SP6641BEK-L-5-0-TR Datasheet, PDF (7/14 Pages) Exar Corporation – 500mA Alkaline DC/DC Boost Regulator in SOT-23

OPERATION
Loop Regulation: continued
The SP6641 feed forward off-time control de-
livers more load current than constant off-time
control because the input battery voltage drops
during its life cycle. The term (I_PK – K_OFF/2L) is
the average current delivered to the output ca-
pacitor during the discharge phase. This is con-
stant with respect to input and output voltage.
With constant off-time control, the average dis-
charge current term becomes
(I_PK-T_OFF*(V_OUT-V_IN)/2L),
which decreases as the input voltage drops.
Table 1 illustrates the average inductor current
delivered to the load during discharge versus the
input voltage. The SP6641 feed forward off-
time control and the constant off-time control
are compared. For purposes of illustration, the
off times of each control scheme are normalized
at a typical two cell alkaline input voltage of
2.6V. The values used in Table 1 are:
I_PK = 0.33A
L = 22µH
V_OUT= 3.3V
T_OFF (SP6641) = 1.5V*µs/(3.3-V_IN)
T_OFF (constant) = 2.14µs
SP6641A
Constant T_OFF
V_IN T_OFF Avg I_L T_OFF Avg I_L
3.0 5.00µs 0.30A 2.14µs 0.32A
2.6 2.14µs 0.30A 2.14µs 0.30A
2.0 1.15µs 0.30A 2.14µs 0.27A
1.3 0.75µs 0.30A 2.14µs 0.23A
1.0 0.65µs 0.30A 2.14µs 0.22A
Table 1- Average I_L vs. Input Voltage
The following equation defines the burst mode
frequency under heavy load conditions:
( ) ( ) FBURST = VOUT – VIN
K_OFF
V_IN – V_C
V_OUT + V_D –V_C
where:
V_D = Forward schottky drop, (0.4V, typ)
V_C = Average charging switch drop,
Rnmos*I_PK, typically 0.1V
Ignoring the conduction losses of V_D and V_C,
the burst frequency equation simplifies to:
F_BURST = ^{(VOUT – VIN)VIN}
K_OFFV_OUT
Startup
The internal regulator circuitry is bootstrapped
to the V_OUT pin. This requires a low voltage
oscillator and charging switch powered from the
V_BATT pin to pump up the output voltage until
the reference is established. The reference pro-
vides a REFREADY signal that determines when
output control is handed over to the regulator.
REFREADY shuts down the startup circuit and
enables the regulator when the reference is valid
and V_OUT is above +1.9V. Once the regulator is
given control it will continue to pump up the
output at full power until regulation is reached.
For two cell alkaline input voltages and above,
the output voltage will be pulled above +1.9V
quickly through the rectifying diode before the
reference has a chance to establish. In this sce-
nario the startup circuit will coarsely regulate
around +2.8V until the REFREADY signal as-
serts. This keeps the output from overshooting
in startup with higher input voltages.
Startup is guaranteed at +0.9V at room tempera-
ture with a 3kΩ load. Heavier loads will require
a higher input voltage.
Shutdown/Enable Control
Pin 4 of the device is a V_BATT referred control
pin that shuts down the converter with the pin
tied to ground, or enables the converter with the
pin tied to V_BATT. When the converter is shut-
down the power switch is opened and all circuit
biasing is extinguished leaving only junction leak-
age currents on supply pins 3 and 5. The output
voltage will droop to one diode drop below the
battery voltage through the rectifying diode.
After pin 4 is brought high, the startup circuit is
enabled and starts pumping up the output until
REFREADY hands over control to the internal
regulator.
Rev. 5/26/05, *Patent Pending
SP6641A/6641B 500mA Alkaline DC/DC Boost Regulator in SOT-23
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© Copyright 2002 Sipex Corporation