LM3532TMX-40ANOPB National Semiconductor Corporation, LM3532TMX-40ANOPB Datasheet - Page 39

• Manufacturer Part Number: LM3532TMX-40ANOPB
• Description: High Efficiency White Led Driver With Programmable Ambient Light Sensing Capability And I2c-compatible Interface
• Manufacturer: National Semiconductor Corporation

Output Capacitor Placement
The output capacitor is in the path of the inductor current dis-
charge current. As a result, C
from 0 to I
diode turns on. Typical turn-off/turn-on times are around 5ns.
Any inductance along this series path from the cathode of the
diode through C
contribute to voltage spikes (V
OUT which can potentially over-voltage the SW pin, or feed
through to GND. To avoid this, C
close as possible to the Cathode of the Schottky diode and
C
LM3532's GND bump. The best placement for C
same layer as the LM3532 so as to avoid any vias that will
add extra series inductance (see Layout Examples).
Schottky Diode Placement
The Schottky diode is in the path of the inductor current dis-
charge. As a result the Schottky diode sees a high current
step from 0 to I
diode turns on. Any inductance in series with the diode will
cause a voltage spike (V
which can potentially over-voltage the SW pin, or feed through
to VOUT and through the output capacitor and into GND.
Connecting the anode of the diode as close as possible to the
SW pin and the cathode of the diode as close as possible to
COUT+ will reduce the inductance (L
voltage spikes (Layout Examples).
Inductor Placement
The node where the inductor connects to the LM3532’s SW
bump has 2 issues. First, a large switched voltage (0 to
V
cycle. This switched voltage can be capacitively coupled into
nearby nodes. Second, there is a relatively large current (in-
put current) on the traces connecting the input supply to the
inductor and connecting the inductor to the SW bump. Any
resistance in this path can cause large voltage drops that will
negatively affect efficiency.
To reduce the capacitively coupled signal from SW into near-
by traces, the SW bump to inductor connection must be
minimized in area. This limits the PCB capacitance from SW
to other traces. Additionally, other nodes need to be routed
away from SW and not directly beneath. This is especially true
for high impedance nodes that are more susceptible to ca-
pacitive coupling such as (SCL, SDA, HWEN, PWM, and
possibly ASL1 and ALS2). A GND plane placed directly below
SW will help isolate SW and dramatically reduce the capaci-
tance from SW into nearby traces.
To limit the trace resistance of the VBATT to inductor con-
nection and from the inductor to SW connection, use short,
wide traces (see Layout Examples).
OUT
OUT
− must be connected as close as possible to the
+ V
F_SCHOTTKY
PEAK
each time the switch turns off and the Schottky
OUT
PEAK
and back into the LM3532's GND pin will
) appears on this node every switching
each time the switch turns off and the
SPIKE
SPIKE
= L
OUT
OUT
PX
sees a high current step
= L
+ must be connected as
× dI/dt) at SW and OUT
PX
PX
) and minimize these
× dI/dt) at SW and
OUT
is on the
39
Input Capacitor Selection and Placement
The input bypass capacitor filters the inductor current ripple,
and the internal MOSFET driver currents during turn on of the
power switch.
The driver current requirement can be a few hundred mA's
with 5ns rise and fall times. This will appear as high dI/dt cur-
rent pulses coming from the input capacitor each time the
switch turns on. Close placement of the input capacitor to the
IN pin and to the GND pin is critical since any series induc-
tance between IN and C
voltage spikes that could appear on the V
the GND plane.
Close placement of the input bypass capacitor at the input
side of the inductor is also critical. The source impedance (in-
ductance and resistance) from the input supply, along with the
input capacitor of the LM3532, form a series RLC circuit. If the
output resistance from the source (R
cuit will be underdamped and will have a resonant frequency
(typically the case). Depending on the size of L
frequency could occur below, close to, or above the LM3532's
switching frequency. This can cause the supply current ripple
to be:
•
•
•
Figure 17
put impedance of the supply and the input capacitor. The
circuit is re-drawn for the AC case where the V
replaced with a short to GND and the LM3532 + Inductor is
replaced with a current source (ΔI
Equation 1 is the criteria for an underdamped response.
Equation 2 is the resonant frequency. Equation 3 is the ap-
proximated supply current ripple as a function of L
C
As an example, consider a 3.6V supply with 0.1Ω of series
resistance connected to C
traces. This results in an underdamped input filter circuit with
a resonant frequency of 712 kHz. Since the switching fre-
quency lies near to the resonant frequency of the input RLC
network, the supply current is probably larger then the induc-
tor current ripple. In this case, using equation 3 from
17, the supply current ripple can be approximated as 1.68
times the inductor current ripple. Increasing the series induc-
tance (L
to around 225 kHz and the supply current ripple to be ap-
proximately 0.25 times the inductor current ripple.
IN
approximately equal to the inductor current ripple when
the resonant frequency occurs well above the LM3532's
switching frequency;
greater then the inductor current ripple when the resonant
frequency occurs near the switching frequency; or
less then the inductor current ripple when the resonant
frequency occurs well below the switching frequency.
.
S
) to 500 nH causes the resonant frequency to move
shows this series RLC circuit formed from the out-
IN
IN
+ or C
through 50 nH of connecting
L
IN
).
− and GND can create
S
) is low enough the cir-
IN
supply line and in
S
the resonant
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IN
S
supply is
, R
Figure
S
, and