Data Sheet

MP2636 –3.0A SINGLE CELL SW MODE BATTERY CHARGER WITH PPM AND 3.0A BOOST
MP2636 Rev.1.01 www.MonolithicPower.com 38
9/16/2015 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
SYS SYS(MAX)
BATT(MAX)
BATT
VI
I
V
×
=
×η
(26)
Where V
BATT
is the minimum battery voltage, f
SW
is the switching frequency, and I
L_MAX
is the
peak-to-peak inductor ripple current, which is
approximately 30% of the maximum battery
current I
BATT(MAX)
, I
SYS(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 (V
SYS
=5V), the inductance is
1.5µH when the efficiency is 90%.
For best results, use an inductor with an
inductance of 2.2uH with a DC current rating that
is not lower than the peak current of MOSFET
For higher efficiency, minimize the inductor’s DC
resistance.
Selecting the Input Capacitor C
IN
The input capacitor C
IN
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 X7R dielectrics because of their
low ESR and small temperature coefficients. For
most applications, a 22µF capacitor will be
sufficient.
Selecting the System Capacitor C
SYS
Select C
SYS
based on the demand of the system
current ripple.
1. Charge Mode
The capacitor C
SYS
acts as the input capacitor of
the buck converter in charge mode. The input
current ripple is:
TC IN _ MAX TC
RMS _ MAX SYS _ MAX
IN _ MAX
V(V V)
II
V
×−
=×
(27)
2. Boost Mode
The capacitor, C
SYS
, is the output capacitor of
boost converter. C
SYS
keeps the system voltage
ripple small and ensures feedback loop stability.
The system current ripple is given by:
TC SYS _ MAX TC
RMS _ MAX SYS _ MAX
SYS _ MAX
V(V V)
II
V
×−
=×
(28)
Since the input voltage is passes to the system
directly, V
IN_MAX
=V
SYS_MAX
, both charge mode and
boost mode have the same system current ripple.
For I
SYS_MAX
=2A, V
TC
=3V, V
IN_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 X7R dielectrics with low
ESR and small temperature coefficients. For
most applications, use three 22µF capacitors.
Selecting the Battery Capacitor C
BATT
C
BATT
is in parallel with the battery to absorb the
high-frequency switching ripple current.
1. Charge Mode
The capacitor C
BATT
is the output capacitor of the
buck converter. The output voltage ripple is then:
BATT SYS
BATT
BATT
2
BATT BATT SW
1V /V
V
r
V8CfL
−
Δ
Δ= =
×××
(29)
2. Boost Mode
The capacitor C
BATT
is the input capacitor of the
boost converter. The input voltage ripple is the
same as the output voltage ripple from equation
(28)
Both charge mode and boost mode have the
same battery voltage ripple. The capacitor C
BATT
can be calculated as:
TC SYS _ MAX
BATT
2
BATT _ MAX SW
1V /V
C
8r f L
−
=
×
Δ××
(30)
To guarantee the ±0.5% BATT voltage accuracy,
the maximum BATT voltage ripple must not
exceed 0.5% (e.g. 0.2%). The worst case occurs
at the minimum battery voltage of the CC charge
with the maximum input voltage.
For V
SYS_MAX
=6V, V
CC_MIN
=V
TC
=3V, L=2.2µH,
f
SW
=600kHz,
%2.0
_
=
Δ
MAXBATT
r
, C
BATT
is 39µF.
Two pieces of 22µF ceramic with X7R dielectrics
capacitor in parallel will suffice.
PCB Layout Guide
PCB layout is very important to meet specified