Low-Cost, Multichemistry Battery-
Charger Building Block
The controller determines the constant off-time period,
which is dependent on BATT voltage. This makes the
ripple current independent of input and battery voltage,
Use the following step-by-step guide:
1) Place the high power connections first, with their
grounds adjacent:
and it should be kept to less than 1A. Calculate ∆I with
L
• Minimize the current-sense resistor trace
lengths, and ensure accurate current sensing
with Kelvin connections.
the following equation:
21Vµs
∆I =
L
(9)
• Minimize ground trace lengths in the high
L µH
(
)
current paths.
Higher inductor values decrease the ripple current.
Smaller inductor values require high saturation current
capabilities and degrade efficiency. Typically, a 22µH
inductor is ideal for all operating conditions.
• Minimize other trace lengths in the high current
paths.
• Use >5mm wide traces.
• Connect C1 and C2 to high-side MOSFET
(10mm max length).
Current-Sense Input Filtering
In normal circuit operation with typical components, the
current-sense signals can have high-frequency tran-
sients that exceed 0.5V due to large current changes
and parasitic component inductance. To achieve prop-
er battery and input current compliance, the current-
sense input signals should be filtered to remove large
common-mode transients. The input current-limit sens-
ing circuitry is the most sensitive case due to large cur-
rent steps in the input filter capacitors (C6, C7) in
Figure 1. Use 0.47µF ceramic capacitors from CSSP
and CSSN to ground. Smaller 0.1µF ceramic capacitors
(C18, C19) can be used on the CSIP and CSIN inputs
to ground since the current into the battery is continu-
ous. Place these capacitors next to the single-point
ground directly under the MAX1772.
• LX node (MOSFETs, rectifier cathode, inductor
(15mm max length)).
Ideally, surface-mount power components are flush
against one another with their ground terminals
almost touching. These high-current grounds are
then connected to each other with a wide, filled
zone of top-layer copper, so they do not go through
vias.
The resulting top-layer subground plane is connect-
ed to the normal inner-layer ground plane at the
output ground terminals, which ensures that the
IC’s analog ground is sensing at the supply’s output
terminals without interference from IR drops and
ground noise. Other high current paths should also
be minimized, but focusing primarily on short
ground and current-sense connections eliminates
about 90% of all PC board layout problems.
Layout and Bypassing
Bypass DCIN with a 1µF to ground (Figure 1). D4 pro-
tects the MAX1772 when the DC power source input is
reversed. A signal diode for D4 is adequate because
DCIN only powers the LDO and the internal reference.
Bypass LDO, BST, DLOV, and other pins as shown in
Figure 1.
2) Place the IC and signal components. Keep the
main switching node (LX node) away from sensitive
analog components (current-sense traces and REF
capacitor). Important: the IC must be no further
than 10mm from the current-sense resistors.
Good PC board layout is required to achieve specified
noise, efficiency, and stable performance. The PC board
layout artist must be given explicit instructions—prefer-
ably, a pencil sketch showing the placement of the
power switching components and high current routing.
Refer to the PC board layout in the MAX1772 evaluation
kit for examples. A ground plane is essential for optimum
performance. In most applications, the circuit will be
located on a multilayer board, and full use of the four or
more copper layers is recommended. Use the top layer
for high current connections, the bottom layer for quiet
connections (REF, CCV, CCI, CCS, DCIN, and GND),
and the inner layers for an uninterrupted ground plane.
Keep the gate drive traces (DHI, DLO, and BST)
shorter than 20mm, and route them away from the
current-sense lines and REF. Place ceramic bypass
capacitors close to the IC. The bulk capacitors can
be placed further away. Place the current-sense
input filter capacitors under the part, connected
directly to the GND pin.
3) Use a single-point star ground placed directly
below the part. Connect the input ground trace,
power ground (subground plane), and normal
ground to this node.
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
