SC4905AIMSTRT Semtech, SC4905AIMSTRT Datasheet - Page 8

• Manufacturer Part Number: SC4905AIMSTRT
• Description: IC, VOLTAGE MODE PWM CTRL, 12V, MSOP-10
• Manufacturer: Semtech

Specifications of SC4905AIMSTRT

• Input Voltage: 48V
• Output Voltage: 500mV
• Output Current: 1mA
• Frequency: 1MHz
• Power Dissipation Pd: 180mW
• Supply Voltage Range: 0V To 12V
• Digital Ic Case Style: MSOP
• No. Of Pins: 10
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

POWER MANAGEMENT
VDD UNDER VOLTAGE LOCK OUT
According to the application, and the voltages available,
the SC4905A (UVLO = 4.4V), or the SC4905B (UVLO =
11.6V) can be used to provide the VDD undervoltage
lock out function to ensure the converters controlled
start up.
Before the VDD UVLO has been reached, the internal ref-
erence, oscillator, OUT driver, and logic are disabled.
REFERENCE
A 4V (SC4905A) or a 5V(SC4905B) reference voltage is
available that can be used to source a typical current
up to 5mA to the external circuitry. The REF can be used
to provide the feed back circuitry with a regulated bias.
OSCILLATOR
The oscillator frequency is set by connecting a RC network
as shown below.
The oscillator has a ramp voltage that will track the volt-
age at the VFF pin (1.2V<VFF<3.6V). The oscillator peak
voltage is derived by charging the oscillator capacitor (Cosc)
to the VFF voltage via the oscillator resistor (Rosc). The
bias current to charge the Cosc is controlled by the Rosc.
Once the RC pin has reached the VFF voltage, the oscil-
lator ramp is discharged by an internal switch hence cre-
ating the triangle oscillator ramp.
Application Information (Cont.)
2006 Semtech Corp.
Vin
RT
RM
RB
280k
2k
8.25k
Rosc
Cosc
499k
220p, 16V
1
2
3
4
5
VDD
FB
VFF
DMAX
RC
SC4905
U1
SYNC
GND
OUT
REF
ILIM
10
9
8
7
6
8
Since the Rosc is referenced to the input supply voltage,
any variation in the supply is directly translated into a
variation in the duty cycle, while maintaining the fixed
frequency operation.
Following equation can be used to calculate the oscilla-
tor frequency:
The recommended range if timing resistors is between
10 kohm and 500kohm and range of timing capacitors
is between 100pF and 1000pF. Timing resistors less
than 10 kohm should be avoided.
F
OSC
R
OSC
C
Vin
OSC
VFF
SC4905A/B
VFF
2
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