S8340 datasheet(17/55 Pages) SII

17 / 55 page

STEP-UP, 600 kHz, PWM CONTROL OR PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

Rev.4.0_01

S-8340/8341 Series



Operation

1. Switching Control Method

1. 1 PWM Control (S-8340 Series)

The S-8340 Series is a DC-DC converter using a pulse width modulation method (PWM).

In conventional PFM DC-DC converters, 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.

The switching frequency does not change, although the pulse width changes from 0 to 82% corresponding to each

load current in the S-8340 Series. The ripple voltage generated from switching can thus be eliminated easily

through a filter. When the pulse width is 0% (when no load is applied or the input voltage is high), pulses are

skipped and the current consumption is low.

1. 2 PWM/PFM Switching Control (S-8341 Series)

The S-8341 Series is a DC-DC converter that automatically switches between a pulse width modulation method

(PWM) and a pulse frequency modulation method (PFM) depending on the load current.

The S-8341 Series operates under PWM control with the pulse duty changing from 27 to 82% (A and B types) and

from 21 to 82% (C and D types) in a high output load current area.

The S-8341 Series operates under PFM control with the pulse duty fixed at 27% (A and B types) and at 21% (C and

D types) in a low load current area, and pulses are skipped according to the load current. The oscillation circuit

thus oscillates intermittently so that the resultant lower self current consumption prevents a reduction in the

efficiency at a low load current. The switching point from PWM control to PFM control depends on the external

devices (coil, diode, etc.), and input and output voltage values. The S-8341 Series is an especially highly efficient

DC-DC converter at an output load current around 1 mA.

2. Soft-Start Function

The S-8340/8341 Series has a built-in soft-start circuit.

This circuit enables the output voltage (VOUT) to rise gradually over the specified soft-start time (tSS) to suppress the

overshooting of the output voltage and the rush current from the power supply when the power is switched on or the

ON/OFF pin is changed to “H”.

Generally, a rush current flows to an output capacitor through an inductor and a diode in the step-up circuit

immediately after the power is turned on as shown in Figure 9. Note that the soft-start function of this IC,

however, does not limit this current.

t (2 m s /d iv )

O u t p u t vo ltag e

(1 V /d iv )

R u s h cu rrent

(0. 5 A /d iv )

3 V

0 V

1.5 A

0 A

S -83 40 A 33A F T (V IN

= 0 → 1.9 V , R L = 300 kΩ )

Figure 9 Waveforms of Output Voltage and Rush Current at Soft-Start

S8340 Datasheet(PDF) 17 Page - Seiko Instruments Inc

S8340 Datasheet(HTML) 17 Page - Seiko Instruments Inc

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