SC4626CEVB-1 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC4626CEVB-1' PDF : 21 Pages View PDF
SC4626
Applications Information
Detailed Description
adjusted to meet the equation shown above.
The SC4626 is a synchronous step-down pulse width
modulated (PWM) voltage mode DC-DC regulator Protection Features
operating at 2.5MHz fixed-frequency. The switching
frequency is chosen to minimize the size of the external
inductor and capacitors while maintaining high
efficiency.
The SC4626 provides the following protection features:
• Thermal Shutdown
• Current Limit
• Over-Voltage Protection
• Soft-Start Operation
Operation
During normal operation, the internal high-
side PMOS device is activated on each ris-
ing edge of the internal oscillator. The voltage
feedback loop uses an internal feedback resistor divider.
The period is set by the on board oscillator when in PWM
mode at average to high loads. The device has an internal
low-side synchronous NMOS device and does not require a
Thermal Shutdown
The device has a thermal shutdown feature to protect
the SC4626 if the junction temperature exceeds 160°C.
During thermal shutdown, the on-chip power devices
are disabled with the LX output floating. When the die
temperature drops by 10°C, the part will initiate a soft
start recovery to normal operation.
Schottky diode on the LX pin.The device operates as a buck
converter in PWM mode with a fixed frequency of 2.5MHz.
Current Limit
The internal PMOS power device in the switching stage is
Output Voltage Selection
protected by current limit feature. If the output is loaded
The SC4626 is designed for fixed output voltage. There above the PMOS current limit for 32 consecutive cycles,
are some options for preset output voltage shown in the SC4626 enters foldback current limit mode and the
Table 1. If the voltage desired is not shown in the Table output current is limited to the current limit holding
1, it can be programmed via an external resistor di- current (ICL_HOLD) of a few hundred milliampere. Under
vider. There will be typical 1uA current flowing into the these conditions the output voltage will be the product
VOUT pin. The typical schematic of adjustable output of ICL_HOLD and the load resistance. The current limit
voltage option from the part with standard 1.0V, the holding current (ICL_HOLD) will be decreased when output
SC4626A, is shown in Figure 1. The CFF is needed for
maintain the performance of the transient response. The
voltage is increased. The load presented must fall below
the current limit holding current for the SC4626 to exit
proper value of CFF can be calculated by the equation
foldback current limit mode. Figure 2 shows the typical
current limit holding current decreasing rate over differ-
( ) C
( ) FF
[nF ]
=
10
×
V
OUT
− 0.5
2
RFB1[kΩ] ⋅
V
OUT
−
V
OSTD
V
× ( ) OSTD
V
OSTD
−
0.5
ent output voltage. The SC4626 is capable of sustaining
a indefinite short circuit without damage and will resume
,where the VOSTD is the standard voltage shown in Table 1.
To simplify the design, it is recommended to program the
normal operation when the fault is removed. The fold-
back current limit mode will be disabled during the
desired output voltage from standard 1.0V as shown in soft-start.
150
Current Limit Holding Current over Vout
FsShicgohuweremn 1aabtwicoivothef.AaFdopjrurpostrpaoebgrlreaCmVFFOmcUaTinlcfgruoltmahteeSdoCuf4rto6pm2u6tAtvho(eSltaetdgqeuVaOfrtUoioTm=n1.0V)
other standard voltage, the RFB1, RFB2 and CFF need to be
TA= 25°C
120
90
VIN= 5.0V
VIN
Enable
CIN
REN
L
VIN
LX
VOUT
GND
RFB1 CFF
COUT
EN
VOUT
RFB2
SC4626A 10k
R
FB1
=
(VOUT
−
1)
×
R
FB
2
Figure 1 — Typical schematic for adjustable output
voltage option from standard 1.0V of SC4626A
60
30
VIN= 3.6V
0
1.0
1.5
2.0
2.5
3.0
3.5
Output Voltage (V)
Figure 2 — Current limit holding current decreasing
rate vs. output voltage
© 2009 Semtech Corp. Note: (1) REN is optional.
(2) RFB2=10k and CFF=10nF for standard design.
15
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