mp: MP2633 — 1.5A SINGLE CELL SWITCH MODE BATTERY CHARGER
MP2633 – 1.5A SINGLE CELL SWITCH MODE BATTERY CHARGER
MP2633 Rev. 1.08 www.MonolithicPower.com 29
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voltage, the CC charge threshold, and the
switching frequency, respectively. IL_MAX is
the maximum inductor ripple current, which is
usually designed at 30% of the CC charge
current.
With a typical 5V input voltage, 30% inductor
current ripple at the corner point between
trickle charge and CC charge (VBATT=3V), the
inductance is 1.85H (for a 1.2MHz switching
frequency), and 3.7µH (for a 600kHz
switching frequency).
2. Boost Mode
When the MP2633 is in boost mode (as a
boost converter), the required inductance
value is calculated as:
MAX_LSSYS
BATTSYSBATT
IfV
)VV(V
L
(24)
)MAX(BATTMAX_L I%)40%30(I (25)
BATT
SYSSYS
)MAX(BATT V
IV
I (26)
Where VBATT is the minimum battery voltage,
fSW is the switching frequency, and IL_MAX is
the peak-to-peak inductor ripple current,
which is approximately 30% of the maximum
battery current, IBATT(MAX). ISYS(MAX) is the
system current and is the efficiency.
In the worst case where the battery voltage is
3V, a 30% inductor current ripple, and a
typical system voltage (VSYS=5V), the
inductance is 1.8H (for the 1.2MHz
switching frequency) and 3.6µH (for the
600kHz switching frequency) when the
efficiency is 90%.
For best results, use an inductor with an
inductance of 1.8H (for the 1.2MHz
switching frequency) and 3.6µH (for the
600kHz switching frequency) with a DC
current rating that is at least 30% higher than
the maximum charge current for applications.
For higher efficiency, minimize the inductor’s
DC resistance.
Selecting the Input Capacitor, CIN
The input capacitor CIN reduces both the surge
current drawn from the input and the switching
noise from the device. The input capacitor
impedance at the switching frequency should be
less than the input source impedance to prevent
high-frequency-switching current from passing to
the input. For best results, use ceramic
capacitors with X5R or X7R dielectrics because
of their low ESR and small temperature
coefficients. For most applications, a 22µF
capacitor will suffice.
Selecting the System Capacitor, CSYS
Select CSYS based on the demand of the system
current ripple.
1. Charge Mode
The capacitor CSYS acts as the input capacitor of
the buck converter in charge mode. The input
current ripple is:
MAX_IN
TCMAX_INTC
MAX_SYSMAX_RMS V
)VV(V
II
(27)
2. Boost Mode
The capacitor, CSYS, is the output capacitor of
boost converter. CSYS keeps the system voltage
ripple small and ensures feedback loop stability.
The system current ripple is given by:
MAX_SYS
TCMAX_SYSTC
MAX_SYSMAX_RMS V
)VV(V
II
(28)
Since the input voltage passes to the system
directly, VIN_MAX=VSYS_MAX, both charge mode and
boost mode have the same system current ripple.
For ICC_MAX=2A, VTC=3V, VIN_MAX=6V, the
maximum ripple current is 1A. Select the system
capacitors base on the ripple-current temperature
rise not exceeding 10°C. For best results, use
ceramic capacitors with X5R or X7R dielectrics
with low ESR and small temperature coefficients.
For most applications, use a 22µF capacitor.
Selecting the Battery Capacitor, CBATT
CBATT is in parallel with the battery to absorb the
high-frequency switching ripple current.
1. Charge Mode
The capacitor CBATT is the output capacitor of the
buck converter. The output voltage ripple is then: