Rainbow Electronics MAX1774 - Manual (Page 10)

MAX1774
Dual, High-Efficiency, Step-Down
Converter with Backup Battery Switchover
10 ______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 1, V
IN
= +5V, V
INC
= +3.3V, T
A
= +25°C, unless otherwise noted.)
TURN-ON RESPONSE
MAX1774-14 5V
0
V OUT
1V/div
2V
1V
100 µ s/div
3V
0
400 µ A
200 µ A
0
INPUT
CURRENT
200mA/div
V SHDN
5V/div
INPUT CURRENT
V MAIN V CORE
BACKUP SWITCHOVER RESPONSE
MAX1774-15
V BKUP
5V/div
I BBATT
50mA/div
V BIN
10mV/div
V MAIN
10mV/div
5 µ s/div
Pin Description
PIN
QSOP QFN
NAME FUNCTION
1 30 SHDNM
Shutdown for Main Regulator. Low voltage on SHDNM shuts off the main output. For normal
operation, connect SHDNM to IN.
2 31 SHDNC
Shutdown for Core Regulator. Low voltage on SHDNC shuts off the core output. For normal
operation, connect SHDNC to CVL.
3 32 BKUP
Open-Drain Backup Input/Output. The device is in backup mode when BKUP is low. BKUP can be
externally pulled low to place the device in backup mode.
4 1 MDRV
Open-Drain Drive Output. MDRV goes low when the ACI voltage drops below the main voltage plus
220mV and device is not in backup. Connect MDRV to the gate of the main battery P-channel
MOSFET to switch the battery to IN when the AC adapter voltage is not present.
5 2 PGNDC Power Ground for the Core Converter. Connect all grounds together close to the IC.
6 3 PGND
Power Ground. Ground for NDRV and core output synchronous rectifier. Connect all grounds
together close to the IC.
7 4 NDRV
N-Channel Drive Output. Drives the main output synchronous-rectifier MOSFET. NDRV swings
between CVL and PGND.
8 5 CVL
Low-Side Bypass. CVL is the output of an internal LDO regulator. This is the internal power supply
for the device control circuitry as well as the N-channel driver. Bypass CVL with a 1.0µF or greater
capacitor to GND. When CS- is above the CVL switchover threshold (2.47V), CVL is powered from
the main output.
9 6 IN Power Supply Input
10 7 PDRV
P - C hannel D r i ve Outp ut. D r i ves the m ai n outp ut hi g h- si d e M OS FE T sw i tch. P D RV sw i ng s b etw een IN and
C V H . The vol tag e at C V H i s r eg ul ated at V
IN - 4.2V unl ess the i np ut vol tag e i s l ess than 5.5V .
11 8 CVH
High-Side Drive Bypass. This is the low-side of the P-channel driver output. Bypass with a 1.0µF
capacitor or greater to IN. When the input voltage is less than 5.5V, CVH is switched to PGND.
12 9 LXB
Backup Converter Switching Node. Connect an inductor from LXB to the backup battery and a
Schottky diode to BIN to complete the backup converter. In backup mode, this step-up converter
powers the main output from the backup battery through BIN.