Rainbow Electronics MAX1635 - Manual (Page 23)

MAX1630–MAX1635
Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
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where t D is the diode-conduction time (120ns typical)
and V FWD is the forward voltage of the diode.
This power is dissipated in the MOSFET body diode if
no external Schottky diode is used.
where I RMS is the input ripple current as calculated in the
Design Procedure and Input Capacitor Value sections.
Light-Load Efficiency Considerations
Under light loads, the PWM operates in discontinuous
mode, where the inductor current discharges to zero at
some point during the switching cycle. This makes the
inductor current’s AC component high compared to the
load current, which increases core losses and I
2 R loss-
es in the output filter capacitors. For best light-load effi-
ciency, use MOSFETs with moderate gate-charge
levels, and use ferrite, MPP, or other low-loss core
material. Avoid powdered-iron cores; even Kool-Mu
(aluminum alloy) is not as good as ferrite.
PC Board Layout Considerations
Good PC board layout is required in order to achieve
specified noise, efficiency, and stability performance.
The PC board layout artist must be given explicit
instructions, preferably a pencil sketch showing the
placement of power-switching components and high-
current routing. See the PC board layout in the
MAX1630 Evaluation Kit manual for examples. A
ground plane is essential for optimum performance. In
most applications, the circuit will be located on a multi-
layer board, and full use of the four or more copper lay-
ers is recommended. Use the top layer for high-current
connections, the bottom layer for quiet connections
(REF, SS, GND), and the inner layers for an uninterrupt-
ed ground plane. Use the following step-by-step guide:
1) Place the high-power components (Figure1, C1, C3,
Q1, Q2, D1, L1, and R1) first, with any grounded
connections adjacent.
Priority 1: Minimize current-sense resistor trace
lengths
and ensure accurate current
sensing with Kelvin connections (Figure 7).
Priority 2: Minimize ground trace lengths in the
high-current paths (discussed below).
Priority 3: Minimize other trace lengths in the high-
current paths.
Use >5mm-wide traces
C
IN to high-side MOSFET drain: 10mm
max length
Rectifier diode cathode to low-side
MOSFET: 5mm max length
LX node (MOSFETs, rectifier cathode,
inductor): 15mm max length
Ideally, surface-mount power components are butted
up to one another with their ground terminals almost
touching. These high-current grounds are then con-
nected to each other with a wide filled zone of top-layer
copper so they don’t go through vias. The resulting top-
layer “sub-ground-plane” is connected to the normal
inner-layer ground plane at the output ground termi-
nals, which ensures that the IC’s analog ground is
sensing at the supply’s output terminals without interfer-
ence from IR drops and ground noise. Other high-
current paths should also be minimized, but
focusing
primarily on short ground and current-sense con-
nections eliminates about 90% of all PC board lay-
out problems
(see the PC board layouts in the
MAX1630 Evaluation Kit manual for examples).
2) Place the IC and signal components. Keep the main
switching nodes (LX nodes) away from sensitive
analog components (current-sense traces and REF
capacitor). Place the IC and analog components on
the opposite side of the board from the power-
switching node.
Important: the IC must be no far-
ther than 10mm from the current-sense resistors.
Keep the gate-drive traces (DH_, DL_, and BST_)
shorter than 20mm and route them away from CSH_,
CSL_, and REF.
3) Use a single-point star ground where the input
ground trace, power ground (sub-ground-plane),
and normal ground plane meet at the supply’s out-
put ground terminal. Connect both IC ground pins
and all IC bypass capacitors to the normal ground
plane.
P(cap) = input capacitor ESR loss = (I ) x R
RMS
2
ESR
MAX1630
SENSE RESISTOR
HIGH CURRENT PATH
Figure 7. Kelvin Connections for the Current-Sense Resistors