BQ24704 Datasheet, PDF (19/31 Pages) Texas Instruments – Low Cost, Host-Controlled Li-Ion and Li-Polymer Battery Charger With Low Iq

bq24704
www.ti.com........................................................................................................................................................................................................ SLUS838 – MAY 2009
I_{RIPPLE_MAX} happens when the duty-cycle, D is mostly near to 0.5 at given V_in, f_s,and L.
The ISYNSET comparator, or charge undercurrent comparator, compares the voltage between SRP-SRN, and
the threshold set by an external resistor R_ISYNSET, which can be calculated by:
250 V
R_ISYNSET
=
I_SYN x R_SENSE
W
(5)
R_SENSE
I_SYN
SRN
SRP
3.3 V
20x
I = 1 V/R_ISYNSET
1V
SYNCH
UCP
ISYNSET
R_ISYNSET
5 kW
Figure 28. ISYNSET Comparator Block
HIGH ACCURACY IADAPT USING CURRENT SENSE AMPLIFIER (CSA)
An industry standard, high accuracy current sense amplifier (CSA) is used to monitor the input current by the
host or some discrete logic through the analog voltage output of the IADAPT pin. The CSA amplifies the input
sensed voltage of ACP – ACN by 16x through the IADAPT pin. The IADAPT output is a voltage source 16 times
the input differential voltage. Once PVCC is above 5 V and ACDET is above 0.6V, IADAPT no longer stays at
ground, but becomes active. If the user wants to lower the voltage, they could use a resistor divider from IOUT to
AGND, and still achieve accuracy over temperature as the resistors can be matched their thermal coefficients.
A 100-pF capacitor connected on the output is recommended for decoupling high-frequency noise. An additional
RC filter is optional, after the 100-pF capacitor, if additional filtering is desired. Note that adding filtering also
adds additional response delay.
INPUT UNDERVOLTAGE LOCK OUT (UVLO)
The system must have a minimum 4 V PVCC voltage to allow proper operation. This PVCC voltage could come
from either input adapter or battery, using a diode-OR input. When the PVCC voltage is below 4 V the bias
circuits REGN and VREF stay inactive, even with ACDET above 0.6 V.
Copyright © 2009, Texas Instruments Incorporated
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