ISL6265A
0.3
0.2
0.1 IP-P,N = 0.75
IP-P,N = 0.5
IP-P,N = 0
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (VIN/VO)
FIGURE 12. NORMALIZED RMS INPUT CURRENT FOR
2-PHASE CONVERTER
MOSFET Selection and Considerations
The choice of MOSFETs depends on the current each
MOSFET will be required to conduct, the switching
frequency, the capability of the MOSFETs to dissipate heat,
and the availability and nature of heat sinking and air flow.
Typically, a MOSFET cannot tolerate even brief excursions
beyond their maximum drain to source voltage rating. The
MOSFETs used in the power stage of the converter should
have a maximum VDS rating that exceeds the sum of the
upper voltage tolerance of the input power source and the
voltage spike that occurs when the MOSFETs switch.
There are several power MOSFETs readily available that are
optimized for DC/DC converter applications. The preferred
high-side MOSFET emphasizes low gate charge so that the
device spends the least amount of time dissipating power in the
linear region. The preferred low-side MOSFET emphasizes low
r DS(ON) when fully saturated to minimize conduction loss.
For the low-side (LS) MOSFET, the power loss can be
assumed to be conductive only and is written as Equation 24:
PCON_LS ≈ ILOAD2 ⋅ rDS(ON)_LS • (1 – D)
(EQ. 24)
For the high-side (HS) MOSFET, the its conduction loss is
written as Equation 25:
PCON_HS = ILOAD2 • rDS(ON)_HS • D
(EQ. 25)
For the high-side MOSFET, the switching loss is written as
Equation 26:
PSW_HS
=
-V----I--N----•---I--V----A----L---L---E----Y-----•--t--O-----N----•---f--S----W--- + -V----I--N----•---I--P----E----A----K----•---t--O----F----F----•---f-S----W----
2
2
(EQ. 26)
Where:
- IVALLEY is the difference of the DC component of the
inductor current minus 1/2 of the inductor ripple current
- IPEAK is the sum of the DC component of the inductor
current plus 1/2 of the inductor ripple current
- tON is the time required to drive the device into
saturation
- tOFF is the time required to drive the device into cut-off
Selecting The Bootstrap Capacitor
All three integrated drivers feature an internal bootstrap
schottky diode. Simply adding an external capacitor across
the BOOT and PHASE pins completes the bootstrap circuit.
The bootstrap function is also designed to prevent the
bootstrap capacitor from overcharging due to the large
negative swing at the PHASE node. This reduces voltage
stress on the BOOT and PHASE pins.
The bootstrap capacitor must have a maximum voltage
rating above PVCC + 4V and its capacitance value is
selected per Equation 27:
CB
O
O
T
≥
---------Q-----g---------
ΔVBOOT
(EQ. 27)
Where:
- Qg is the total gate charge required to turn on the
high-side MOSFET
- ΔVBOOT, is the maximum allowed voltage decay across
the boot capacitor each time the high-side MOSFET is
switched on
As an example, suppose the high-side MOSFET has a total
gate charge Qg, of 25nC at VGS = 5V, and a ΔVBOOT of
200mV. The calculated bootstrap capacitance is 0.125µF; for
a comfortable margin, select a capacitor that is double the
calculated capacitance. In this example, 0.22µF will suffice.
Use a low temperature-coefficient ceramic capacitor.
PCB Layout Considerations
Power and Signal Layers Placement on the PCB
As a general rule, power layers should be close together,
either on the top or bottom of the board, with the weak analog
or logic signal layers on the opposite side of the board. The
ground-plane layer should be adjacent to the signal layer to
provide shielding. The ground plane layer should have an
island located under the IC, the compensation components,
and the FSET components. The island should be connected
to the rest of the ground plane layer at one point.
Component Placement
There are two sets of critical components in a DC/DC
converter; the power components and the small signal
components. The power components are the most critical
because they switch large amount of energy. The small
signal components connect to sensitive nodes or supply
critical bypassing current and signal coupling.
The power components should be placed first and these
include MOSFETs, input and output capacitors, and the
inductor. It is important to have a symmetrical layout for each
power train, preferably with the controller located equidistant
from each power train. Symmetrical layout allows heat to be
dissipated equally across all power trains. Keeping the
21
FN6884.0
May 11, 2009