the total gate charge for each synchronous
MOSFET.


Html Pages

1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37 


Datasheet Download




Link URL




Privacy Policy
ALLDATASHEET.COM
Does ALLDATASHEET help your business so far?  [ DONATE ]  

About Alldatasheet   |   Advertisement   |   Datasheet Upload   |   Contact us   |   Privacy Policy   |   Alldatasheet API   |   Link Exchange   |   Manufacturer List
All Rights Reserved© Alldatasheet.com


Mirror Sites
English : Alldatasheet.com  |   English : Alldatasheet.net  |   Chinese : Alldatasheetcn.com  |   German : Alldatasheetde.com  |   Japanese : Alldatasheet.jp
Russian : Alldatasheetru.com  |   Korean : Alldatasheet.co.kr  |   Spanish : Alldatasheet.es  |   French : Alldatasheet.fr  |   Italian : Alldatasheetit.com
Portuguese : Alldatasheetpt.com  |   Polish : Alldatasheet.pl  |   Vietnamese : Alldatasheet.vn