AN4138 Datasheet, PDF (9/20 Pages) STMicroelectronics – This application note presents the HSP061-2

APPLICATION NOTE
AN4138
where V_o is the output voltage, V_OP is opto-diode forward
voltage drop, which is typically 1V and I_FB is the feedback
current of FPS. With I_FB=0.25mA (FSD210), R_d and R_bias
are determined as 56Ω and 510Ω, respectively.
Constant Current (CC) control : The current control
circuit is shown in detail in Figure 14. The CC control is
implemented using a transistor. Because the transistor base-
emitter voltage drop varies with the temperature, negative
thermal coefficient (NTC) thermistor is used for a
temperature compensation.
I_FB /2
+
V_OP
-
R_d
56Ω
1Ω
R_sense
I_o = 0.65A
10kΩ
R_TH
V_sense
510Ω
R_bias
e
c
I_C
V_BE
b
Q
I_B
KSP2222
I_RTH
R_base
510Ω
KA431
I_C
=
(_{---2---5----0---}µ_----A------⋅_{-5--5-1--6-0---}Ω_-Ω----)_---⁄_---2-----+_{-----1----V--}
+
1--
2
⋅
250µA
=
2.1mA
By assuming that the current gain (β) of Q is 100, the
transistor base current is obtained as
I_B
=
I-_β-C--
=
2----.-1----m-----A---
100
=
21uA
(32)
The voltage drop in the sensing resistor (V_sense) should be set
to be 40-100mV higher than the transistor base-emitter
voltage (V_BE) at room temperature (25°C). The actual
transistor base-emitter voltage (V_BE) temperature is
measured at room temperature as 0.608V with I_C of 2.1mA
and V_sense is determined to be 0.650V.
With the V_sense chosen, the sensing resistor (R_sense) is
obtained as
R_sense
=
V-----_{s--e---n---s---e-}
I_o
=
0----.--6---5---V---
0.65A
=
1Ω
(33)
where I_o is SMPS output current.
It is typical to design the NTC thermistor so that the current
through the thermistor would be about 3-6 times of the
transistor base current at room temperature. The resistance
of the thermistor at room temperature (R_TH) is determined as
10 kΩ. The current through the thermistor is obtained as
Figure 14. Current control circuit in detail
When the voltage across the sensing resistor is sufficient to
turn on the transistor, CC controller is enabled while CV
controller is disabled. Then, the KA431 consumes very small
current and most of the currents through R_d and R_bias flow
into the collector of the transistor Q. By assuming that the
feedback voltage of FPS (V_FB) is in the middle of its
operating range, half of the FPS feedback current (I_FB) sinks
into the opto-coupler transistor. Since it is also assumed that
the CTR of the opto-coupler is 100%, the transistor collector
current is given by
I_C
=
(_{---I--F---B-----}⋅_---R----d---)_----⁄_--2-----+_{------V----o---p-}
R_bias
+
1--
2
⋅
I_FB
(31)
where I_FB is the feedback current of FPS, V_OP is opto-diode
forward voltage drop, which is typically 1V.
I_RTH
=
-V----^B---^E--
R_TH
=
0---1-.--60---0-k--8-_Ω---V--
=
61µA
(34)
The base resistor is determined by
R_base
=
V-----_{s--e---n---s---e----}–_{-----V----B---E--}
-V----^B---^E--
R_TH
+
I_B
=
-0---.--6---5----V-----^–----0----.--6---0---8----V--
0---1-.-6-0---0-k--8-_{Ω---V-- + 21µA}
=
513Ω
(35)
Variations in the junction temperature of Q will cause
variations in the value of controlled output current (I_o). The
base-emitter voltage decreases with increasing temperature
at a rate of approximately 2mV/°C. When the base-emitter
voltage is changed to V_BET as the temperature changes to T
°C, the thermistor resistance at T °C required to compensate
this variation is given by
R_T
T
H
=
------------------V----^B----^E---^T-------------------
V-----_{s--e---n---s---e----}–_{-----V----B---E----}^T_-
R_base
–
I_B
(36)
From the circuit in Figure 14, I_C is obtained as
With -2mV/°C, V_BE reduces to 0.508V from 0.608V as
temperature increases from 25°C to 75°C. From equation
©2003 Fairchild Semiconductor Corporation
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