七,功能概述
Main Control Loop
The JTMA7178 is a constant frequency, synchronous rectifier and current-mode switching regulato
r. In normal operation, the internal P-channel power MOSFET is turned on each cycle. The peak
inductor current at which ICMP turn off the P-FET is controlled by the voltage on the COMP node,
which is the output of the error amplifier (EAMP). An external resistive divider connected between
VOUT and ground allows the EAMP to receive an output feedback voltage VFB at FB pin. When
the load current increases, it causes a slightly decrease in VFB relative to the 0.6V reference, which
in turn causes the COMP voltage to increase until the average inductor current matches the new load current.
Enable/Shutdown
Driving RUN to the ground places the JTMA7178 in shutdown mode. When in shutdown, the internal power
MOSFETs turn off, all internal circuitry shuts down and the quiescent supply current reduces to 0.5µA maximum.
 
Pulse Frequency Modulation Mode (PFM)
The JTMA7178 is a fixed frequency, peak current mode PWM step-down converter. At light loads, the JTMA7178
will automatically enter in pulse frequency mode operation to reduce the dominant switching losses. In PFM
operation, the inductor current may reach zero or reverse on each pulse. A zero current comparator turn off the N-
FET, forcing DCM operation at light load. These controls get very low quiescent current, help to maintain high effi-
ciency over the complete load range.
 
Slope Compensation and Inductor Peak Current
The JTMA7178 is a peak current mode PWM step downconverter. To prevent sub-harmonic oscillations, the
JTMA7178 sense the peak current and add slope compensation to stable the converter. It is accomplished in-
ternally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally,
this results in a reduction of maximum inductor peak current for duty cycles > 40%. However, the JTMA7178 uses a
special scheme that counteracts this compensating ramp, which allows the maximum inductor peak current
to remain unaffected throughout all duty cycles.
Dropout Operation
As the input supply voltage decreases to a value ap-proaching the output voltage, the duty cycle increases
toward the maximum on time. Further, reduction of thesupply voltage forces the main switch to remain on for
more than one cycle until it reaches 100% duty cycle. The input voltage minus the voltage drop will determine the
output voltage across the P-FET and the inductor.An important detail to remember is that on resistance of
P-FET switch will increase at low input supply voltageTherefore, the user should calculate the power dissipa-
tion when the JTMA7178 is used at 100% duty cycle with low input voltage.
Over-Temperature Protection (OTP)
The over-temperature circuit limits the junction tempera-ture of the JTMA7178. When the junction temperature ex-
ceeds 150oC, a thermal sensor turns off the both power MOSFETs, allowing the devices to cool. The thermal sen-
sor allows the converters to start a soft-start process and regulate the output voltage again after the junction tem-
perature cools by 30oC. The OTP is designed with a 30oC hysteresis to lower the average Junction Temperature
(TJ) during continuous thermal overload conditions, in-creasing the lifetime of the device.

八,相关产品

Part_No	Package & Pins	Topology	Architecture	No. of PWM Outputs	Drive Lout (A)	Vin (V)		Fmax(Khz)	Vref (V)	R-Top (milohm)	R-Sync (milohm)	Iq (No load) (uA)
						min	max
jtma7083	TQFN5x6-28	Buck	CM	1	5	4.5	26	380	0.8	50	20	1000
jtma7085	SOP-8	Buck	CM	1	2	4.5	26	380	0.8	100		1000
jtma7089	SOP-8P	Buck	CM	1	4	4.5	26	380	0.8	80		1000
jtma7104	SOT-23-5	Buck	CM	1	1	2.7	6	1500	0.6	280	250	25
	TSOT-23-5
jtma7104A	SOT-23-5	Buck	CM	1	1	2.7	6	1500	0.6	280	250
jtma7142	SOP-8	Buck	CM	1	3	4.3	14	500	0.8	70	70	500
jtma7145	SOP-8P	Buck	CM	1	3	4.3	14	500	0.8	55	45	500
jtma7153B	TDFN3x3-10				3	2.6	5.5	2000	0.8	110	110	460
jtma7176	TDFN3x3-10	Buck	CM	2	1	2.7	6	1500	0.6	280	250	500
jtma7176A	TDFN3x3-10	Buck	CM	2	1	2.7	6	1500	0.6	280	250
jtma7178	TDFN2x2-6	Buck	CM	1	1	2.7	6	1500	0.6	280	250	25
jtma7178A	TDFN2x2-6	Buck	CM	1	1	2.7	6	1500	0.6	280	250
jtma7179	TDFN3x3-12	Buck	CM	2	1	2.7	6	1500	0.6	280	250	25
jtma7179A	TDFN3x3-12	Buck	CM	2	1	2.7	6	1500	0.6	280	250
jtma7290	TDFN2x2-8		CM	1	1	3.3	8	1500	0.6	280	250	25
jtma7300	SOP-8	Buck	CM	1	2	4.5	24	340	0.925	110	110	1900
jtma7301	SOP-8P	Buck	CM	1	3	4.5	24	340	0.925	110	110	1900
jtma7302	SOP-8	Buck	CM	1	2	4.5	24	340	0.92	130	130	1900
jtma7302B	SOP-8P	Buck	CM	1	2	4.5	24	340	0.92	130	130	1900
jtma7303	SOP-8P	Buck	CM	1	3	4.5	24	340	0.92	85	85	1900
jtma7304	SOP-8P	Buck	CM	1	4	4.5	24	340	0.8	85	85	1900
jtma7311	SOP-8P	Buck	CM	1	3	4.5	24	1300	0.925	110	110	1900
jtma7312	SOP-8	Buck	CM	1	2	4.5	24	340	0.925	110	110	1900
jtma7313	SOP-8P	Buck	CM	1	3	4.5	24	340	0.925	110	110	1900
jtma7314	TSOT-23-8	Buck	CM	1	2	4.5	24	500	0.8	150	70	500
jtma7322	SOP-8P	Buck	CM	1	2	2.5	6	1000	0.8	85	75	460
	TDFN3x3-10
jtma7323	SOP-8P	Buck	CM	1	3.2	2.5	6	1000	0.8	85	75	460
	TDFN3x3-10
jtma7324	SOP-8P	Buck	CM	1	4	2.5	6	500	0.8	85	75	460
	TDFN3x3-10
jtma7325	SOP-8P	Buck	CM	1	5	2.5	6	1000	0.6	65	55	460
jtma7334	SOP-8	Buck	CM	1	2	4.5	24	340	0.8	110	110	1900
jtma7335	SOP-8P	Buck	CM	1	3	4.5	24	340	0.8	110	110	1900
jtma7336	TDFN3x3-10	Buck	CM	1	2	4.5	24	340	0.8	110	110	1900
jtma7337	TDFN3x3-10	Buck	CM	1	3	4.5	24	340	0.8	110	110	1900
jtma8713	TQFN4x4-23	Buck	COT	1	6	2.7	28	100~1000	0.8	30	9	750
jtma8802	TQFN5x6-28	Buck	COT	1	8	3	25	500	0.75	12	12	400
jtma8804	TDFN3x3-10	Buck	CM	1	3	2.6	6	1000	0.6	65	55	460
jtma8805	TDFN3x3-10	Buck	CM	1	3	2.6	6	1000	0.6	75	55	460
jtma8807	TQFN3x3-16	Buck	COT	1	8	6	28	500	0.6	24	14	80
jtma8824	TSOT-23-6	Buck	CM	1	1	2.7	5.5	1500	0.6	280	250	25
jtma8835	TDFN3x3-10	Buck	CM	1	3	4.5	24	600	0.8	70	70	400
jtma7414	TSOT-23-8A	Buck	CM	1	2	4.5	24	500	0.788	150	70	1000
jtma7319	TSOT-23-8A	Buck	CM	1	2.5	4.5	24	500	0.807	140	60	1000
jtma8715	TQFN 4x4 23	Buck	COT	1	6	2.7	28	100~1000	0.8	30	12	700
jtma8716A	TQFN 5x5 30	Buck	COT	1	12	2.7	28	300~1000	0.8	14.6	7.8	750
jtma8715	TQFN 4x4 23	Buck	COT	1	6	2.7	28	100~1000	0.8	30	12	700
jtma8805B	TDFN 3x3 10	Buck	CM	1	3	2.6	6	1000	0.8	75	55	460
jtma8805A	TDFN 3x3 10	Buck	CM	1	3	2.6	6	1000	0.8	75	55	460