SC4503EVB SEMTECH [Semtech Corporation], SC4503EVB Datasheet - Page 8

• Manufacturer Part Number: SC4503EVB
• Description: 1.3MHz Step-Up Switching Regulator with 1.4A Switch
• Manufacturer: SEMTECH [Semtech Corporation]

Operation
The SC4503 is a 1.3MHz peak current-mode step-up
switching regulator with an integrated 1.4A (minimum)
power transistor. Referring to the block diagram, Figure
2, the clock CLK resets the latch and blanks the power
transistor Q
edge of the clock.
Switch current is sensed with an integrated sense resistor.
The sensed current is summed with the slope-compensat-
ing ramp and fed into the modulating ramp input of the
PWM comparator. The latch is set and Q
terminated when the modulating ramp intersects the error
amplifi er (EA) output. If the switch current exceeds 1.9A (the
typical current-limit), then the current-limit comparator ILIM
will set the latch and turn off Q
width modulating and current limiting paths, cycle-by-cycle
current limiting is not affected by slope compensation.
The current-mode switching regulator is a dual-loop feed-
back control system. In the inner current loop the EA output
controls the peak inductor current. In the outer loop, the
error amplifi er regulates the output voltage. The double
reactive poles of the output LC fi lter are reduced to a single
real pole by the inner current loop, allowing the internal loop
compensation network to accommodate a wide range of
input and output voltages.
Applying 0.9V at the
ence. The signal “REF NOT READY” does not go low until
V
2.2V). Assume that an external resistor is placed between
the IN and the
reference is enabled when the
0.9V. Before V
Q
age). The optional external resistor limits the current drawn
during under-voltage lockout.
When V
off, releasing
from the
will ramp up slowly. The error amplifi er output, which is
 2007 Semtech Corp.
SHDN
POWER MANAGEMENT
Applications Information
IN
2
turns on and the Zener diode Z
exceeds its under-voltage lockout threshold (typically
SS
IN
exceeds 2.2V, “REF NOT READY” goes low. Q
voltage to 1V (above the reference enabling volt-
SHDN
3
conduction. Q
SHDN
IN
SS
SHDN
reaches 2.2V, “REF NOT READY” is high.
pin to the ground, the
SS
SHDN
SS
. If an external capacitor is connected
pins during startup. The voltage
SS
3
is switched on at the trailing
pin enables the voltage refer-
3
SHDN
. Due to separate pulse-
1
loosely regulates the
SS
SHDN
voltage rises to
3
conduction is
SS
voltage
2
turns
8
clamped by D1 and Q1, follows the voltage at the
pin. The input inductor current, which is in turn controlled
by the error amplifi er output, also ramps up gradually.
Soft-starting the SC4503 in this manner eliminates high
input current and output overshoot. Under fault condition
(V
discharged to 1V. When the fault condition disappears, the
converter again undergoes soft-start.
Setting the Output Voltage
An external resistive divider R
tied to the FB pin (Figure 3) sets the output voltage.
The input bias current of the error amplifi er will introduce
an error of:
The percentage error of a V
100kΩ and R
This error is much less than the ratio tolerance resulting
from the use of 1% resistors in the divider string.
IN
< 2.2V or over-temperature), the soft-start capacitor is
∆
∆
Figure 3. R
=
=
=
−
−
2
= 301kΩ is
1
- R
−
VOUT
VOUT
VOUT
VOUT
•
•
2
(
(
Divider Sets the Output Voltage
R1
R1
R1
R1
R2
R2
R2
R2
25nA
25nA
25nA
25nA
)
•
OUT
3
3
3
3
1
SC4503
SC4503
SC4503
SC4503
and R
= 5V converter with R
FB
FB
FB
FB
)
•
2
with its center tap
=
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SC4503
−
SHDN
1
(2)
(1)
SS
=