LTC3620
9
3620fa
OPERATION
The LTC3620 is a variable frequency buck switching regu-
lator with a maximum output current of 15mA. At high
loads the LTC3620 will supply constant peak current pulses
through the output inductor at a frequency dependent on
the load current.
A switching cycle is initiated by a pulse from the error
amplifi er, EAMP. The top FET is turned on and remains on
until the peak current threshold is sensed by ICMP (35mA
at full loads). When this occurs, the top FET it is turned off
and the bottom FET is turned on. The bottom FET remains
on until the inductor current drops to 0A, as sensed by the
reverse-current comparator, RCMP. The time interval before
another switching cycle is initiated is adjusted based on
the output voltage error, measured by the EAMP to be the
difference between V
FB
and the 0.6V reference.
As the load current decreases, the EAMP will decrease
the switching frequency to match the load, until the mini-
mum switching frequency (internally or externally set) is
reached. With the FMIN/MODE pin pulled low, the minimum
frequency is internally set to 50kHz. Further decreasing
the load will cause the phase frequency detector (PFD) to
decrease the peak inductor current in order to maintain
the switching frequency at 50kHz.
The minimum switching frequency can be externally set
by clocking the FMIN/MODE pin at the desired minimum
switching frequency. The load current below which the
switching frequency will be clamped is dependent on the
externally set frequency and the value of the inductor used.
A higher externally set minimum frequency will result in
a higher load current threshold below which the part will
lock to this minimum frequency. The relationship between
load current and minimum frequency is described by the
following equation:
I
MAX(LOCK)
=
V
IN
()
f
MIN
()
L
()
35mA
()
2
2V
OUT
V
IN
–V
OUT
()
The LTC3620 will switch at this externally set frequency
at load currents below this threshold; though in general,
neither this minimum nor this synchronization will be
maintained during load transients.
At very light loads, the minimum PFET on time will be
reached and the peak inductor current can no longer
be reduced. In this situation, the LTC3620 will resume
decreasing the regulator switching frequency to prevent
the output voltage from climbing uncontrollably.
For those applications which are not sensitive to the spectral
content of the output ripple, the minimum frequency clamp
can be defeated by pulling the FMIN/MODE pin high. In this
mode the inductor current peaks will be held at 35mA and
the switching frequency will decrease without limit.
Figure 1. Switching Frequency vs Load Current, FMIN/MODE
LOAD CURRENT (mA)
100
SWITCHING FREQUENCY (kHz)
1000
0.01 1 10 20
3620 F01
10
0.1
T
A
= 25°C
V
IN
= 3.6V
V
OUT
= 1.1V
200kHz, EXTERNAL
FMIN/MODE = 0V
FMIN/MODE = V
IN
