AN3319 Datasheet, PDF (16/25 Pages) STMicroelectronics – The SPV1040 is a high efficiency, low power and low voltage DC-DC converter

External component selection
AN3319
According to the maximum current (I_SC) provided by the PV panel connected at the input,
the following formula can be used to select the proper capacitance value (C_OUT) for a
specified maximum output voltage ripple (V_{OUT_rp_max}):
I
C
OUT
≥
F^----_S^----_WS^----C--⋅^---V^--_{OUT_rp_max}
Note:
Maximum voltage of this capacitor is strictly dependent on the output voltage range.
SPV1040 can support up to 5.2 V, so the suggested maximum voltage for these capacitors
is 10 V.
Low-ESR capacitors are a good choice to increase the whole system efficiency.
Output voltage partitioning
R₁ and R₂ are the two resistors used for partitioning the output voltage.
The said V_{OUT_max} the maximum output voltage of the battery, R₁ and R₂ must be selected
according to the following rule:
RV
-----¹- = -----^O----^U----^T-^_max -1
R 1.25
2
Also, in order to optimize the efficiency of the whole system, when selecting R₁ and R₂, their
power dissipation must be taken into account.
Assuming a negligible current flowing into the V_CTRL pin, maximum power dissipation on the
series R₁+R₂ is:
(V
)²
P
V
C
T
R
L_sns
=
-------^O----^U----^T--^_-^m----^a---^x------
R +R
12
As an empirical rule, R₁ and R₂ should be selected to get:
P
« 0.01 ⋅ (V
⋅I
)
V C T R L_s n s
OUT_max OUT_max
In order to guarantee proper functionality of the V_CTRL pin, the current flowing into the series
R₁+R₂ should be in the range between 2 µA and 20 µA.
Output voltage sensing capacitor
C₂ is placed in parallel to R₂ and as close as possible to the V_CTRL pin.
Its role is to reject the noise on the voltage sensed by the V_CTRL pin.
Capacitance value depends on the time constant resulting from R₂ (τ_OUT= C₂*R₁//R₂) and
from the system switching frequency (100 kHz), as follows:
τ ≅ 10 ∗ ¹
out
Fssw
C ≅ 10 * ¹ * ¹
2
F R //R
ssw 1 2
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