DS9535A Datasheet, PDF (14/18 Pages) Richtek Technology Corporation – High Efficiency Switching Mode Battery Charge

RT9535A
Full-Scale Charge Current Programming
The basic formula for full-scale charge current is (see
Block Diagram) :
I_BATT =^{}
V_REF
R4





RS2 
RS1 ^
Where R4 is the total resistance from ISET pin to
ground. For the sense amplifier CA biasing purpose,
RS3 should have the same value as RS2 with 1%
accuracy. For example, 2A full-scale charging current is
needed. For low power dissipation on RS1 and enough
signal to drive the amplifier CA, let RS1 = 100mV/2A
= 50m. This limits RS1 power to 0.2W. Let R4 = 10k,
then :
RS2 = RS3 = ^{IBATT R4RS1} = ^{2A 10k 0.05} = 400Ω
V_REF
2.5V
Note that for charge current accuracy and noise
immunity, 100mV full scale level across the sense
resistor RS1 is required. Consequently, both RS2 and
RS3 should be 402. The R4 should be set to between
5k and 15k for the best operation.
It is critical to have a good Kelvin connection on the
current sense resistor RS1 to minimize stray resistive
and inductive pickup. RS1 should have low parasitic
inductance (typical 3nH or less). The layout path from
RS2 and RS3 to RS1 should be kept away from the fast
switching SW node. A 1nF ceramic capacitor can be
used across SNSH and SNSL and be kept away from
the fast switching SW node.
Battery Voltage Regulation
The RT9535A uses high-accuracy voltage bandgap
and regulator for the high charging-voltage accuracy.
The charge voltage is programmed via a resistor
divider from the battery to ground, with the midpoint
tied to the VFB pin. The voltage at the VFB pin is
regulated to 2.5V, giving the following equation for the
regulation voltage:
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14
V_BATT
=
2.5

₁₊
RF2
RF1


where RF2 is connected from VFB to the battery and
RF1 is connected from VFB to GND.
Charging
The 2A Battery Charger (typical application circuit)
charges lithium-ion batteries at a constant 2A until
battery voltage reaches the setting value. The charger
will then automatically go into a constant voltage mode
with current decreasing to near zero over time as the
battery reaches full charge.
Dropout Operation
The RT9535A can charge the battery even when VIN
goes as low as 2V above the combined voltages of the
battery and the drops on the sense resistor as well as
parasitic wiring. This low VIN sometimes forces 100%
duty cycle and high-side power switch stays on for
many switching cycles. While high-side power switch
stays on, the voltage VBOOT across the capacitor C8
drops down slowly because the current sink at BOOT
pin. C8 needs to be recharged before VBOOT drops too
low to keep the topside switch on.
A unique design allows the RT9535A to operate under
these conditions. If SW pin voltage keeps larger than
1.3V for 32 oscillation periods, high-side power
MOSFET will be turned off and an internal MOSFET
will be turned on to pull SW pin down. This function
refreshes VBOOT voltage to a higher value. It is
important to use 0.1F to hold VBOOT up for a sufficient
amount of time.
Shutdown
When adapter power is removed, VIN will drift down.
As soon as VIN goes down to 0.1V above VBATT, the
RT9535A will go into sleep mode drawing only ~10A
from the battery. There are two suggest ways to stop
switching: pulling the EN pin low or pulling the VC pin
low. Pulling the EN pin low will shut down the whole
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DS9535A-04 February 2016