SC415 Semtech Corporation, SC415 Datasheet - Page 14

• Manufacturer Part Number: SC415
• Description: Dual Synchronous Buck Controller
• Manufacturer: Semtech Corporation

www.DataSheet4U.com
Applications Information (continued)
The 5μA current into the capacitor creates a linear voltage
ramp. The internal FB comparator tracks this SS ramp,
which forces VOUT to also track the SS ramp. The time
in msec needed for the SS ramp to reach the 750mV
reference is:
At the end of this time, the SS pin has reached 750mV
and the output voltage is at its nominal value. The FB
comparator then switches over to the internal 750mV REF,
and the SS pin is thereafter ignored.
The 5μA current source remains on, so the Css capacitor
continues to charge up to +5V.
The startup waveforms when using a capacitor for the
voltage ramp are shown in Figure 7.
VOUT
EN
SS
T
START
T
(T
START
START
= Css × 150
in μsec, Css in nF)
Figure 7
750mV
CSS
SS linear ramp - 5uA current
source into CSS
EN
SS
5V
SC415
Shutdown
When the EN pin is set low, the SS pin is discharged and
the output DH/DL drivers are pulled low. There is a small
delay between EN going low and DH/DL turning off . The
shutdown delay is given by the equation:
With DH/DL set low, there is no switching, and the output
capacitor is discharged into the VOUT pin discharge
resistance of 16 ohms. This provides a soft-discharge with
no need for a clamp diode across the output capacitor.
When DH/DL stop switching, then that side is placed into
an inactive, low-power state.
MOSFET Gate Drivers
The DH and DL drivers are optimized to drive moderate
high-side and larger low-side power MOSFETs. An adaptive
dead-time circuit monitors the DL output and prevents
the high-side MOSFET from turning on until DL is fully off ;
another circuit monitors the DH output and prevents the
low-side MOSFET from turning on until DH is fully off .
Note: be sure there is low resistance and low inductance
between the DH and DL outputs to the gate of each
MOSFET.
SmartDrive
Each side uses Semtech’s proprietary SmartDrive to
reduce switching noise. The DH drivers will turn on the
high-side MOSFET at a lower rate initially, allowing a
softer, smooth turn-off of the low-side diode. Once the
diode is off , the SmartDrive circuit automatically drives
the high-side MOSFET on at a rapid rate. This technique
reduces switching less while maintaining high effi ciency,
and also avoids the need for snubbers or series resistors
in the gate drive.
TM
T
(T
SD_DELAY
SD_DELAY
= Css × 19
in usec, Css in nF)
SC415
14