DS9535A Datasheet, PDF (13/18 Pages) Richtek Technology Corporation – High Efficiency Switching Mode Battery Charge

RT9535A
Application Information
Input and Output Capacitors
In the typical application circuit, the input capacitor (C2)
is assumed to absorb all input switching ripple current
in the converter, so it must have adequate ripple
current rating. Typically, at high charging currents, the
converter will operate in continuous conduction mode.
In this case, the RMS current IRMSIN of the input
capacitor C2 can be estimated by the equation :
I_RMSIN = I_BATT  D-D²
Where IBATT is the battery charge current and D is the
duty cycle. In worst case, the RMS ripple current will be
equal to one half of output charging current at 50% duty
cycle. For example, IBATT = 2A, the maximum RMS
current will be 1A. A low-ESR ceramic capacitor such
as X7R or X5R is preferred for the input-decoupling
capacitor and should be placed to the drain of the
high-side MOSFET and source of the low-side
MOSFET as close as possible. The voltage rating of
the capacitor must be higher than the normal input
voltage level. Above 20F capacitance is suggested for
typical of 2A charging current.
The output capacitor (CBATT) is also assumed to
absorb output switching current ripple. The general
formula for capacitor current is :
I_RMSCB =
V_BATT
 ^{}1-
V_BATT
V_VIN


2 3 L1 f_osc
For example, VVIN = 19V, VBATT = 8.4V, L1 = 10H,
and fOSC = 500kHz, IRMS = 0.15A.
EMI considerations usually make it desirable to
minimize ripple current in the battery leads. Beads or
inductors may be added to increase battery impedance
at the 500kHz switching frequency. Switching ripple
current splits between the battery and the output
capacitor depending on the ESR of the output capacitor
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
DS9535A-04 February 2016
and the battery impedance. If the ESR of COUT is 0.2
and the battery impedance is raised to 4 with a bead
or inductor, only 5% of the ripple current will flow in the
battery.
Inductor
The inductor value will be changed for more or less
current ripple. The higher the inductance, the lower the
current ripple will be. As the physical size is kept the
same, typically, higher inductance will result in higher
series resistance and lower saturation current. A good
tradeoff is to choose the inductor so that the current
ripple is approximately 30% to 50% of the full-scale
charge current. The inductor value is calculated as :
L1 = ^{VBATT   VVIN-VBATT }
V_VIN  f_OSC  ΔI_L
Where IL is the inductor current ripple. For example,
VVIN = 19V, choose the inductor current ripple to be
40% of the full-scale charge current in the typical
application circuit for 2A, 2-cell battery charger, IL =
0.8A, VBATT = 8.4V, calculate L1 to be 12H. So
choose L1 to be 10H which is close to 12H.
Soft-Start and Under-Voltage Lockout
The soft-start is controlled by the voltage rise time at
SS pin. There are internal soft-start and external
soft-start in the RT9535A. With a 0.01F capacitor,
time to reach full charge current is about 20ms and it is
assumed that input voltage to the charger will reach full
value in less than 20ms. The capacitor can be
increased if longer input start-up times are needed.
For the RT9535A, it provides Under-Voltage Lockout
(UVLO) protection. If 5V5LDO output voltage is lower
than 3.5V, high-side internal power MOSFET. This will
protect the adapter from entering a quasi “latch” state
where the adapter output stays in a current limited state
at reduced output voltage.
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