FAN5308 Datasheet, PDF (8/11 Pages) Fairchild Semiconductor – 800mA High-Efficiency Step-Down DC-DC Converter

UVLO and Soft Start
The reference and the circuit remain reset until the V_IN crosses
its UVLO threshold.
The FAN5308 has an internal soft-start circuit that limits the in-
rush current during start-up. This prevents possible voltage
drops of the input voltage and eliminates the output voltage
overshoot. The soft-start is implemented as a digital circuit
increasing the switch current in four steps to the P-channel cur-
rent limit (1500mA). Typical start-up time for a 20µF output
capacitor and a load current of 800mA is 800µs.
Short Circuit Protection
The switch peak current is limited cycle-by-cycle to a typical
value of 1500mA. In the event of an output voltage short circuit,
the device operates with a frequency of 400kHz and minimum
duty cycle, therefore the average input current is typically
200mA.
Thermal Shutdown
When the die temperature exceeds 150°C, a reset occurs and
will remain in effect until the die cools to 130°C, at that time the
circuit will be allowed to restart.
Applications Information
Setting the Output Voltage
The internal reference is 0.8V (Typical). The output voltage is
divided by a resistor divider, R1 and R2 to the FB pin. The out-
put voltage is given by:
I_OUT = V_OUT × ^1-----–-----⁽-2-V----_×-^O---^U-L---^T--_×---^⁄--f-V----^I--^N----⁾
Where R₁ + R₂ < 800KΩ.
According to this equation, and assuming desired output volt-
age of 1.5096V, and given R2 = 10KΩ, the calculated value of
R1 is 8.87KΩ. If quiescent current is a key design parameter a
higher value feedback resistor can be used (e.g. R2 = 100KΩ)
and a small bypass capacitor of 10pF is required in parallel with
the upper resistor as shown in Figure 4.
C_IN
10µF
V_IN
1
PGND
2
EN
3
P1
(AGND)
SW
6
3.3µH
5 NC
R1
FB
4
C_f
5KΩ
R2 10KΩ
V_OUT
1.2V (800mA)
C_OUT
2 x 10µF
This is calculated as follows:
∆I_L = V_OUT × -1----^–-----⁽--V----L-^O---^U-_×---^T-f---^⁄---V----^I--^N----⁾
where:
∆I_L = Inductor Ripple Current
f = Switching Frequency
L = Inductor Value
Some recommended inductors are suggested in the table
below:
Inductor Value
3.3µH
3.3µH
Vendor
Panasonic
Murata
Part Number
ELL6PM3R3N
LQS66C3R3M04
Table 1: Recommended Inductors
Capacitors Selection
For best performances, a low ESR input capacitor is required. A
ceramic capacitor of at least 10µF, placed as close to the V_IN
and AGND pins of the device is recommended. The output
capacitor determines the output ripple and the transient
response.
Capacitor
Value
10µF
Vendor
Taiyo Yuden
TDK
Murata
Part Number
JMK212BJ106MG
JMK316BJ106KL
C2012X5ROJ106K
C3216X5ROJ106M
GRM32ER61C106K
Table 2: Recommended Capacitors
PCB Layout Recommendations
The recommended PCB layout is shown in Figure 5. The inher-
ently high peak currents and switching frequency of power sup-
plies require a careful PCB layout design.
Figure 4. Setting the Output Voltage
Inductor Selection
The inductor parameters directly related to the device’s perfor-
mances are saturation current and dc resistance. The FAN5308
operates with a typical inductor value of 3.3µH. The lower the dc
resistance, the higher the efficiency. For saturation current, the
inductor should be rated higher than the maximum load current
plus half of the inductor ripple current.
Figure 5. Recommended PCB Layout
FAN5308 Rev. 1.0.1
8
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