Datasheet LTC3421 (Analog Devices) - 7

link to page 14 link to page 14 LTC3421
PIN FUNCTIONS RT (Pin 5):
Connect a resistor to ground to program the
GND, Exposed Pad (Pins 10 and 25):
Signal Ground Pin. oscillator frequency according to the formula: Connect to ground plane near the RT resistor, error amp 28,100 compensation components and feedback divider. The f = exposed pad must be soldered to the PCB and is typi- OSC RT cally connected through the power GND plane. where fOSC is in kHz and RT is in kΩ.
PGND (Pins 11 to 13):
Source Terminal of Power Internal
SS (Pin 6):
Soft-Start Pin. Connect a capacitor from this N-Channel MOSFET. pin to ground to set the soft-start time according to the
SW (Pins 14 to 16):
Switch Pin for Inductor Connection. formula: For applications where VOUT > 4.3V, a Schottky diode from t(ms) = C SW to V SS(µF) • 320 OUT or to a snubber circuit is required to maintain absolute maximum rating for SW. (see Application Circuits The nominal soft-start charging current is 2.5µA. The for 5V). active range of SS is from 0.8V to 1.6V.
V SYNC (Pin 7):
Oscillator Synchronization Pin. A clock
OUT (Pins 17, 19 and 20):
The output of the synchro- nous rectifier and bootstrapped power source for the IC. pulse width of 100ns to 2µs is required to synchronize the A ceramic bypass capacitor is required to be very close internal oscillator. If not used SYNC should be grounded. to the VOUT and PGND pins of the IC.
ILIM (Pin 8):
Current Limit Adjust Pin. Connect a resis-
V
tor from this pin to ground to set the peak current limit
OUTS (Pin 18):
VOUT Sense Pin. Connect VOUTS directly to an output filter capacitor. The top of the feedback divider threshold for the N-channel MOSFET according to the network should also be tied to this point. formula (note that this is the peak current in the inductor):
VIN (Pin 21):
Input Supply Pin. Connect this pin to the 150 I input supply and decouple with at least a 4.7µF ceramic LIM = R capacitor. where I is in amps and R is in kΩ.
LBO (Pin 22):
Open-Drain Output. This pin pulls low when
BURST (Pin 9):
Burst Mode Threshold Adjust Pin. A resis- the LBI input is below 0.6V. The open-drain output can tor/capacitor combination from this pin to ground pro- sink up to 20mA. During Burst Mode operation LBO is grams the average load current at which automatic Burst only active during the time the IC wakes up to service Mode operation is entered, according to the formula: the output.
LBI (Pin 23):
Low-Battery Comparator Input. Typical 2 R = threshold voltage is 0.6V with 30mV hysteresis. This BURST IBURST function is always enabled in non-back-fed applications. where R To enable this comparator when V BURST is in kΩ and IBURST is in amps. OUT is back-fed, the ENB pin must be high. The low-battery comparator will operate C • V C ≥ OUT OUT off VIN or VOUT, whichever is greater. BURST 10,000
VC (Pin 24):
Error Amp Output. A frequency compensa- where CBURST(MIN) and COUT are in µF. tion network is connected from this pin to ground to com- pensate the loop. See the section Closing the Feedback For manual control of Burst Mode operation, ground the Loop for guidelines. BURST pin to force Burst Mode operation or connect it to VOUT to force fixed frequency PWM mode. Note that the
Exposed Pad (Pin 25):
Ground. This pin must be soldered BURST pin must not be pulled higher than VOUT. to the PCB and is typically connected through the power GND plane. Rev. A For more information www.analog.com 7 Document Outline Description Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Package Description Typical Application Related Parts Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Package/Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Low Voltage Start-Up Low Noise Fixed Frequency Operation Burst Mode Operation Output Disconnect and Inrush Limiting Applications Information Component Selection Typical Application Package Description Revision History Typical Application Related Parts