SC2542EVB SEMTECH [Semtech Corporation], SC2542EVB Datasheet - Page 9

• Manufacturer Part Number: SC2542EVB
• Description: High Performance Wide Input Range Dual Synchronous Buck Controller
• Manufacturer: SEMTECH [Semtech Corporation]

General Description
The SC2542 is a fixed frequency dual voltage mode step-
down PWM controller for adapter or battery operated
systems. The two channels of the controller operate 180
degrees out of phase, which results in lower input cur-
rent ripple and reduces the amount of input filtering ca-
pacitance needed.
Synchronous and “pulse skip” mode of operation are
adopted in both switching channels to increase overall
efficiency. The internal light-load current detection cir-
cuit and minimum pulse setting circuit determine the
mode of operation and the “pulse skip” duration.
To extend battery life, the SC2542 features shutdown
mode, where the quiescent current is 100nA (typical),
and standby mode, where the 5V linear standby regula-
tor is active and quiescent current is typically 200uA.
The functional block diagram is shown in Figure 1.
Figure 1. The functional block diagram.
Skip Mode Operation
When the RMIN pin is left open, the SC2542 operates
as a synchronous PWM (SPWM) controller with the bot-
tom MOSFET ON whenever the top MOSFET is OFF. Fig-
ure 2 shows the typical SPWM waveforms.
© 2005 Semtech Corp.
POWER MANAGEMENT
Applications Information
INPUT
5VSBY
VO1
5V
Linear
Regulator
Channel 1
SMPS
Power in
Sequence
Channel 2
SMPS
Power
Good
EN
STBY
PWG
VO2
9
Figure 2. Typical waveforms of SPWM mode.
To enhance light-load efficiency, “pulse skip” mode (SKIP)
can be enabled by placing a resistor from the RMIN pin
to ground. For normal loads the controller will operate in
SPWM mode, but when the load is light enough, the pulse
widths of the top and bottom MOSFETs will be adjusted
by the load condition. The SC2542 employs a simple
circuit to detect the load condition by monitoring the
PHASE node of the circuit. The PHASE node will start to
turn positive if the inductor current begins to reverse (I
< 0) signaling a light-load condition.
If a light-load condition is detected the SC2542 will op-
erate in SKIP mode. Similar to a buck converter in dis-
continuous conduction mode (DCM), the converter will
try to reduce its duty cycle as the load decreases. For
example, it would start out with continuous conduction
mode (CCM) duty cycle values, but as the load decreases,
the duty cycle would try to shrink to zero as shown in
Figure 3. However, the SC2542 clamps the DCM duty
cycle to a minimum percentage (set by the RMIN resistor)
of the CCM value. If the DCM duty cycle falls below this
percentage, SKIP mode will be active.
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SC2542
DRVH 1
DRVH 1
PHASE 1
PHASE 1
IL 1
IL 1
DRVL 1
DRVL 1
L