SC2618 Semtech, SC2618 Datasheet - Page 4

• Manufacturer Part Number: SC2618
• Description: Miniature PWM Controller for Buck Regulator
• Manufacturer: Semtech

Switching F
Switching F
UVL
UVL
The SC2618 is a hysteretic mode PWM controller. It uses
a comparator to generate PWM wave with minimum on
time and off time control. As shown in the block diagram,
the output voltage is fed back to the comparator nega-
tive input and compared to a setting voltage. If the out-
put voltage is below its set point, the top gate drive will
turn on and remain on until the minimum on time has
expired AND the output voltage has risen above the set
point. Similarly, if the output voltage is above its set point,
top gate drive will be turned off and bottom gate will turn
on and stay on until the minimum off time has expired
AND the output is below its set point. Because of this
minimum time control scheme, the comparator hyster-
esis is not required and is internally set to zero.
Switching F
Switching F
Switching Freq
This control scheme will force a buck converter to oper-
ate either at minimum on time, or minimum off time, or
both. The SC2618 has minimum time of 1us. Its switch-
ing frequency, peaking at 500kHz, can be found in Fig. 1
as long as the voltage ratio of the buck converter has
been decided.
UVL
UVL
UVLO, Star
To initiate the SC2618, supply voltages are applied to
Vcc and BST pins. The top gate (DH) and bottom gate
(DL) are held low until Vcc exceed UVLO threshold, typi-
cally 4.5V. At that point, the internal soft start capacitor
begins to charge, the top gate remains low, and the bot-
tom gate is pulled high to turn on the bottom FET. When
the voltage at soft start cap reaches a setting level, the
top and bottom gates of PWM controller begin to switch.
The switching regulator output is slowly ramping up for a
POWER MANAGEMENT
Theory of Operation
 2004 Semtech Corp.
500
400
300
200
100
O, Star
O, Star
O, Star
O, Start up and Shut do
0
Fig. 1. Switching frequency vs. duty cycle
0.1
t up and Shut do
t up and Shut do
t up and Shut down
t up and Shut do
req
req
req
requency vs. Duty Cy
0.2
uency vs. Duty Cy
uency vs. Duty Cy
uency vs. Duty Cy
uency vs. Duty Cycle
0.3
0.4
Duty Cycle
0.5
0.6
wn
wn
wn
wn
0.7
cle
cle
cle
cle
0.8 0.9
1.0
4
soft turn on. If, however, one or more supply voltage rails
are late or absent, the output voltage will not rise and
the part will behave as if there was a short circuit at the
output, that is, it will go into hiccup mode. It will remain in
this mode until all the necessary voltage rails are present,
at which time normal operation will start.
If the supply voltage Vcc falls below UVLO threshold dur-
ing normal operation, the soft start capacitor begins to
discharge. When the voltage reaches the setting level,
the PWM controller control the switching regulator out-
put to ramp down slowly for a soft turn off.
Hiccup Mode Shor
Hiccup Mode Shor
Hiccup Mode Shor
Hiccup Mode Shor
Hiccup Mode Short Cir
Short circuit protection is implemented by comparing the
feedback node with the soft start and reference volt-
age. If the FB voltage falls more than about 200mV be-
low it’s correct voltage the short circuit latch is set which
immediately disables the top drive. If the short circuit
occurred during soft start, the soft start capacitor con-
tinues to charge to it’s full voltage before discharging
towards zero. If the short occurred during steady state
operation, the internal soft start capacitor begins to dis-
charge immediately, at the same rate as it charged, to-
wards zero. Once the soft start capacitor reaches its
minimum voltage, the latches are cleared and a normal
soft start is attempted, if the short circuit condition has
not been cleared, the fault latch will again be set once
the soft start voltage reaches about 200mV and the cycle
will repeat until the short is cleared.
It is also possible, with large output capacitor values,
that the output voltage will be unable to rise fast enough
to accurately follow the soft start voltage and a short
circuit trip may result. During soft start the bottom gate
drive is disabled to prevent discharging the output ca-
pacitor and so the output capacitor will retain the volt-
age achieved before the trip and will charge to a higher
voltage during the next hiccup cycle. In this way the out-
put voltage will achieve the set point value but may take
several hiccup cycles to do so.
t Cir
t Cir
t Cir
t Circuit Pr
cuit Pr
cuit Pr
cuit Pr
cuit Pro o o o o t t t t t ection
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SC2618
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