SC410 SEMTECH [Semtech Corporation], SC410 Datasheet - Page 13

• Manufacturer Part Number: SC410
• Description: 3A EcoSpeed TM Step-Down Regulator with LDO and Ultrasonic Power Save
• Manufacturer: SEMTECH [Semtech Corporation]

Applications Information (continued)
This method automatically produces an on-time that is
proportional to V
Under steady-state operating conditions in continuous
conduction mode, the switching frequency can be deter-
mined from the on-time by the following equation.
The SC410 uses an external resistor to set the on-time
which indirectly sets the frequency. The on-time can be
programmed to provide operating frequencies from
200kHz to 1MHz using a resistor between the TON pin
and ground. The resistor value is selected by the follow-
ing equation.
The maximum R
ing equation.
Immediately after the on-time, the DL (the 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:
FB
Ref.
V
V
LX_FILTER
IN
•
•
f
R
SW
R
R
TON
V
The zero cross detector senses that the voltage
on the LX node is below ground. PSAVE is acti-
vated 8 periods after the zero cross is detected.
TON
TON
-
+
FB
PWM
falls below the 750mV reference
_
t
One-Shot
MAX
ON
V
Timer
25
Figure 2 — On-Time Generation
OUT
pF
Time = K x (V
R
V
S
10
TON
1
IN
V
Q
OUT
f
value allowed is shown by the follow-
IN
SW
1
_
5 .
and inversely proportional to V
MIN
Lo-side
Drivers
Hi-side
Gate
and
400
A
OUT
/V
IN
)
V
V
IN
V
Q1
V
Q2
OUT
IN
LX
L
C
LPF
ESR
OUT
+
V
LX_FILTER
V
OUT
FB
IN
.
V
The switcher output voltage is regulated by comparing
V
internal 750mV reference voltage, see Figure 3.
Note that this control method regulates the valley of the
output ripple voltage, not the DC value. The DC output
voltage V
the following equation.
Enable Input
The EN input is used to enable or disable the switching
regulator. When EN is low (grounded), the switching regu-
lator and LDO are disabled and the SC410 is in its lowest
power state. When disabled, the output power switches
are tri-stated. When EN is higher than 0.8V, the internal
LDO will be activated. The switching regulator remains off
until the voltage at the EN pin exceeds 1.5V.
The EN pin can be used for implementing UVLO for the
input voltage by confi guring a voltage divider from VIN to
EN to PGND.
Continuous Mode Operation
The SC410 operates in CCM (Continuous Conduction
Mode) at larger load currents when the load is greater
than or equal to half of the inductor ripple current (Figure
4). In this mode one of the power MOSFETs is always on,
with no intentional dead time other than to avoid cross-
conduction. This mode of operation results in uniform
frequency.
OUT
OUT
V
as seen through a resistor divider at the FB pin to the
Voltage Selection
OUT
V
Figure 3 — Output Voltage Selection
OUT
OUT
. 0
is off set by the output ripple according to
75
1
R
R
1
2
R
1
V
RIPPLE
2
R
2
To FB pin
SC410
13