FAN5601 Datasheet, PDF (12/14 Pages) Fairchild Semiconductor – Regulated Step-Down Charge Pump DC/DC Converter

PRODUCT SPECIFICATION
FAN5601
Applications Information
Proper operation of the FAN5601 requires one ceramic
bucket capacitor in the 0.1µF to 1µF range; one 10µF output
bypass capacitor and one 10µF input bypass capacitor. In
order to obtain optimum output ripple and noise perfor-
mance, use of low ESR (<0.05Ω) ceramic input and output
bypass capacitors is recommended. The X5R and X7R
rated capacitors provide adequate performance over the
-40ºC to 85ºC temperature range.
The value of the bucket capacitor is dependent on load cur-
rent requirements. A 1µF bucket capacitor will work well in
all applications at all load currents, while a 0.1µF capacitor
will support most applications under 100mA of load current.
The choice of bucket capacitor values should be verified in
the actual application at the lowest input voltage and highest
load current. A 30% margin of safety is recommended in
order to account for the tolerance of the bucket capacitor and
the variations in the on-resistance of the internal switches.
One of the key benefits of the ScalarPump™ architecture is
that the dynamically scaled on-resistance of the switches
effectively reduces the peak current in the bucket capacitor
and therefore input and output ripple current is also reduced.
Nevertheless, due to the ESR of the input and output bypass
capacitors, these current spikes generate voltage spikes at the
input and output pins. However, these ESR spikes can be
easily filtered because their frequency lie at up to 12 times
the clock frequency.
In applications where conductive and radiated EMI/RFI
interference has to be kept as low as possible, the user may
consider the use of additional input and output filtering. For
example, adding an L-C filter to the standard output bypass
configuration is very effective in reducing both the output
ripple and the voltage spikes. Figure 2 shows an L-C filter
using a 100nH chip inductor and a 1µF capacitor. The chan-
nel 1 of Figure 3 shows the ripple voltage at the output of the
device while Channel 2 shows the ripple voltage at the out-
put of the filter at V_IN = 3.3V,
V_OUT = 1.3V and I_OUT = 100mA. Similar filtering method
will greatly reduce the current spikes at the input. The user
should be mindful of considering resistive voltage drops in
the inductors connected serially in the input and output
leads.
I_OUT = 100mA
V_IN = 3.3V
V_OUT = 1.3V
Time (10µs/div)
Figure 3. Effect of L-C Filter on output ripple
While evaluating the FAN5601 (or any other switched
capacitor DC-DC converter) the user should be careful to
keep the power supply source impedance low; use of long
wires causing high lead inductances and resistive losses
should be avoided. A carefully laid out ground plane is
essential because current spikes are generated as the bucket
capacitor is charged and discharged. The input and output
bypass capacitors should be placed as close to the device
pins as possible.
CB = 1µF
INPUT 2.2V to 5.5V ^C+
VIN
C–
VOUT
CIN = 10µF
ENABLE GND
LF = 100nH Output 1.0V to 1.8V
COUT = 10µF
CF = 1µF
Figure 2. Optional L-C Filter
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