SC4620 Просмотр технического описания (PDF) - Semtech Corporation
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SC4620
Semtech Corporation
SC4620 Datasheet PDF : 19 Pages
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SC4620
POWER MANAGEMENT
Application Information
Overview
The SC4620 is a programmable high switching frequency,
integrated 2.5A MOSFET, synchronous step down regula-
tor. 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.
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
SC4620 is controlled by an external capacitor.
The SC4620 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.
The SC4620 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 VCC finally.
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.
Enable
The SC4620 is enabled by applying a voltage greater than
2V (typical) to the VCC and SYNC/EN pin. The voltage on
the VCC pin determines the operation of the SC4620. As
VCC 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 VCC reaches 2V. If
no faults are present, the SC4620 will initiate a soft start
when VCC 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 SC4620 is disabled when VCC 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 VCC 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.
145
135
125
115
105
95
85
75
65
55
45
35
25
15
5
200
Switching Frequency Setting
5VIN
2.5VIN
400
600
800 1000 1200 1400 1600 1800 2000
Fosc (kHz)
Figure 15. Switching Frequency vs. RFS
The operation frequency can be programmed up to 2MHz,
but there is a minimum on-time limitation which is around
110ns. Users should take care of minimum limitation on the
operating duty cycle under high frequency application.
2008 Semtech Corp.
11
www.semtech.com
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