S-8340A25AFT-T2-X Datasheet, PDF (19/55 Pages) Seiko Instruments Inc – SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-UP, 600 kHz, PWM CONTROL OR PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.4.0_01
S-8340/8341 Series
4. Current Limit Circuit
The current limit circuit of the S-8340/8341 Series protects the external transistors from being damaged by heat due
to an overload or magnetic saturation of coils. Inserting a SENSE resistor (RSENSE) between the external FET
source or external NPN bipolar transistor emitter and Vss and entering a connection point with a sensor resistor into
the SENSE pin enables the current limit to function. Refer to “ Standard Circuit”.
A current limiting comparator in the IC monitors the SENSE pin for reaching the current limit detection voltage
(VSENSE = 120 mV (typ.)). Upon detection of the voltage, the external transistor is held off for one clock of the
oscillator so that the current flowing in the external transistor is limited. At the ON signal of the next clock, the
external transistor is turned on and the current limit detection function is resumed.
However, this current limit circuit contains a CR filter with a time constant (τ = 220 ns (typ.)) between the SENSE
pin and the current limiting comparator in the IC to prevent detection errors caused by the spike voltage generated
at the SENSE pin. If the time (pulse width tON : “H” level time at the EXT pin) after the external transistor turns on
until the current limit circuit operates is short, the current value that is actually limited becomes higher than the
current limit setting value determined by VSENSE/RSENSE as a side effect. The actual limit current value (ILIMIT) is
expressed by the following equation :
ILIMIT
=
VSENSE
÷


1
−
−
e
ton × 0.5
CR


RSENSE _

Remark CR in the equation is determined by the internal CR filter and varies in the range 116 to 470 ns (220 ns
(typ.).)
Caution Therefore, this current limit function does not guarantee full protection of external parts by ILIMIT =
VSENSE/RSENSE under all operating conditions. Perform a thorough evaluation using the actual
devices.
For example, usage when the current value that the current limit circuit actually functions to raise the current limit
set value decided by VSENSE/RSENSE that includes usage under the conditions that the input voltage become close to
the output voltage or situations when the output voltage falls due to the activation of the current limit circuit and
become close to the input voltage.
Figure 12 shows an example of the actually measured increase of the peak current flowing through the coil when
the current limit circuit functions while the input voltage is nearing the output voltage.
Figure 13 shows an example of the actually measured increase of the peak current flowing through the coil when
the output voltage drops and approaches the input voltage by increasing the output current after the current limit
circuit functions.
Input Voltage (VIN) vs. Coil Peak Current (ILPEAK)
4 ^{S-8340A50 (RSENSE = 51 mΩ)}
3
2
V_SENSE / R_SENSE
1
0
1
2
3
4
5
V_IN (V)
Figure 12 ILPEAK Measured at Activation of Current Limit
(VOUT Starts to Fall)
Output Current (IOUT) vs. Coil Peak Current (ILPEAK)
5 ^{S-8340A50 (VIN = 3V, RSENSE = 51 mΩ)}
4
Current limit circuit
is activated
Influence of CR filter
3
2
V_SENSE / R_SENSE
1
0
0
1
2
3
I_OUT (A)
Figure 13 Measuring Coil Peak Current (ILPEAK)
If the current limit circuit is not used, remove RSENSE and connect the external transistor source or the emitter and
the SENSE pin to VSS.
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