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SC4624EVB-SO Datasheet(PDF) 11 Page - Semtech Corporation

Part # SC4624EVB-SO
Description  Low Input Voltage, High Efficiency, 4A Integrated FET Synchronous Step down DC/DC Regulator
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC4624EVB-SO Datasheet(HTML) 11 Page - Semtech Corporation

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 2008 Semtech Corp.
www.semtech.com
SC4624
POWER MANAGEMENT
Application Information
Overview
The SC4624 is a programmable high switching frequency,
integrated 4A MOSFET, synchronous step down regulator.
This reduces external component count and makes it
effective for applications which are low in cost and sized
small. A non-overlap protection is provided for the gate
drive signals to prevent shoot through of the internal
MOSFET pair.
The SC4624 is capable of producing an output voltage as
low as 0.5V and Its operation frequency is programmable
up to 2MHz by an external resistor. It features lossless
current sensing of the voltage drop across the internal
drain to source resistance of the high side MOSFET during
its conduction period.
The quiescent supply current in shutdown mode is
typically lower than 1µA. An external soft start is provided
to prevent output voltage overshoot during start-up. Over
Temperature Protection, Power Good Indicator, External
Clock Synchronization are some of the internal added
features.
Enable
The SC4624 is enabled by applying a voltage greater than
2V (typical) to the V
CC and SYNC/EN pin. The voltage on
the V
CC pin determines the operation of the SC4624. As
V
CC increases during start up, the UVLO block senses VCC
and keeps the high side and low side MOSFETs off and
the internal soft start voltage low until V
CC reaches 2V. If
no faults are present, the SC4624 will initiate a soft start
when V
CC exceeds 2V. A typical 120mV hysteresis in the
UVLO comparator provides noise immunity during its start
up. (refer to Figure 1 to 2).
Shutdown
The SC4624 is disabled when V
CC falls below 1.88V (typical)
or shutdown mode operation is invoked by clamping the
SYNC/EN pin to a voltage below 0.8V. During the shutdown
mode, A typical 0.2µA current draw through the V
CC pin,
the internal soft start voltage is held low and the internal
MOSFETs are turned off. (refer to Figure 3 to 4).
Soft Start
The soft start function is required for step down controllers
to prevent excess in-rush current through the DC bus
during start up. An external capacitor is necessary for
the soft start function and is connected from SS pin to
AGND.
During start up or restart, A typical 4µA sourcing current
charges the capacitor and then the voltage of capacitor
ramp up the error amp reference slowly. The closed
loop creates narrow width driver pulses while the output
voltage is low and allows these pulses to increase to their
steady state duty cycle as the output voltage reaches
its regulated value. The duration of the soft start in the
SC4624 is controlled by an external capacitor.
The SC4624 starts up in asynchronous mode before SS
voltage reaches to 0.5V, and the bottom FET diode is used
for circulating current during the top FET off time. Ths SS
voltage level is clamped at V
CC finally.
Pre-biased Output
The SC4624 is able to start into pre-biased output by
adding external RC circuit, where R(10K) is between V
CC
pin and EN pin, C(0.1uF) is between EN pin and AGND.
If there is a pre-biased load on the output of SC4624
during start-up, the internal low-side MOSFET of SC4624
is always disabled before SS reach to 0.5V, the output
voltage is maintained. The great feature avoids negative
voltage spikes or short circuit on the output, which could
cause damage to the down-stream IC during start-up.
Timing between V
CC and EN is very important for pre-biased
output. V
CC must lead EN. When VCC and EN voltage rise
at same time(tied together), the pre-biased output voltage
is pull low before V
CC reach to the voltage of UVLO. If this
isn’t desirable, RC(10Kand 0.1uF) must be added at EN
to prevent this from happening.
Oscillator
The FS pin is used to set the PWM oscillator frequency
through an external resistor that is connected from the
FS pin to the AGND. The internal ramp is a triangle at the
PWM frequency with a peak voltage of 1.25V and a valley
voltage of 0.25V. The approximate operating frequency is
determined by the value of an external resistor as shown
in Figure 15.


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