ISL78210 Datasheet, PDF (13/17 Pages) Intersil Corporation – Automotive PWM DC/DC Voltage Controller

ISL78210
1000
Q_U = 100nC
900 Q_L=200nC
800
Q_U = 50nC
Q_L = 100nC
Q_U = 50nC
Q_L= 50nC
700
600
500
Q_U = 20nC
Q_L= 50nC
400
300
200
100
00 _{200 400 600 800 1k 1.2k 1.4k 1.6k 1.8k 2k}
FREQUENCY (Hz)
FIGURE 11. POWER DISSIPATION vs FREQUENCY
There are several power MOSFETs readily available that
are optimized for DC/DC converter applications. The
preferred high-side MOSFET emphasizes low switch
charge so that the device spends the least amount of
time dissipating power in the linear region. Unlike the
low-side MOSFET, which has the drain-source voltage
clamped by its body diode during turn-off, the high-side
MOSFET turns off with V_IN -V_OUT, plus the spike, across
it. The preferred low-side MOSFET emphasizes low
r _DS(ON) when fully saturated to minimize conduction
loss.
For the low-side MOSFET, (LS), the power loss can be
assumed to be conductive only and is written as
Equation 22:
P_{CON_LS} ≈ I_LOAD2 ⋅ r_{DS(ON)_LS} ⋅ (1 – D)
(EQ. 22)
For the high-side MOSFET, (HS), its conduction loss is
written as Equation 23:
P_{CON_HS}
=
I_LO
A
2
D
⋅
r_D
S
(
ON)_
H
S
⋅
D
(EQ. 23)
For the high-side MOSFET, its switching loss is written as
Equation 24:
P_{SW_HS}
=
V_{-----I--N-----}⋅_---I_{--V----A----L---L---E----Y-----}⋅_---t_{--O-----N-----}⋅_---F_{----S----W---}
2
+
-^V----^I--^N-----^â‹---^I--^P----^E----^A----^K-----^â‹---^t-^O-----^F----^F----^â‹---^F----^S----^W---
2
(EQ. 24)
Where:
- I_VALLEY is the difference of the DC component of
the inductor current minus 1/2 of the inductor
ripple current
- I_PEAK is the sum of the DC component of the
inductor current plus 1/2 of the inductor ripple
current
- t_ON is the time required to drive the device into
saturation
- t_OFF is the time required to drive the device into
cut-off
Layout Considerations
The IC, analog signals, and logic signals should all be on
the same side of the PCB, located away from powerful
emission sources. The power conversion components
should be arranged in a manner similar to the example in
Figure 12 where the area enclosed by the current
circulating through the input capacitors, high-side
MOSFETs, and low-side MOSFETs is as small as possible
and all located on the same side of the PCB. The power
components can be located on either side of the PCB
relative to the IC.
G^GN^ND
++
O_OU_UT_TP_PU_UT _T
C_CA_AP_PA_AC_CIT_ITO_ORS_RS
V^VO^OU^UTT
PPHHASSEE
NNOODDEE
H_HIIG_GH_H--_SS_IID_DEE
M_MO_OS_SF_FE_ET_TSS
V_VI_IN
L^LO^OWW^--^SS^II^DD^EE
MOOSSFFEETSTS
I^IN^NP^PU^UTT
CCAAPPAACCITITOORSRS
FIGURE 12. TYPICAL POWER COMPONENT
PLACEMENT
Signal Ground
The GND pin is the signal-common also known as analog
ground of the IC. When laying out the PCB, it is very
important that the connection of the GND pin to the
bottom feedback voltage-divider resistor and the C_SOFT
capacitor be made as close as possible to the GND pin on
a conductor not shared by any other components.
In addition to the critical single point connection
discussed in the previous paragraph, the ground plane
layer of the PCB should have a single-point-connected
island located under the area encompassing the IC,
feedback voltage divider, compensation components,
C_SOFT capacitor, and the interconnecting traces among
the components and the IC. The island should be
connected using several filled vias to the rest of the
ground plane layer at one point that is not in the path of
either large static currents or high di/dt currents. The
single connection point should also be where the VCC
decoupling capacitor and the GND pin of the IC are
connected.
Power Ground
Anywhere not within the analog-ground island is Power
Ground.
VCC AND PVCC PINS
Place the decoupling capacitors as close as practical to
the IC. In particular, the PVCC decoupling capacitor
should have a very short and wide connection to the
PGND pin. The VCC decoupling capacitor should not
share any vias with the PVCC decoupling capacitor.
EN AND PGOOD PINS
These are logic signals that are referenced to the GND
pin. Treat as a typical logic signal.
13
FN7583.0
March 8, 2010