S-8340A25AFT-T2-X Datasheet, PDF (20/55 Pages) Seiko Instruments Inc – SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-UP, 600 kHz, PWM CONTROL OR PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
S-8340/8341 Series
Rev.4.0_01
 Series Products and External Parts Selection
1. Method for Selecting Series Products
The S-8340/8341 Series is classified into eight types, according to the control systems (PWM and PWM/PFM
switching), oscillation frequencies, and output voltage setting types.
The following describes the features of respective types. Select the type according to the applications.
1. 1 Control Systems
Two different control systems are available : PWM control system (S-8340 Series) and PWM/PFM switching control
system (S-8341 Series).
For applications for which the load current greatly differs between standby and operation, if the efficiency during
standby is important, applying the PWM/PFM switching system (S-8341 Series) realizes high efficiency during
standby.
For applications for which switching noise is critical, applying the PWM control system (S-8340 Series) whereby
switching frequency does not change due to load current allows the ripple voltage to be easily eliminated by using a
filter.
1. 2 Oscillation Frequencies
Either oscillation frequencies, 600 kHz (A and B types) or 300 kHz (C and D types), can be selected.
The A and B types whereby high operation frequency allows the L value to be reduced, so a small inductor can be
used. In addition, use of small output capacitors is effective for downsizing devices.
The C and D types, whereby lower oscillation frequency realizes smaller self-consumption current, are highly
efficient under light loads. In particular, the C type, when combined with a PWM/PFM switching control system,
drastically improves the operation efficiency when the output load current is approximately 1 mA.
1. 3 Output Voltage Setting
Either fixed output type (A and C types) or external setting type (B and D types) can be selected.
The A and C types, whereby output voltage can be internally set between 2.5 and 6.0 V in the 0.1 V steps, realizes
highly accurate output voltage of ±2.0% with internal highly resistant and highly accurate resistors.
In the B and D types, the output voltage can be adjusted in the range 2.5 to 6.0 V by adding external resistors (RFB1
and RFB2) and a capacitor (CFB).
A temperature gradient can be provided by installing a thermistor in series to RFB1 and RFB2.
The resistance of RFB1 + RFB2 must not exceed 2 MΩ, and set the ratio of RFB1 to RFB2 so that the FB pin is at 1.0 V.
Add CFB in parallel with RFB1 to prevent unstable operation due to output oscillation.
Set CFB so that fOSC = 1/(2 × π × CFB × RFB1) is 0.1 to 20 kHz (normally, 10 kHz).
Example : VOUT = 3.0 V, RFB1 = 200 kΩ, RFB2 = 100 kΩ, CFB = 100 pF
The accuracy of the output voltage VOUT set with resistors RFB1 and RFB2 is affected by the absolute precision of
external resistors RFB1 and RFB2, the FB pin input current (IFB) and IC power supply voltage (VDD) as well as the
precision of the voltage at FB pin (1 V ±2.0%).
When it is assumed that IFB is 0 nA, the maximum absolute value variations of external resistors RFB1 and RFB2 are
RFB1max. and RFB2max., the minimum absolute value variations of external resistors RFB1 and RFB2 are RFB1min. and
RFB2min., and the shift of the output voltage due to the dependence of voltage on VDD is ΔV, the minimum value
(VOUT min.) and maximum value (VOUT max.) of variations of VOUT are expressed by the following formulas :
20