SD46520 Datasheet, PDF (5/8 Pages) Silan Microelectronics Joint-stock – 400KHZ 2A STEP-DOWN DC-DC CONVERTER

SD46520
higher than 2.495V, the chip works normally.
Short protection
When the output voltage is short connected to the ground, FB feedback voltage is pulled down to the ground, and
the oscillator frequency will decrease to 50kHz from normal 400kHz, which also decreases the inductor current
average value, that is the output current is decreased.
APPLICATION NOTE
Output voltage setting
As right figure shows, the output voltage is determined by the feedback
resistors ratio of R1 and R2. Generally, R2＝10KΩ, R1 is decided by:
R1=R2.(VO/1.22-1).
Inductor selection
When output current is large, the regulator is working under continuous
current mode that is inductor current is continuous and will not
decrease to zero. The inductor value will affect the ripple of inductor current, and the relationship between them
is as follows:
L = ^{VO ⋅(VI − VO ) .}
V_I ⋅ f_S ⋅ ΔI
Where, VI is input voltage, VO is output voltage, fs is on-off frequency, ΔI is the peak-peak value of inductor
current ripple. In general, ΔI is no larger than 30% of the maximum output current, thus to decide the inductor
value.
At the same time, the peak inductor current should be less than 2.4A, the value is decided by:
I_LMAX
= I_O
+
1
2
⋅
ΔI_L
= I_O
+
1_â‹
2
V_O ⋅(V_I − V_O )
V_I ⋅ f_S ⋅L
.
Input capacitor selection
The input current of step-down DC-DC is not successive, an additional input capacitor C_I is needed to keep input
voltage stable. The input capacitor should be low ESR, and ceramic capacitor is the best choice, tantalum
capacitor or low ESR electrolytic capacitor is all right. The input capacitor value should be larger than 10μF.
Output capacitor selection
The output capacitor C_O is decided by the ripple requirement of output voltage. The output voltage ripple is
decided by:
ΔV_O
=
ΔI_L (ESR
+
⋅1
8f_S ⋅ C_O
)
.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.0 2009.04.07
Page 5 of 8