S-8550AA-M5T1X Datasheet, PDF (8/31 Pages) Seiko Instruments Inc – RECTIFICATION, PWM CONTROL SWITCHING REGULATORS

STEP-DOWN, BUILT-IN FET, SYNCHRONOUS RECTIFICATION, PWM CONTROL SWITCHING REGULATORS
S-8550 Series
Rev.5.0_01
 Operation
1. Synchronous rectification PWM control step-down switching regulator
1. 1 Synchronous rectification
The synchronous rectification method lowers voltage drop to greatly reduce power dissipation since an Nch
power MOS FET, having resistance much lower than conventional switching regulators, is used.
In conventional switching regulators, current flows in the diode connected between the GND and CONT pins
when the Pch power MOS FET is off. The forward drop voltage (Vf) of such diodes is large, between 0.3 V
to 0.7 V, so the power dissipation used to be very large. Synchronous rectification ultra-low resistance Nch
transistors repeat on and off, in synchronization with the operation of the Pch driver, in the reverse cycle of
the Pch driver. Moreover, the built-in P and N through prevention circuit helps much reduction of power
consumption during operation.
1. 2 PWM control
The S-8550 Series is a switching regulator using a pulse width modulation method (PWM) and features low
current consumption.
In conventional PFM control switching regulators, pulses are skipped when the output load current is low,
causing a fluctuation in the ripple frequency of the output voltage, resulting in an increase in the ripple
voltage.
In the S-8550 Series, the switching frequency does not change, although the pulse width changes from 0% to
100% corresponding to each load current. The ripple voltage generated from switching can thus be
removed easily using a filter because the switching frequency is constant.
2. Soft-start function
The soft-start circuit built in the S-8550 Series controls the rush current and the overshoot of the output voltage
when powering on, the ON/OFF pin is switched from the “L” level to the “H” level, or the UVLO operation is
released. A reference voltage adjustment method is adopted as the soft-start method.
3. Shutdown pin
This pin stops or starts step-up operations.
Switching the shutdown pin to the “L” level stops operation of all the internal circuits and reduces the current
consumption significantly. DO NOT use the shutdown pin in a floating state because it is not pulled up or
pulled down internally. DO NOT apply voltage of between 0.3 V and 0.9 V to the shutdown pin because
applying such a voltage increases the current consumption. If the shutdown pin is not used, connect it to the
VIN pin.
Shutdown Pin
“H”
“L”
Table 6
CR Oscillation Circuit
Operates
Stops
Output Voltage
Set value
Hi-Z
VIN
ON/OFF
VSS
Figure 8
8