sc4608mltrt Semtech Corporation, sc4608mltrt Datasheet - Page 9

• Manufacturer Part Number: sc4608mltrt
• Description: High Efficiency Synchronous Step Down Controller
• Manufacturer: Semtech Corporation

width driver pulses while the output voltage is low and
allows these pulses to increase to their steady state duty
cycle as the output voltage reaches its regulated value.
With this, the inrush current from the input side is con-
trolled. The duration of the soft start in the SC4608 is
controlled by an external capacitor. SS, the start-up time
is defined as:
where, C is the value of the external capacitor in nF, and
SS is the start-up time in ms.
Over Current Protection
The SC4608 detects over current conditions by sensing
the voltage across the drain-to-source of the high side
MOSFET. The SC4608 determines the high side MOS-
FET current level by sensing the drain-to-source conduc-
tion voltage across the high side MOSFET via the V
the Typical Application Circuit on page 1) and PHASE pin
during the high side MOSFETs conduction period. This
voltage value is then compared internally to a user pro-
grammed current limit threshold. Note that user should
place Kelvin sensing connections directly from the high
side MOSFET source to the PHASE pin.
The current limit threshold is programmed by the user
based on the RDS(on) of the high side MOSFET and the
value of the external set resistor RSET (where RSET is
represented by R3 in the applications schematics of this
document). The SC4608 uses an internal current source
to pull a 50µA current from the input voltage to the ISET
pin through external resistor RSET.
The current limit threshold resistor (RSET) value is calcu-
lated using the following equation:
The R
tional feature that enhances the performance of the over
current protection. Because the R
temperature coefficient, the 50
positive coefficient of about 0.3%/C° providing first or-
der correction for current sensing vs temperature. This
compensation depends on the high amount of thermal
transferring that typically exists between the high side N-
MOSFET and the SC4608 due to the compact layout of
the power supply.
POWER MANAGEMENT
Application Information (Cont.)
SS
2007 Semtech Corp.
DS(ON)
. 0
09
sensing used in the SC4608 has an addi-
C
R
SET
I
MAX
50
R
µ
DS
A
A current source has a
(
ON
DS(ON)
)
has a positive
in
(see
9
When the converter detects an over current condition
takes is to enter the cycle by cycle protection mode (Point
B to Point C), which responds to minor over current cases.
Then the output voltage is monitored. If the over current
and low output voltage (set at 70% of nominal output
voltage) occur at the same time, the Hiccup mode op-
eration (Point C to Point D) of the SC4608 is invoked
and the internal soft start capacitor is discharged. This is
like a typical soft start cycle:
Figure 2. Over current protection characteristic of
SC4608
Power MOSFET Drivers
The SC4608 has two drivers which are optimized for driv-
ing external power N-Channel MOSFETs. The driver block
consists two 1 Amp drivers. DRVH drives the high side
N-MOSFET (main switch), and DRVL drives the low side
N-MOSFET (synchronous rectifier transistor).
The output drivers also have gate drive non-overlap
mechanism that provides a dead time between DRVH
and DRVL transitions to avoid potential shoot through
problems in the external MOSFETs. By using the proper
design and the appropriate MOSFETs, the SC4608 is
capable of driving a converter with up to 12A of output
current. As shown in Figure 3,
top MOSFET off to the bottom MOSFET on is adaptive by
detecting the voltage of the phase node.
from the bottom MOSFET off to the top MOSFET on is
fixed, is 40ns for the SC4608. This control scheme guar-
antees avoidance of cross conduction or shoot through
(I > I
V
V
V
V
O
O
O
O
MAX
) as shown in Figure 2, the first action the SC4608
0
0
0
0
6 .
7 .
6 .
7 .
V
V
V
V
V
V
V
V
O
O
O
O
O
O
O
O
nom
nom
nom
nom
nom
nom
nom
nom
D
D
D
D
A
A
A
A
I
I
I
I
O
O
O
O
t
d1
,
I
I
I
I
MAX
MAX
MAX
MAX
the delay from the
B
B
B
B
C
C
C
C
SC4608
www.semtech.com
t
d2
, the delay