SC414 Ver la hoja de datos (PDF) - Semtech Corporation
Número de pieza
componentes Descripción
Fabricante
SC414 Datasheet PDF : 29 Pages
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SC414/SC424
Applications Information (continued)
This method automatically produces an on-time that is
proportional to V and inversely proportional to V .
OUT
IN
Under steady-state conditions, the switching frequency
can be determined from the on-time by the following
equation.
fSW
VOUT
TON u VIN
The SC414/SC424 uses an external resistor to set the on-
time which indirectly sets the frequency. The on-time can
be programmed to provide an operating frequency from
200kHz to 1MHz using a resistor between the TON pin and
ground. The resistor value is selected by the following
equation.
RTON
1
400: u VIN
25pF u fSW
VOUT
The maximum R value allowed is shown by the follow-
TON
ing equation.
RTON _ MAX
VIN _ MIN
10 u1.5PA
Immediately after the on-time, the DL (drive signal for the
low side FET) output drives high to turn on the low-side
MOSFET. DL has a minimum high time of ~320ns, after
which DL continues to stay high until one of the following
occurs:
• VFB falls below the 750mV reference
• The Zero Cross Detector senses that the voltage
on the LX node is below ground. Power Save is
activated when a zero crossing is detected.
TON limitations and V5V Supply Voltage
For V5V below 4.5V, the TON accuracy may be limited by
the input voltage.
The original RTON equation is accurate if V satisfies the
IN
below relation over the entire V range:
IN
VIN < (V5V - 1.6V) x 10
If VIN exceeds (V5V - 1.6V) x 10, for all or part of the VIN
range, the RTON equation is not accurate. In all cases
where V > (V5V - 1.6V) x 10, the RTON equation must be
IN
modified as follows.
RTON
1 400: u (V5V 1.6V) u10
25pF u fSW
VOUT
Note that when V > (V5V - 1.6V) x 10 , the actual on-time
IN
is fixed and does not vary with V . When operating in this
IN
condition, the switching frequency will vary inversely with
V rather than approximating a fixed frequency.
IN
V Voltage Selection
OUT
The switcher output voltage is regulated by comparing
V as seen through a resistor divider at the FB pin to the
OUT
internal 750mV reference voltage (see Figure 3).
VOUT
R1
R2
To FB pin
Figure 3 — Output Voltage Selection
Note that this control method regulates the valley of the
output ripple voltage, not the DC value. The DC output
voltage V is offset by the output ripple according to the
OUT
following equation.
VOUT
0.75
u
¨¨©§1
R1
R2
¸¸¹·
¨§
©
VRIPPLE
2
¸·
¹
When a large capacitor is placed in parallel with R1 (C )
TOP
V is shown by the following equation.
OUT
VOUT
0.75
u
¨¨©§1
R1
R2
¸¸¹·
¨§
©
VRIPPLE
2
¸·
¹
u
1 (R1ZCTOP )2
1
¨¨©§
R2
R2
u
R1
R1
ZCTOP
¸¸¹·2
Where ω is the angular switching frequency.
Enable and Power-save Inputs
The EN/PSV and ENL inputs are used to enable or disable
the switching regulator and the LDO. When EN/PSV is low
(grounded), the switching regulator is off and in its lowest
power state. When off, the output of the switching regula-
tor soft-discharges the output into a 10Ω internal resistor
via the VOUT pin. When EN/PSV is allowed to float, the pin
voltage will float to 33% of the voltage at V5V. The switch-
ing regulator turns on with power-save disabled and all
switching is in forced continuous mode. For V5V < 4.5V, it
15
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