SC4607
POWER MANAGEMENT
current protection. Because the RDS(ON) has a positive
temperature coefficient, the 50µA current source has a
positive coefficient of about 0.28%/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 SC4607 due to the compact layout of
the power supply.
Application Information (Cont.)
slew rate of the output voltage, then preventing over-
shoot and limiting inrush current during its start up. Dur-
ing start up of a converter with a big capacitive load, the
load current demands large supply current. To avoid this
an external soft start scheme can be implemented as
shown in Figure 2. Cs can be adjusted for different appli-
cations.
When the converter detects an over current condition (I
> IMAX) as shown in Figure 3, the first action the SC4607
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 SC4607 is invoked
and the internal soft start capacitor is discharged. This is
like a typical soft start cycle:
Pin COMP
Cs
Rs
Q
MMBT2222A-7
2.05k
330n
Vo
Output of a converter
Rp
47.5k
Figure 2
Over Current Protection:
The SC4607 detects over current conditions by sensing
the voltage across the drain-to-source of the high side
MOSFET. The SC4607 determines the high side MOSFET
current level by sensing the drain-to-source conduction
voltage across the high side MOSFET via the Vin (see the
Typical Application Circuit on page 1) and PHASE pin dur-
ing the high side MOSFET’s conduction period. This volt-
age 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.
A
B
VO −nom
0.6 ⋅VO−nom
C
0.7
VO
IMAX
D
IO
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 SC4607 uses an internal current source
to pull a 50µA current from the input voltage to the ISET
pin through external resistor RSET.
Figure 3. Over current protection characteristic of
SC4607
Power MOSFET Drivers:
The current limit threshold resistor (RSET) value is calcu-
lated using the following equation:
The SC4607 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 SC4607 is
capable of driving a converter with up to 12A of output
IMAX RDS(ON)
RSET
=
50µA
The RDS(ON) sensing used in the SC4607 has an addi-
tional feature that enhances the performance of the over
2005 Semtech Corp.
9
www.semtech.com
SEMTECH [ SEMTECH CORPORATION ]
