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參數(shù)資料
| 型號： | SC416MLTRT |
| 廠商： | Semtech Corporation |
| 英文描述： | Dual Synchronous Buck Controller with Tracking Start-up/Shutdown |
| 中文描述： | 雙同步降壓控制器跟蹤Start-up/Shutdown |
| 文件頁數(shù)： | 9/34頁 |
| 文件大?。?/td> | 1131K |
| 代理商： | SC416MLTRT |
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SC416
9
Applications Information
SC416 Synchronous Buck Controller
The SC416 is a dual synchronous controller which sim-
plifies the task of designing a dual-output power sup-
ply with synchronized or tracking ramps for startup and
shutdown.
VIN and +5V Bias Supplies
The SC416 requires an external +5V bias supply in ad-
dition to the VIN supply. If stand-alone capability is re-
quired, the +5V supply can be generated with an exter-
nal linear regulator.
Pseudo-Fixed Frequency Constant On-Time PWM
Controller
The PWM control method for each output is a constant-
on-time, pseudo-fixed frequency PWM controller, see
Figure 1. The ripple voltage seen across the output ca-
pacitor’s ESR provides the PWM ramp signal. The on-time
is determined by an internal one-shot whose period is
proportional to output voltage and inversely proportion-
al to input voltage. A separate one-shot sets the mini-
mum off-time (typically 330ns). The two converters are
designed to operate at different frequencies to reduce
interaction; side2 frequency is set typically 20% higher
than side1.
Q1
Q2
L
C
OUT
VIN
ESR
+
C
IN
V
OUT
FB Threshold
750mV
V
FB
V
LX
V
LX
TON
FB
R1
R2
Figure 1
On-Time One-Shot (TON)
Each internal on-time one-shot comparator has two in-
puts. One input looks at the output voltage via the VOUT
pin, while the other input samples the input voltage
via the TON pin and converts it to a proportional cur-
rent. This current charges an internal on-time capacitor.
The TON on-time is the time required for this capacitor
to charge from zero volts to VOUT, thereby making the
on-time directly proportional to output voltage and in-
versely proportional to input voltage. This implementa-
tion results in a fairly constant switching frequency with
no clock generator.
The nominal frequency is set through an external resis-
tor RTON connected between VIN and the TON pin. To
minimize interaction between the two converters, side2
is set to operate at a slightly higher frequency. The gen-
eral equations for the side1 and side2 on-times are:
TON1 = 3.30 × (RTON + 37) × (VOUT/VIN) + 35
TON2 = 2.75 × (RTON + 37) × (VOUT/VIN) + 35
(TON in nsec, RTON in kΩ)
Switch-Mode Operation
The switch-mode operation is explained below, and is
identical for both sides except for the TON timing.
The output voltage is sensed at the FB pin and is com-
pared to the internal 750mV reference. (The output volt-
age can also be sensed at the VOUT pin which uses an
internal resistor divider, see VOUT Voltage Selection.)
When the sensed voltage drops below 750mV, this trig-
gers a single TON pulse, which is fed to the DH high-side
driver. The DH pulse-width follows TON according to the
TON equation, and after that time DH drives low to shut
off the high-side MOSFET. After DH drives low, the DL
output drives high to energize the low-side MOSFET.
Once high, DL has a minimum pulse width of typically
330nsec which is the minimum off-time. At the end of
the minimum off-time, DL continues to stay high until
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