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參數(shù)資料
| 型號： | SC475AMLTRT |
| 廠商： | Semtech Corporation |
| 英文描述： | Synchronous Buck Controller with Dual-Level VOUT Transition Support |
| 中文描述： | 同步降壓控制器與雙級VOUT電源支持過渡 |
| 文件頁數(shù)： | 14/27頁 |
| 文件大小： | 603K |
| 代理商： | SC475AMLTRT |
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14
2006 Semtech Corp.
SC475A
www.semtech.com
POWER MANAGEMENT
Applications Information
(continued)
Note that the presence of Rs/Cs will affect the effective
resistance at the FB pin, and therefore modi
fi
es the VOUT
setpoints. If Rs is used, the following table shows the
calculated values for VOUT.
VOUT Equation
G0
0.75 (1 + R1/R2)
1
0.75 (1 + R1/R2 + R1/(R3+Rs))
0
Enable Input
The EN is used to disable or enable the SC475A. When
EN is low (grounded), the SC475A is off and in its lowest-
power state. When EN is high the controller is enabled
and switching will begin.
PSAVE Operation
The SC475A provides automatic power save operation at
light loads. The internal Zero-Cross comparator looks for
inductor current (via the voltage across the lower MOSFET)
to fall to zero on eight consecutive switching cycles. Once
observed, the controller enters power save and turns off
the low-side MOSFET on each cycle when the current
crosses zero. To add hysteresis, the on-time is increased
by 25% in power save. The ef
fi
ciency improvement at light
loads more than offsets the disadvantage of slightly higher
output ripple. If the inductor current does not cross zero
on any switching cycle, the controller immediately exits
power save. Since the controller counts zero crossings,
the converter can sink current as long as the current does
not cross zero on eight consecutive cycles. This allows the
output voltage to recover quickly in response to negative
load steps, or to voltage transitions from a higher to a
lower voltage where the change exceeds 8%.
Smart Power Save Protection
In some applications, active loads on VOUT can leak
current from a higher voltage and thereby cause VOUT to
slowly rise and reach the OVP threshold, causing a hard
shutdown; the SC475A uses Smart Power Save to prevent
this. When FB exceeds 8% above nominal (810mV), the
IC exits power save (if already active) and DL drives high to
turn on the low-side MOSFET, which starts to draw current
from VOUT via the inductor. When FB drops to the 0.75V
trip point, a normal TON switching cycle begins. This cycles
energy from VOUT back to VBAT and prevents a hard OVP
shutdown, and also minimizes operating power by avoiding
continuous conduction-mode operation. If a light load is
present, the switching continues for 8 consecutive clock
cycles and then the IC will re-enter power save to reduce
operating power.
Current Limit Circuit
Current limiting can be accomplished in two ways. The
RDSON of the lower MOSFET can be used as a current
sensing element, or a sense resistor at the lower MOSFET
source can be used if greater accuracy is needed. RDSON
sensing is more ef
fi
cient and less expensive. In both
cases, the R
resistor sets the over-current threshold.
The R
connects from the ILIM pin to either the lower
MOSFET drain (for RDSON sensing) or the high side of the
current-sense resistor. R
connects to a 10
μ
A current
source from the ILIM pin which turns on when the low-
side MOSFET turns on, after the on-time DH pulse has
completed. If the voltage drop across the sense resistor
or low-side MOSFET exceeds the voltage across the R
resistor, current limit will activate. The high-side MOSFET
is held off until the voltage drop across the sense element
(resistor or MOSFET) falls below the voltage across the
R
ILIM
resistor.
This current sensing scheme actually regulates the
inductor valley current, (see Figure 10). This means that
if the current limit is set to 10A, the peak current through
the inductor would be 10A plus the peak ripple current,
and the average current through the inductor would be
10A plus 1/2 the peak-to-peak ripple current.
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