SC1211VXSTRT SEMTECH [Semtech Corporation], SC1211VXSTRT Datasheet - Page 9

• Manufacturer Part Number: SC1211VXSTRT
• Manufacturer: SEMTECH [Semtech Corporation]

Thermal Shut Down
Thermal Shut Down
COMPONENT SELECTION
COMPONENT SELECTION
Switching F
Switching F
VPN and Vout. During Q1 turn-on, Qcst turns on as well.
The voltage drop across Q1 and Lo charges Ccs. During
Q2 turn-on, Qcsb turns on as well. The voltage drop across
Q2 and Lo discharges Ccs. Both voltage drops are pro-
portional to the inductor current and a resistance equal
to FET’s Rdson plus ESR of the inductor. If the time con-
stant Rcs x Ccs is close to the Lo/Ro of the inductor,
where Ro is given by
the signal developed across Ccs will be proportional to
the inductor current, where Ro is the equivalent current
sensing resistance. In the above equation, Rinductor is
ESR of the inductor, Rdson_hs and Rdson_ls are the top
and bottom FET’s Rdson, and D is the duty cycle of the
converter.
Since a perfect timing match down to the nanosecond is
impossible, the VPN totem pole is held in tri-state during
the communtations of DRN in the SC1211VX. This avoids
errors and offset on the current detection which can be
significant since the timing mismatch is multiplied by the
input voltage. An optional capacitor between VPN and
DRN allows these two nodes to be AC coupled during the
tri-state window, hence yields a perfect timing match.
Refer to Semtech SC2643VX Combi-Sense® Current
Mode Controller about the details of the Combi-Sense®
technique.
Thermal Shut Down
Thermal Shut Down
Thermal Shut Down
The SC1211VX will shut down by pulling both driver out-
puts low if its junction temperature, T
COMPONENT SELECTION
COMPONENT SELECTION
COMPONENT SELECTION
Switching F
Switching Freq
Switching F
The SC1211VX is capable of providing up to 3.5A peak
drive current, and operating up to 1.5MHz PWM frequency
without causing thermal stress on the driver. The selec-
tion of switching frequency, together with inductor and
FETs is a trade-off between the cost, size, and thermal
management of a multi-phase voltage regulator. In mod-
ern microprocessor applications, these parameters could
Ro
POWER MANAGEMENT
Applications Information (Cont.)
2005 Semtech Corp.
=
R
inductor
req
req
req
requency
+
R
uency
uency
uency, Induct
uency
dson
_
hs
, Induct
, Induct
, Inductor and MOSFET
, Induct
*
D
+
R
dson
or and MOSFET
or and MOSFET
or and MOSFETs s s s s
or and MOSFET
_
ls
*
j
, exceeds 155°C.
1 (
-
D
)
9
be in the range of:
Switching Frequency 100kHz to 500kHz per phase
Inductor Value
FETs
Bootstrap Circuit
Bootstrap Circuit
Bootstrap Circuit
Bootstrap Circuit
Bootstrap Circuit
The SC1211VX uses an external bootstrap circuit to pro-
vide a voltage for the top FET drive. This voltage, refer-
ring to the Phase Node, is held up by a bootstrap capaci-
tor. The capacitor value can be calculated based on the
total gate charge of the top FET, Q
voltage ripple on the capacitor, V
C
Typically, it is recommended to use a 1uF ceramic ca-
pacitor with 25V rating and a commonly available diode
IN4148 for the bootstrap circuit. In addition, a small re-
sistor (one ohm) has to be added in between DRN of the
SC1211VX and the Phase Node. The resistor is used to
allievate the stress of the SC1211VX from exposing to
the negative spike at the Phase node. A negative spike
could occur at the Phase Node during the top FET turn-
off due to parasitic inductance in the switching loop. The
spike could be minimized with a careful PCB layout. In
those applications with TO-220 package FETs, it is rec-
ommended to use a clamping diode on the DRN pin to
mitigate the impact of the excessive phase node nega-
tive spike.
Filters for Supply Power
Filters for Supply Power
Filters for Supply Power
Filters for Supply Power
Filters for Supply Power
For VCC pin of the SC1211VX, it is recommended to use
a 1uF to 4.7uF, 25V rating ceramic capacitor for
decoupling.
LA
LA
LAY Y Y Y Y OUT GUIDELINES
LA
LA
The switching regulator is a high di/dt power circuit. Its
Printed Circuit Board (PCB) layout is critical. A good lay-
out can achieve an optimum circuit performance while
minimized the component stress, resulting in better sys-
tem reliability. For a multi-phase voltage regulator, the
SC1211VX driver, FETs, inductor, and supply decoupling
capacitors in each phase have to be considered as a
BST >
OUT GUIDELINES
OUT GUIDELINES
OUT GUIDELINES
OUT GUIDELINES
Q
TOP
/ V
BST
0.2uH to 2uH
4m-ohm to 20m-ohm R
20nC to 100nC total gate charge
BST
TOP
SC1211VX
, in one PWM cycle:
, and an allowed
www.semtech.com
dson