A8426 Просмотр технического описания (PDF) - Allegro MicroSystems
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A8426 Datasheet PDF : 16 Pages
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A8426
High Performance Photoflash Capacitor Charger
with IGBT Driver
Selection of Switching Current Limit
The A8426 features continuously adjustable peak
switching current between 1.0 and 3.2 A. This is done
by selecting the value of the external resistor RSET
(connected between the ISET pin and GND), which
determines the ISET bias current, and therefore the
switching current limit, ISWlim.
To the first order approximation, ISWlim is related to
ISET and RSET by the following equation:
ISWlim = ISET × K
= (VSET × RSET ) × K ,
(6)
where K ≈ 60000 when the IC bias voltage, VIN , is
3.6 V.
In real applications, the switching current limit is
affected by bias voltage, battery voltage, and the
transformer primary inductance, LP. If necessary, the
following expressions can be used to determine ISWlim
more accurately:
ISET = VSET / (RSET + RSET(INT) – K × RG(INT) ) ,(7)
where RSET(INT) is the internal resistance of the ISET
pin (330 Ω typical), RG(INT) is the internal resistance
of the bonding wire for the GND pin (27 mΩ typical),
and:
ISWlim = ISET × (K' + VIN × K")
+ (VBAT / LP ) × td ,
(8)
where K' = 47500, K" = 3500, and td = delay in SW
turn-off (0.12 μs typical).
Figure 2 shows the relationship between RSET and
ISWlim at different bias voltages, VIN, when battery
voltage, VBAT, is fixed at 3.6 V.
Peak Current Limit versus ISET Resistance at Various Bias Voltages
VBAT = 3.6 V, Transformer LP = 7.5 μH, TA=25°C
3.4
3.2
3.0
2.8
2.6
VIN = 5.0 V
2.4
VIN = 3.6 V
2.2
VIN = 3.0 V
2.0
1.8
1.6
1.4
1.2
1.0
0.8
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
RSET (kΩ)
Figure 2. Chart of current versus limit settings, at fixed battery voltage
Allegro MicroSystems, Inc.
11
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
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