SC4530 Datasheet, PDF (8/23 Pages) Semtech Corporation – 30V, 300mA Output Micropower Step-Down Switching Regulator

SC4530
General Description and Operation
The SC4530 is a micropower, hysteretic current-mode
step-down switching regulator. As shown in the block
diagram in Figure 2, the converter is controlled by an error
amplifier EA and two current-sensing comparators IPK and
IVLY. IPK and IVLY monitor the switch (Q₁) collector current
and the freewheeling diode (D₁) current respectively. The
EA amplifies the differential voltage between the FB and
the bandgap reference, and produces a current, I_CNTL,
proportional to its output voltage. I_CNTL, in turn, adjusts the
switching thresholds of both the peak and valley current
comparators. The EA output voltage is high at heavy
loads, as is the peak inductor current. The Zener diode
D_Z clamps the amplifier output and sets the switch peak
current limit.
When the switch Q₁ is turned on, the current through Q₁
ramps up until it reaches the peak threshold set by I_CNTL. The
output of the IPK comparator, OC, goes high. This resets
the latch and turns off the switch. With Q₁ off, the inductor
current ramps down through the freewheeling diode
D₁. When D₁ current ramps below the valley threshold
established by I_CNTL, the output of the IVLY comparator,
UC, goes high. If Q₁ has been turned off for more than
530ns, then the latch will be set and Q₁ will again turn on,
starting a new cycle.
The RUN/IDLE comparator, CMP, monitors the output of
the error amplifier. If the EA output falls below the RUN/
IDLE threshold, then Q₁ and all control circuits except the
reference and EA will be shut off. The output capacitor
will then supply the load, causing the output voltage
to fall. When the EA output rises above the RUN/IDLE
threshold, the control circuit wakes up and the part starts
to switch, delivering power to the output. The offset
voltage V_OS at the input of the IPK comparator ensures
that any current pulse delivered to the output has some
minimum amplitude.
At very light loads, even a single minimum charge packet
delivered to the output will cause the FB voltage to rise
above the reference voltage. This causes the EA output
voltage to fall and the part to idle. The part resumes
switching when the output current discharges the FB
voltage below the reference. At light loads, the part
switches only as needed to keep the output in regulation.
By reducing the supply current drawn when idling, high
efficiency is maintained at light loads. At heavier loads,
it may take a number of consecutive minimum pulses to
bring the FB above the reference voltage. The part enters
continuous conduction mode when the amplifier output
never falls below the RUN/IDLE threshold.
The inductor ripple current in continuous-conduction
mode is independent of I_CNTL and is primarily determined
by V_OS and V_HYS. Continuous mode switching frequency,
therefore, depends on V_IN, V_OUT, the inductance L and the
propagation delay times of the current comparators.
If the regulator output is shorted to ground, then the
amplifier output will rise to D_Z clamp voltage. Q₁ turns
off as the inductor current reaches the peak current limit.
With the output shorted to ground, the inductor current
ramps down at a slower rate through D₁. Q₁ turns on again
when the inductor current crosses the valley threshold.
Therefore, short-circuiting the output merely lowers the
converter switching frequency. The inductor current
remains bounded by the peak switch current limit.
Driving the base of the power transistor above the input
power supply rail minimizes the power transistor turn-
on voltage and maximizes efficiency. A bootstrap circuit
[formed by an internal bootstrap diode D₂ (Figure 2) and
an external capacitor connected between BST and SW]
generates a voltage higher than V_IN at the BST pin. The
bootstrapped voltage becomes the supply voltage of the
power transistor driver.
The internal control circuit takes its power from either the
input or from the BIAS pin if V_BIAS > 2.3V. For applications
with output voltage higher than 2.5V, the BIAS pin should
be tied to the regulator output to maximize efficiency.