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13
2007 Semtech Corp.
www.semtech.com
SC804
PRELIMINARY
POWER MANAGEMENT
current and the charge path resistance from VOUT to the
Kelvin sense point should not exceed 150mV to ensure
normal mode operation.
The SC804 Adjust Mode schematic is shown in Figures 3A
and 3B. Referring to these schematics, the equation for
setting the output voltage is:
The capacitor C3 across R8 in the feedback network
introduces zero-pole frequency compensation for stability.
Place the zero according to the following equation to
ensure stability:
NOTE: When using Adjust Mode to program a CV regulation
voltage greater than V
CV, care must be taken when CC
charging with a charging adapter operating in current
limit.
Adapter current-limited operation occurs when
the adapter current limit is less than the programmed
SC804 fast charge current, such that the adapter voltage
is pulled down to V
VOUT plus the SC804 dropout voltage. A
low adapter current limit multiplied by the low minimum
path resistance of the main pass transistor and current
sense resistor (as low as 290m
Ω total at extremely low
temperature) can result in a voltage drop from VCC to
VOUT of less than 150mV if the Adjust Mode CV regulation
voltage is programmed above V
CV + 50mV. If VVCC - VVOUT <
150mV, Adjust Mode may not operate correctly. Adjust
Mode will operate correctly whenever the programmed
VOUT CV voltage is less than V
CV + 50mV, regardless of the
adapter current limit, because the regulated minimum
dropout voltage is always greater than 150mV in this
case. It will also operate correctly with an adapter current
limit greater than 550 mA, regardless of the programmed
output voltage, because the I-R dropout voltage will
exceed 150mV at even the lowest specied operating
temperature. Normal mode (that is, not Adjust Mode) has
a regulated minimum dropout voltage of approximately
200mV, which is constant for any V
VOUT, and so operates
correctly for any adapter current limit.
Over current and Max Temperature Protection
Over current protection is inherent in all modes of
R11 × C3 =
1
2 × 100kHz
Applications Information (Cont.)
VOUT = V
BSEN-ADJ_Typ ×
(1 + R11 )
R12
operation. When the device is in charge mode the output
is current-limited to either the pre-charge current limit
value or the fast charge current limit value depending on
the voltage at the output. Max die temperature protection
is also included. This feature allows the SC804 to operate
with maximum power dissipation by disabling the output
current when the die temperature reaches the maximum
operating temperature. The result is that the SC804 will
operate as a pulse charger in extreme power dissipation
applications, delivering the maximum allowable output
current while regulating the internal die temperature to
a safe level.
Indicator Flags
There are four indicator outputs/LED drivers on the SC804:
CPB (Charger Present), CHRGB (Charge Active), OVPB
(Over Voltage Fault), and FLTB (Fault). These outputs are
all active-low, open drain NMOS drivers capable of sinking
up to 10mA. The following table denes each indicator’s
output state.
The CPB output can be used as a VCC-present indicator.
Regardless of the state of NTC, the CPB output reects
the VCC voltage. When V
VCC is between the UVLO and OVP
thresholds the CPB output is low. If V
VCC is outside these
limits this output is high impedance.
3.2
3.4
3.6
3.8
4
4.2
4.4
0.2
0.4
0.6
0.8
Output voltage, V
Mi
ni
mum
dr
opout
v
ol
tage,
V
Figure 4 - Adjust Mode Minimum Dropout Voltage
The actual dropout voltage is the greater of the Minimum Dropout
Voltage at various programmed V
CV and instantaneous VOUT
voltages (shown here, with several programmed V
CV voltages
indicated in the gure by ‘o’), and the IR drop due to the product
of IOUT and R
DS-ON (not shown here).
Adjust mode operation is
ensured for any IOUT current at programmed V
CV voltages up to
approximately 4.25V.